Main active faults and seismic activity along the Yangtze River Economic Belt: Based on remote sensing geological survey

1.   Introduction

The Yangtze River Economic Belt (YREB) is one of the three development strategies recently initiated by China, namely, the Belt and Road Initiative, the Coordinated Development of Beijing, Tianjin and Hebei, and the YREB. The YREB spans Shanghai, Jiangsu, Zhejiang, Anhui, Jiangxi, Hubei, Hunan, Chongqing, Sichuan, Yunnan and Guizhou provinces by virtue of the waterways of the Yangtze River, covering an area of approximately 2.05 ×10⁶ km². The YREB is a combination of “one axis, two flanks, three poles and multiple points”. “One axis” refers to a riverside green development axis centered around Shanghai, Wuhan and Chongqing, all interlinked by the waterways of the Yangtze River. “Two flanks” refers to the Shanghai-Ruili and Shanghai-Chengdu transportation corridors, along which transport will be interlinked to provide access to the local populace and for industrial cohesion of important node cities in the northern and southern bottomlands. “Three poles” refers to the Yangtze River delta, the middle Yangtze, and the Chengdu-Chongqing urban agglomerations, which will become the three poles of growth along the YREB by synergizing the diverse capacities of the central cities. “Multiple points” refers to the prefecture-level cities not included in the three urban agglomerations, which will serve as supporting pillars for the economic connections and interactions with the central cities, in order to drive the regional economy.

The YREB measures about 2300 km by 1400 km, encompassing the first, second, and third altitudinal steps of Mainland China. The altitude of the area ranges from an average of 4000 m on the southeastern margin of the Qinghai-Tibet plateau, down to a few meters in the eastern coastal area. It spans the southeastern Qinghai-Tibet plateau, the Yunnan-Guizhou plateau, and the uplands, alluvial plains, and basins (lakes) of Central East China. Diverse topography, considerable geotectonic variability, different present-day crustal deformation, and geohazard-triggering conditions characterize the YREB. Hence, it is essential to investigate the geological tectonic conditions; to examine present-day crustal activity; and to carry out a comprehensive survey and assessment of the geo-environment in this area, in order to support the implementation of the country’s economic development strategies. Late Cenozoic active faults are the most important geological factors affecting the regional economic planning of China, especially the YREB, which is a critical part of China’s development. Systematic investigation of the active faults and seismic activity, and a comprehensive assessment of the present-day crustal stability will provide a geoscientific basis for the development of core cities, as well as for the planning and development of important transport lines, important urban agglomerations, and towns therein, as also other major infrastructure projects along the YREB.

In this paper, the particularly noteworthy active fault zones and their effects on the crustal stability of the urban agglomerations were identified through the process of investigating the active faults and seismic activity along the YREB. In addition, the active tectonic systems and potential future earthquake exposure of the area have been discussed. A preliminary assessment of present-day stability of the area has also been conducted.

2.   Geotectonic background

The geological structure of the Yangtze River Economic Belt is complex. Geotectonically, it runs across three geological units: The Sichuan-Yunnan-Tibet orogen, the Yangtze block, and the Wuyi-Yunkai-Taiwan orogen (also called the South China orogenic belt or fold belt); thus involving a huge number of secondary tectonic blocks with different evolution histories (Ma LF, 2002; Pan GT et al., 2009). During the Neotectonic period about 10–8 Ma, the older orogens and tectonic features in mainland China were reactivated under the control of two dynamic systems: The NNE low-angle intracontinental subduction of the Indian Plate and the westward high-angle subduction of the Western Pacific Plate, which led to the emergence of many active faults and the occurrence of active tectonic systems. The YREB traverses the southern part of Mainland China. Inevitably, it is home to many active faults of varying sizes and activity intensities as its present-day crustal activity is jointly subject to these two dynamic systems in the east and the west.

Spatially, the YREB is approximately divided into a western part, a central part, and an eastern part by the nearly NS-trending Chengdu-Xichang-Kunming line (which roughly corresponds with the central southern section of the North-South seismic belt of Chinese mainland) and the NNE Suqian-Hefei-Jiujiang line (which roughly corresponds with the southern section of the Tanlu fault zone). The western part is in the Qinghai-Tibet active tectonic region; the central and eastern parts are in the South China active tectonic region.

The western YREB is located in the southeastern Qinghai-Tibet plateau, the Longmenshan Mountains, and the Yunnan-Guizhou plateau. It mainly contains the Sichuan-Yunnan active block, the most active structure on the eastern margin of the Qinghai-Tibet plateau. Its present-day crustal activity is typically controlled by the continent-continent collision between the Indian and Eurasian plates. The intense extrusion of materials from the Qinghai-Tibet plateau is responsible for the complex and extremely active crustal activity in this area. The present-day crustal tectonic stresses and crustal activity, both featuring clockwise rotation around the Eastern Himalayan syntaxis, gave rise to a number of large active structure zones, which include the Longmenshan structure belt and the Xianshuihe-Xiaojiang fault system, as well as many medium and other size active fault systems within them (Wang EC et al., 1998; Zhang PZ et al., 2013). Among these faults, the most active ones are typically found on the boundary of the Sichuan-Yunnan block, where the highest activity can be in the range of 10–15 mm/a (Zhang PZ et al., 2004, 2013). Boundaries of the secondary blocks are mostly home to medium-sized faults with fault activity rate mostly distributed in the 0.5–2.0 mm/a range (Wu ZH et al., 2015; Shen ZK et al., 2005). This provides an area with the largest active fault density, as well as the highest earthquake frequency and the most significant crustal instability in China.

Large NE orientation subsidence and uplift zones of the Neocathaysian tectonic system run across the central-eastern YREB, which is controlled by the collision between the Indian and Eurasian plates, as well as the westward subduction of the Western Pacific Plate. The principal part of this area belongs to the relatively stable South China active tectonic region and includes the secondary Mid-lower Yangtze fault block and the Sichuan-Guizhou-Hunan-Jiangxi fault block regions (Deng QD et al., 2002). Focal mechanism solutions indicate a nearly E-W compression of the present-day tectonic stress field of the area (United Group of Focal Mechanism, 1981; Wang SY and Xu ZH, 1985; Wang SY et al., 1987; Xu ZH et al., 1997; Zheng YJ et al., 2006). GPS data demonstrate that the Yangtze block is moving N130°–150° E, relative to the stable Siberian plate in the north, at the rate of 8–14 mm/a; the differential movement is insignificant and no particularly obvious velocity gradient exists inside the block (Zhang PZ et al., 2002; Ye ZR et al., 2004; Li YX et al., 2006).

Previous studies of the active faults in China and along the YREB (Deng QD et al., 1994; Zhu JA et al., 1984; Ding BT., 1985; Wang B et al., 2008) have revealed that the fault activity is relatively weak across South China since the Neogene, whereas significant fault activity and differential activity due to faulted blocks have existed in the central-eastern YREB during the Quaternary.

3.   Methodology and concepts

3.1   Concepts related to active tectonics and active faults

Active tectonics is generally defined as: Tectonics those are active since the Late Quaternary and will continue to be active in the future (Wallace RE, 1986; Yeats RS, et al., 1997). Active faults, as the main type of active tectonics, are defined in a similar fashion. To be precise, they are faults that have been active during the Late Quaternary (mainly since about 150 Ka, or since the Late Pleistocene) and will continue to be active in the future (Yeats RS et al., 1997; Deng QD et al., 1994, 2008). Given that active faults are defined directly in connection with seismic risk assessment, one of the essential goals of active fault investigation is to determine whether a fault will be active and trigger seismic disasters in future (Wu ZH et al., 2014). As far as East China, especially South China, is concerned, the activity of Late Quaternary, Quaternary and even Cenozoic faults is often notably consistent and inheritable (Deng QD, 1982). The crustal deformation and fault activity of the central-eastern YREB have been much weaker than that of the west YREB since the Late Quaternary. Hence, any discussion on the active tectonics in this area only with respect to faults that are active since the beginning of the Late Quaternary, would be limited. Rather, it is necessary to look at faults and tectonics that were notably active during the Quaternary, and even those that were active during the Cenozoic, especially since the Neogene.

3.2   Methodology

3.2.1   Remote sensing interpretation and recognition of active faults

In this paper, the main active faults that directly or indirectly threaten the crustal stability of important urban agglomerations, state-level new areas, and regionally important cross-river transport corridors were identified after careful review of the active tectonic systems and the historical seismic activity of known main active faults along the YREB. This was based on available active tectonics data in China (Deng QD et al., 2007), as well as through our new findings from comprehensive remote-sensing interpretation of various resolutions and surface surveys of the active faults in the area. The active structure characteristics and crustal stability of the YREB have also been macroscopically summarized. Analysis and findings of our study can provide some guidance for subsequent research and studies in this respect.

Our active fault review involved active faults that: (1) Constitute the main fault-blocks boundaries or play a critical role in the regional active tectonic system; (2) have induced M >5.0 destructive earthquakes; or (3) are topographically significant or control the boundary of a main Quaternary depression basin in the area.

The authors can observe both similarities and differences in the activity performance of different fault zones, including the frequency and intensity of seismic activity, however, the indicators used to indicate the fault activity should be different. Therefore, it is necessary to classify the activity according to different fracture types. Combined with the differences in fault activity, they mainly manifest in: the scale of the fault, continuity of spatial distribution, amplitude and significance of dislocation quaternary or Late Quaternary geological geomorphic body, and the seismicity (including the frequency of the largest earthquake and major earthquake that can occur) and average slip rate of the fault. Based on this, the activity of the faults can be divided into strong, medium, and weak levels.

Remote sensing image can not only study the occurrence, development and spatiotemporal evolution of fault structure from a macro and dynamic point of view, but also significantly improve the efficiency of active fault investigation through comprehensive interpretation of remote sensing image, and help to determine the active amplitude or displacement of key sections of the fault, so as to provide an important basis and reference for the analysis of geometric and kinematic characteristics of the fault zone. The special linear signs or features in remote sensing images are the important basis for identifying active faults. These features include (Fig. 1): Abnormal color tones, boundaries and color bands of image structural units; abnormal zones of sharp changes in different geomorphic units and drainage system types; structural landforms such as linear extended scarps and fault triangular facets; linear arrangement and composite superposition of proluvial skirts; and the drainage system is distributed in a straight line and lattice shape; the mountain ridges and valleys are dislocated and distorted; the sharp turns and synchronous deflection of a series of rivers; the river is offset; the linear arrangement, extension and distortion of the modern sedimentary basins and lakes; the linear distribution and boundary of the modern sedimentary center; the rupture, dislocation and fold deformation of the Quaternary sediments (See Table 1 for details).

Figure  1.  The remote sensing image feature on some important active faults in the Yangtze River Economic Belt.

a–the linear image of braided structure in the southern section of Xiaojiang left lateral strike slip active fault zone (Yiliang-Tonghai section, Yunnan Province) and the left lateral pull apart fault basin between them; b–about 64 km left lateral deflection of Jinsha River across the Yushu-Garzê fault zone; c–linear basin mountain boundary indicating quaternary normal fault activity in the western margin of the nearly N-S Baoshan basin in western Yunnan Province; d–linear images of Zhongdian fault and Jianchuan fault in western Yunnan Province and their beadlike distribution of basin-valley topography; e–the geomorphology of fault triangle facets in the western margin of Baoshan basin; f–earthquake surface rupture on Kangding left lateral strike slip fault; g–the main geomorphological indicators of the Late Quaternary sinistral strike slip faulting along Yushu-Garzê fault zone (modified from Taylor M and Yin A, 2009).

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Table  1.  Main remote sensing image indicators for identification of regional fault activity in the Yangtze River Economic Belt.

Type	Main signs	The concrete expression of remote sensing image
Linear sign	The regular distribution of landform or surface features along a certain direction of space	For example, the connection line of drainage system synchronous inflection point, the connection line of a series of alluvial proluvial fans, basins with en echelon or beaded distribution, the linear distribution of cliffs and scarps, the linear distribution of lakes and negative terrain, the linear distribution of hot springs and islands, Linear intermittent surface rupture caused by earthquakes, and the linear distribution of rocks, dikes and craters, etc.
	The line between hue and shape formed in a particular direction	For example, the boundary between uplift area and subsidence area, between bedrock and Quaternary loose sediment, between vegetation difference, between water rich degree of loose sediment, and between different geomorphic areas along a straight line.
Vertical offset sign	Fault scarp and fault triangle facet	On the image, fault scarps are linear traces with deep hue and obvious shadow, ranging from several kilometers to tens of kilometers. The multi-stage active faults often form stepped scarps, each of which represents a vertical differential movement of the fault, and it appears as a dark tone protrusion on the high-resolution remote sensing image. The triangular surface of faults is mostly triangular, and some of them are trapezoid, and arranged orderly along the faults. The features of the images are obvious.
	Uplift area and fault depression area	Due to the denudation of external forces, the uplifted area has undulating terrain, complex image, obvious shadow, deep color, obvious ridge line, narrow and deep valley bottom; the faulted area is often covered by Quaternary loose sediments, with slight undulation, no shadow, and shallow image color.
	Formation displacement	In stereoscopic observation, if the near horizontal strata cannot be connected after the natural extension of the space on both sides of the valley up and down dislocation.
	Change of drainage system distribution characteristics	The vertical differential dislocation of active faults often results in the change of drainage pattern. In the upthrow plate, the drainage system is generally in the form of deep branches or lattices; in the downthrow plate, the drainage system is mostly in the form of branches, parallels, feathers or fans. If the relative downthrow plate is tilted, the parallel drainage system is generally formed; if it is a groove type fault basin, the feather drainage system is mostly formed. At the edge of the fault block uplift with large vertical difference dislocation amplitude, the vertical section gradient is large, and the river valley across the fault is hanging valley or bottle shaped valley. The riverbed from the ascending plate to the descending plate often suddenly widens, or becomes fan-shaped drainage system, or even wandering river. The meander section formed by several rivers is arranged in a belt along a certain direction, suddenly widened or narrowed, the straight-line section and meander section of the river are suddenly transformed, the riverbed is suddenly changed or the water drop (waterfall) suddenly occurs in the longitudinal direction, the “elbow” bend of the drainage system, etc.
	Mark of alluvial-proluvial fan	On the satellite image, the older alluvial proluvial fans have poor water yield, lighter color, more damaged fan body shape, and slightly gentle surface undulation; the newer alluvial proluvial fans have better water yield, darker color, intact fan shape, and flat surface.
Horizontal offset sign	Horizontal displacement of formation	Fault activity can cause traction, drag, deformation, fold and fault of new strata in two plates. They are linear and striking in remote sensing image, and the corresponding points of translation of two plates are clear and easy to interpret. Tracing the contact line of a rock stratum, dyke, orebody or intrusions along the strike, the image is suddenly interrupted, dislocated-resulting in the connection with another group of strata with different hues, topography and other characteristics.
	Repetition or absence of strata	For the strata with certain sequence, there are sudden repetition or absence of hue, landform, drainage system, texture, etc. along the inclined direction.
	Structural dislocation	Some structural lines (outcrops such as folds, faults and unconformities) are suddenly interrupted and offset.
	Ridge offset	Faults with strong strike slip activity can often offset the modern geomorphic forms developed in Cenozoic, such as ridge, spur, strip denudation surface, etc. On remote sensing images, the strike slip direction of faults can be determined according to the offset of the ridge line.
	Pull apart basin	Because of the difference of horizontal movement between the two plates, the strike slips active faults often lead to the formation of certain geometry shape of the basins controlled by the faults, most of which are banded or rhombic pull apart basins, which can be seen clearly on the image.
	Synchronous offsetting of drainage system and terrace	The horizontal offset of the active fault and the direction of strike slip movement are different, which can lead to the different geometry of the river in the plane. The protruding direction of the arc generally represents the direction of translation of the plate, such as the “S”, “L” and “Z” type of the river, indicating the left-hand strike slip movement of the fault; otherwise, indicating the right-hand strike slip movement of the fault. The dislocation of river terraces usually occurs at the same time with the drainage system. The dislocation distance of terraces at different stages in different periods reflects the horizontal activity amplitude, activity frequency and activity nature of faults.
	Synchronous asymmetry of valley and valley slope	In the process of horizontal dislocation of rivers across faults, synchronous side erosion often occurs, resulting in the asymmetry of valley slope on both banks, while the erosion bank and steep valley slope formed by side erosion reflect the movement direction of the plate. On the remote sensing image, the hue of gentle valley slope is lighter and well developed in the Quaternary; the hue of steep valley slope is darker and the common erosion walls and cliffs are developed with obvious shadow.
	Gully dislocation, beheaded stream and betrunked stream	Gully dislocation is a common geomorphic feature of strike slip active faults. The gully dislocation developed in different times is different. The old gully experienced more fault activities and larger dislocation; the new gully experienced less fault activities and smaller dislocation. When the amplitude of horizontal dislocation of a fault exceeds the width of the valley, a beheaded stream and betrunked stream will be formed. These marks can be clearly displayed on the image.
	Form change of alluvial-proluvial fan	The acute angle direction of oblique intersection between the central axis of fan-shaped landform and the fault can indicate the horizontal movement direction of the fault, and the lateral stacking of diluvial fans in different periods can also be used as evidence of the horizontal movement of the fault, and the direction from the old fan body to the new fan body can reflect the translation direction of the plate.

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3.2.2   Methods for seismic activity assessment

The seismic activity of the YREB, particularly that of the central-eastern YREB, was characterized after review of the historical and instrument-recorded seismic data and based on the results of paleo earthquake surface surveys. The seismic zones were demarcated and the seismically empty regions in East China were also investigated based on the seismic gap concept.

Paleoearthquakes, also called prehistoric earthquakes, are earthquake events identified through geological or topographic survey, or unrecorded earthquakes discovered through archaeological efforts. The latter are also referred to as archaeological earthquakes. Paleoearthquake research can reveal or restore large earthquake event sequences in an active fault zone or a specific area during its geological history as a complement to the short period of the historical earthquake record. It helps improve the understanding of the processes, characteristics and rules of regional large earthquakes, and also provides a scientific basis for forecasting potential large earthquakes in future.

4.   Main results

4.1   Main active faults in the Yangtze River Economic Belt

A total of 165 main active faults along the YREB, including 79 in the western YREB and 86 in the central-eastern YREB, were reviewed (Fig. 2).

Figure  2.  Main active faults, important urban agglomerations, and major projects in the central-eastern Yangtze River Economic Belt.

F1–Chenjiabao-Xiaohai fault; F2–Suqian-Yangzhou fault; F3–Wuxi-Suzhou fault; F4–Zhenhai-Wenzhou fault; F5–Nanjing-Hushu fault; F6–Maoshan fault; F7–Jintan-Nandu fault; F8–Maanshan-Wuhu fault; F9–Jiangnan fault; F10–Chaohu-Hangzhou fault; F11–Guzhen-Huaiyuan fault; F12–Xinyang-Jinzhai fault; F13–Gegong fault; F14–Ganjiang fault; F15–Luxi-Yichun fault; F16–Jiujiang-Jing’an fault; F17–Tonggu-Ruichang fault; F18–Macheng-Tuanfeng fault; F19–Chongyang-Ningxiang fault; F20–Yueyang-Wuhan fault; F21–Changde-Changsha fault; F22–Xiangxiang-Shaodong fault; F23–Cili-Chengbu fault; F24–Changde-Jingzhou fault; F25–Xiannvshan fault; F26–Xiangfan-Guangji fault; F27–Enshi-Xianfeng fault; F28–Huayingshan fault; F29–Weiyuan fault; F30–Longquanshan fault; F31–Xinjin-Deyang fault; F32–Weining-Shuicheng fault.① Zhenhai-Wenzhou fault zone; ② Yangzhou-Chenzhou fault zone; ③ Lu’an-Changsha fault zone; ④ Cili-Jingzhou fault zone; ⑤ Qianjiang-Xianfeng fault zone; ⑥ Huayingshan fault zone; ⑦ Deyang-Leshan fault zone.(1) Suqian-Ningbo fault zone; (2) Bengbu-Hushu fault zone; (3) Gushi-Hangzhou fault zone; (4) Xinyang-Tongling fault zone; (5) Xiangfan-Guangji fault zone; (6) Changde-Changsha fault zone; (7) Liupanshui fault zone.

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Given that the western YREB mainly comprises of the Sichuan-Yunnan active block, an area in Southwest China infamous for its strong earthquakes, it has been extensively studied by previous authors with respect to active faults and seismic-geological conditions. The authors have recently published papers on neotectonics and active tectonics examining the active tectonic systems and regional crustal stability of this area (Wu ZH et al., 2012, 2015; Yao X et al., 2015), and thus the analysis here is more focused on the central-eastern YREB to the east of the Yibin-Chongqing line.

4.1.1   Main active faults in the western YREB (Sichuan-Yunnan)

Total of 79 main active faults in and around Sichuan-Yunnan were reviewed based on available data, as well as our own findings from active fault surveys along the Yunnan-Xizang and Dali-Ruili railway lines, and active tectonic system research in Southwest China since 2005 (Wu ZH et al., 2009, 2012, 2015). These active faults were then grouped according to their geometry, kinematics, active intense classification and combination patterns, and then attributed to nine main active tectonic zones (Wu ZH et al., 2015) as further detailed below:

Two first-order active structural boundary zones, including:

(i) The 1400 km long, nearly NS-trending dextral strike-slip fault system the Sagaing fault system.

(ii) The 1800 km long Yushu-Xianshuihe-Xiaojiang sinistral strike-slip fault system, which strikes from NWW-NW to nearly N-S from the northwest to the southeast as an arc convex to the northeast.

Three second-order active structure zones, including:

(i) The 1400 km long sinistral strike-slip transtensional deformation zone that strikes NWW-NW-nearly N-S from the northwest toward the southeast as an arc convex to the northeast (the Litang-Dali-Ruili fault system).

(ii) The 280 km long, NW-trending dextral strike-slip transtensional deformation zone the Gengma-Jinghong fault system.

(iii) The 560 km span shear deformation region composed of a series of NE-NEE arc sinistral strike-slip fault zones aligned along the nearly NW direction the “pectinate” rotational-shear tectonic deformation zone along the boundary of China-Myanmar-Laos.

Three third-order active tectonic zones: The Dayao-Chuxiong transtensional deformation zone; the Tengchong transtensional shear deformation zone; and the Simao “broom-like” rotational-shear deformation zone.

A few relatively independent active faults and faults whose assemblage relations could not be not identified as yet were categorized as “other active faults inside main blocks”.

4.1.2   Main active faults in the central-eastern YREB

Among the numerous active faults of varying strikes in the central-eastern YREB, there are 86 NE-NNE and NW-NWW active faults that are approximately conjugate to each other. Among these 86 main active faults, the 32 most active faults were further identified along the Chengdu-Shanghai riverside area (Fig. 2; Table 2). It is to be noted however, that these faults do not include the Tanlu fault, as it is a well-known important active fault in the area having been extensively discussed and reported over the past years. These 32 active faults can be grouped into nine fault zones, including:

Table  2.  The 32 representative active faults in the central-east area of the Yangtze River Economic Belt and their influencing city groups and major engineering projects.

No.	Fault zone or tectonic zone	Fault name	Length/km	Strike	Slip type	Latest activity time	Faults to influence city groups or economic zones
1	Zhenhai-Wenzhou fault zone	Zhenhai-Wenzhou fault	270	NE	Right-lateral strike-slip	Quaternary	Yangtze River city group
2	Jiaxing-Xuzhou NW tectonic zone	Suqian-Yangzhou fault	300	NW	Left-lateral strike-slip	Quaternary	Yangtze River delta city group
3		Wuxi-Suzhou fault	455	NW	Left-lateral strike-slip	Quaternary	Sunan modernization construction demonstration area
4		Nanjing-Nandu fault	325	NW	Left-lateral strike-slip	Quaternary	Sunan modernization construction demonstration area
5	Xinyu-Jiujiang-Yangzhou NE tectonic zone	Maoshan fault	97	NE	Right-lateral strike-slip	Quaternary	Sunan modernization construction demonstration area
6		Maanshan-Wuhu fault	65	NNE	Right-lateral strike-slip	Quaternary	Wan River economic zone, Sunan modernization construction demonstration area
7		Chenjiabao-Xiaohai fault	130	NE	Right-lateral strike-slip	Quaternary	Yangtze River delta city group
8		Jintan-Nandu fault	50	NNE	Right-lateral strike-slip	Quaternary	Sunan modernization construction demonstration area
9		Jiangnan fault	260	NE	Right-lateral fault	Quaternary	Wan River economic zone
10		Gegong fault	227	NNE	Left-lateral strike-slip	Quaternary	Wan River economic zone
11		Jiujiang-Jingan fault	150	NNE	Right-lateral strike-slip	Quaternary	Ring Boyang Lake city group
12		Tonggu-Wuning-Rechang fault	100	NE	Left-lateral strike-slip	Quaternary	top Yangtze River city group
13		Gan River fault	227	NE	Left-lateral strike-slip	Quaternary	Ring Boyang Lake city group
14		Luxi-Yichuan fault	97	NE	Left-lateral strike-slip	Quaternary	Shanghai-Kunming railway
15	Xinyang-Hangzhou NWW tectonic zone	Chao Lake-Hangzhou fault	65	NW	Left-lateral strike-slip	Quaternary	Wan River economic zone
16		Xinyang-Jinzai fault	260	NW	Left-lateral strike-slip	Quaternary	Wan River economic zone
17	Yueyang-Wuhan NE tectonic zone	Guzhen-Huiyuan fault	357	NE	Right-lateral strike-slip	Quaternary	Wuhan city group and Wan River economic zone
18		Macheng-Tuanfeng fault	227	NE	Right-lateral strike-slip	Quaternary	Wuhan city group
19		Yueyang-Wuhan fault	390	NE	Right-lateral strike-slip	Quaternary	Top Yangtze River city group
20		Chongyang-Ningxiang fault	130	NE	Right-lateral strike-slip	Quaternary	Top Yangtze River city group
21		Xiangxiang-Shaodong fault	97	NE	Normal fault	Quaternary	Changsha-Zhuzhou-Xiangtan city group
22	Xiangfan-Guangji fault zone	Xiangfan-Guangji fault	650	NW	Left-lateral strike-slip	Quaternary	Wuhan city group. Ring Boyang Lake city group
23	Changde-Jingzhou NE tectonic zone	Changde-Jingzhou fault	260	NE	Right-lateral strike-slip	Quaternary	Top Yangtze River city group
24		Cili-Chengbu fault	195	NE	Right-lateral strike-slip	Quaternary	Top Yangtze River city group
25	Ya’an-Zigong thrust-fold tectonic zone	Weiyuan fault	97	NEE	Thrust	Quaternary	Chengdu-Chongqing city group
26		Longquanshan fault	195	NE	Thrust	Late Quaternary	Chengdu-Chongqing city group
27		Xinjin-Deyang fault	97	NE	Thrust	Quaternary	Chengdu-Chongqing city group
28	Other faults	Changde-Changsha fault	240	NW	Left-lateral strike-slip	Quaternary	Top Yangtze River city group
29		Xiannüfeng fault	97	NW	Right-lateral strike-slip	Quaternary	Shanghai-Chengdu railway
30		Enshi-Xianfeng fault	292	NE	Right-lateral strike-slip	Quaternary	Shanghai-Chengdu railway
31		Huanyingshan fault	292	NE	Right-lateral strike-slip	Quaternary	Chengdu-Chongqing city group
32		Weining-Shuicheng fault	292	NW	Strike-slip	Quaternary	Central Guizhou city group

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(i) The 270 km long, NE-trending Zhenhai-Wenzhou fault zone, which is a dextral strike-slip fault that potentially affects the Yangtze River delta urban agglomeration.

(ii) The NW-trending Jiangxing-Xuzhou structure belt, comprising of the Suqian-Yangzhou, Wuxi-Suzhou and Nanjing-Nandu sinistral strike-slip faults, which is about 300–455 km long and potentially affects the Yangtze River delta urban agglomeration and the South Jiangsu modernization demonstration area.

(iii) The NE-NNE-trending Xinyu-Jiujiang-Yangzhou structure belt, comprising of the Maoshan, Ma’anshan-Wuhu, Chenjiabao-Xiaohai, Jintan-Nandu, Jiangnan, Hegong, Jiujiang-Jing’an, Tonggu-Wuning-Ruichang, Ganjiang and Luxi-Yichun dextral strike-slip faults that are 50–260 km long, and that potentially affect the South Jiangsu modernization demonstration area, Wanjiang economic belt, Yangtze River delta urban agglomeration, around-Poyang Lake urban agglomeration, and cities in the middle reaches of the Yangtze River.

(iv) The NWW-trending Xinyang-Hangzhou structure belt, comprising of the Chaohu-Hangzhou and Xinyang-Jinzhai sinistral strike-slip faults, which are 65 km and 260 km long, respectively, and potentially affect the Wanjiang economic belt.

(v) The NE-trending Yueyang-Wuhan structure belt, comprising of the Guzhen-Huaiyuan, Macheng-Tuanfeng, Yueyang-Wuhan, Chongyang-Ningxiang and Xiangxiang- Shaodong dextral faults or dextral faults with normal faulting components, which are 97–390 km long and which potentially affect the Wuhan urban agglomeration, Wanjiang economic belt, Middle Yangtze urban agglomeration, and Changsha- Zhujiang-Xiangtan urban agglomeration.

(vi) The Xiangfan-Guangji fault zone, which is a NW-trending sinistral strike-slip fault about 650 km long and potentially affects the Wuhan urban agglomeration and around Poyang Lake urban agglomeration.

(vii) The NE-trending Changde-Jingzhou structure belt, mainly comprising of Changde-Jingzhou and Cili-Chengbu dextral strike-slip faults, which are 260 km and 195 km long, respectively, and potentially affect the Middle Yangtze urban agglomeration.

(viii) The Ya’an-Zigong thrust-fold structure belt, mainly comprising of Weiyuan, Longquanshan and Xinjin-Deyang blind thrust faults, which are 97–195 km long and which potentially affect the Chengdu-Chongqing urban agglomeration.

(ix) Other faults, mainly including the NW-trending Changde-Changsha sinistral strike-slip fault, which is about 240 km long and potentially affects the Middle Yangtze urban agglomeration; the NE-trending Xiannushan dextral strike-slip fault, which is about 97 km long and potentially affects the Shanghai-Chengdu railway line that runs across this area; the NE-trending Enshi-Xianfeng dextral strike-slip fault, which is about 292 km long and potentially affects the Shanghai-Chengdu railway line that runs across this area; the NE-trending Huayingshan dextral strike-slip fault, which is about 292 km long and potentially affects the Chengdu-Chongqing urban agglomeration; and the NW-trending Weining-Shuicheng sinistral strike-slip fault, which is about 292 km long and potentially affects the Central Guizhou urban agglomeration.

Regionally, the 32 main active faults can be summarized as 14 regional structure belt, i.e. seven NE-NNE active faults and seven NW-NWW ones (collectively the “7-longitudinal, 7-horizontal” pattern), according to their spatial correlation (Fig. 2). The NE-NNE and NW-NWW faults make up a conjugate strike-slip fault system that is active in shear deformation but can be accompanied with compressive or tensional components. The NE-NEE active faults are characterized by dextral strike-slip activity while the NW-NWW active faults are mostly characterized by sinistral strike-slip activity. The NE and NNW faults seem to be more active. As most of the historical seismicity is distributed along these two groups of faults, and these two groups of faults have more significant control over the current river and other landforms, and as the current near East-West compression of the tectonic stress field is more conducive to these two groups of faults, except the area near the southern extension of the Tanlu fault zone the Suqian-Hefei-Jiujiang line which shows higher fault and seismic activity, the faults in most parts of the area are low-rate active faults with activity rate generally below 0.5–1.0 mm/a.

Fault or depression basins of varying sizes and varying Quaternary subsidence rates, including the Jianghan-Dongting, Poyanghu, Subei and South Yellow Sea basins, are also located in the central-eastern YREB (Deng QD et al., 1994; Deng QD, 1982). These basins are all complex fault basins that are controlled by the NE-NNE, NEE and NW-NWW faults and have gradually developed since the Cretaceous. The Jianghan-Dongting basin is typically filled with Cretaceous-Paleogene formations. In the Jianghan basin, the Paleogene deposits are up to 4000 m thick; the Neogene and Quaternary deposits are much thinner, 800 m and 150 m thick, respectively. In the Dongting basin, the Cretaceous-Paleogene formations are up to 3000 m thick; the maximum Quaternary depositional thickness is 250 m in the southwest near the Taiyangshan Mountains. In the Subei basin, which is controlled by the NE-trending normal faults, the depositional thickness since Cretaceous is 5000–6000 m. The Neogene and Quaternary formations are 700–1100 m, and more than 300 m thick, respectively. The NE-trending Huaiyin-Xiangshui fault (also known as the Xiangshuihe fault) is a well-recognized basin-controlling fault in the north of the basin (Deng QD, 1982). Faults occurring on the periphery of these basins usually have more significant Quaternary activity in the regions. Typical examples include the NE-trending dextral Taiyangshan fault in the west of the Jianghan-Dongting basin, the NNE-trending Macheng-Tuanfeng fault and Shahu-Xiangyin fault in the east, and the NWW-trending Xiangfan-Guangji fault and Changde-Yiyang-Changsha fault, which run through the northern and southern boundaries of the basin, respectively. Furthermore, the Wuduhe, Yuan’an, Nanzhang, and Zhongxiang faults, which extend into the basin from the mountains to the northwest of the basin and feature NNW-trending sinistral strike-slip activity, all have weak activity (Deng QD et al., 1994). In the Subei basin, the active faults are mostly NE-trending dextral strike-slip faults with normal fault components. Eastward to the South Yellow Sea, the main active faults change to near-E-W faults featuring normal fault activity with strike-slip components.

Overall, compared with the western YREB, neotectonic deformation is significantly weaker in the central-eastern YREB and the faulting is also weak activity. Furthermore, given the different geotectonic background and the present-day plate position, the central and eastern YREB also differ in present-day tectonic activity in terms of historical seismic activity and active fault development, with the east being more active than the center. This is because the majority of the center belongs to the more integral and rigid Yangtze Block and is located in an area where the interaction of the eastern and western blocks of mainland China are significantly weaker, while the east principally belongs to the less integral South China orogen and is close to the boundary of the Western Pacific Plate.

Based on the findings of previous researchers, seven regional active fault zones with more development potential in the central-eastern YREB were identified by combining remote-sensing interpretation results with historical earthquake records. Following is a more detailed description of these active faults.

4.1.2.1   NE-trending faults

(i) Qianjiang-Jianshi fault zone

The Qianjiang-Jianshi fault zone (also known as the Qianjiang fault or the Enshi-Xianfeng faults), is a NE-trending sinistral strike-slip fault zone hosted in the Early Yanshanian fold zone along east Sichuan-northwest Hubei, and includes three secondary faults: The Enshi, Pengshui, and Qianjiang faults (Fig. 3). These three secondary faults display clear remarkable linearity in remote-sensing images. Furthermore, between the last two faults, an M6¼ Qianjiang earthquake took place and triggered the Xiaonanhai landslide in 1856. The cores of the syncline to the northwest of this fault zone are mostly located in a sub-horizontal thick limestone formation containing weak rock layers that feature fairly developed vertical joints of ruptures. Deep canyon landforms have mostly occurred in places where the Qianjiang River estuaries cut across this formation. For these reasons, large collapses and landslides are very likely to occur when the thick limestone formation overlying the weak layers becomes unstable if the estuary drainage systems trace back to the source and erode this formation. Given the complex mountain environment and special geotectonic properties, any medium to strong earthquake can trigger very serious crustal stability problems.

Figure  3.  The remote sensing image feature of the Qianjiang-Jianshi fault zone.

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(ii) Changde-Jingzhou fault zone

The Changde-Jingzhou fault zone strikes N40° E as a whole. It originates from the south of the Taoyuan section of the Yuanjiang River in Hunan Province and runs northeastward past Jinshi (Hunan Province), Gon’an, Jingzhou and Shayang (Hubei Province) before right-stepping along the Tianmen-Anlu line (Fig. 4). Its northern end is intercepted somewhere along the NW-trending Xiangfan-Guangji fault zone. The whole fault extends about 430 km and exhibits notable spatial segmentation. The most remarkable active marks there include the synchronous bending of a number of rivers, including the Hanjiang, Yangtze, Lishui, and Yuanjiang Rivers, and the distinctly different Quaternary thicknesses in the Jianghan-Dongting basin the depositional thickness on the eastern reaches of the fault is significantly larger than that on the west. Landform divergences are obvious on the sides of this fault: On the west are low-undulation hills featuring exposed bedrocks; on the east are thick plains comprised of thick Late Cenozoic deposits. The largest earthquake ever recorded here is Changde M6¾ earthquake in 1631.

Figure  4.  The remote sensing image feature of the Changde-Jingzhou fault zone.

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(iii) Yueyang-Wuhan fault zone

The Yueyang-Wuhan fault zone originates from somewhere near Ningxiang (Hunan Province) and runs past Yueyang and western Wuhan to northern Macheng, striking N30°–35° E as a whole over a distance of about 450 km. It constitutes the eastern boundary fault of the Jianghan-Dongting basin and includes the southern and northern sections that are distributed in a right-step en-echelon alignment (Fig. 5). The southern section, which corresponds with the formerly named Shahu-Xiangyin fault (also known as the Xiangjiang fault) (Ding BT, 1985), is around 360 km long. This section constitutes the eastern linear boundary of the Dongting Lake and shows distinguishable characteristics in the remote sensing images. It affects or controls the flow direction of the Yangtze River’s Yueyang-Wuhan section in the NE direction. The largest earthquake recorded in this section historically is Yueyang M5 earthquake in 1556 with maximum seismic intensity area of VII. The northern section is the formerly named Macheng-Tuanfeng fault (Chen LD et al., 2014), which originates from somewhere near the Zhangduhu Lake in the west of Mafeng County (Hubei Province) and runs northeastward across Xinzhou and Macheng approximately along the Jushuihe River into the Dabieshan Mountains, extending about 130 km. Some M4–6 earthquakes have been recorded historically, the largest being the 1932 Huangtugang (Macheng, Hubei Province) M6 earthquake with maximum seismic intensity area of VIII. This fault runs through a number of populous, important cities of the Middle Yangtze urban agglomeration. Hence it is absolutely necessary to study the Quaternary activity and potential seismic risks along this fault zone in greater detail.

Figure  5.  The remote sensing image feature of the Yueyang-Wuhan fault zone.

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(iv) Southern Tanlu fault zone (Suqian-Feidong-Huangmei section)

The Tanlu fault zone strikes NNE as a whole. Its northern end is located near the southwestern margin of the Okhotsk Sea, and runs through Khabarovsk Krai and Jewish Autonomous Oblast, Russia before entering China somewhere around the northern Luobei County, Hegang of Heilongjiang, from where it further runs past Heilongjiang, Jilin, Liaoning, Bohai Bay, Shandong, Jiangsu, Anhui, and Hubei provinces. Its southern end joins the NW-trending Xiangfan-Gangji fault somewhere near Wuxue City (formerly Guangji County), Hubei Province. Totaling about 3300 km in length, including approximately 2400 km in China, the Tanlu fault zone marks the boundary between the Circum-Pacific active tectonic domain in the east and the Central Mainland China active tectonic domain in the west. It is also the most famous seismic fault zone in East China. The strongest earthquake in East China has been recorded along the Shandong section the 1668 M8.5 earthquake with maximum seismic intensity area of XII. The Holocene dextral strike-slip rate of this seismic fault is 1.7–2.8 mm/a and its vertical displacement rate is 0.2–0.5 mm/a (Wang HL, 1996).

The Tanlu fault zone section to the south of Shandong Province, which can be considered as its southern section, strikes NNE-NE. It runs through Jiangsu, Anhui and Hubei provinces, extending about 580 km, and includes about 145 km in Jiangsu Province, about 370 km in Anhui Province, and about 65 km in Hubei province. After running past western Lujiang, the Anhui-Hubei section typically stretches along the piedmont to the west of the Wanjiang River valley which refers to the Anhui section of the Yangtze River. Besides dextral strike-slipping, this section has quite notable dip-slipping components and should be responsible for the uplifting of the mountains to its west and subsidence of the valleys to its east, giving rise to the fairly distinct basin-mountain landform of the Wanjiang River valley section.

Although some authors suggest that the most recent activity of the southern section of the Tanlu fault zone took place at the end of Mid-Pleistocene, no direct evidence of it dislocating Q₃ or Q₄ has been found as yet, probably because its active intensity was weaker during the Late Pleistocene-Holocene (Tang YB et al., 1988, 1990a; Liu B et al., 2015). However, the prehistoric seismic remains from many geological stages discovered in this section (Tang YB et al., 1990b; Yao DQ et al., 2012; Zheng YP et al., 2012) still signal negligible seismic risks in future.

(v) Anqing fault zone

The Anqing fault zone, also known as the Yanjiang fault, stretches approximately along the southeastern hilly boundary of the Tongling-Susong line in the Wanjiang River valley to the west of the Yangtze River and strikes NE. Its southwestern section starts somewhere near Wuxue, Hubei Province, and runs northeastward past Huangmei, Susong, Anqing, and Zongyang before joining the Wanjiang fault zone somewhere near Wuhu, extending about 310 km. This fault zone shows obvious linearity in remote-sensing images. It not only makes up the southeastern boundary of the NE-trending hilly geomorphic unit with a distinguishable triangular fault facet landform along the Anqing-Shipai town section, but also has linear waterlogged depressions in the Wuxue-Shipai town section, to the southeast of the fault, which include the Longganhu, Daguanhu, Pohu, and Wuchanghu lakes, distributed from the southwest to the northeast. All these are indicative of the possible activity of this fault during the Quaternary.

(vi) Wanjiang fault zone

The Wanjiang fault zone, a NNE-trending fault zone freshly identified according to remote-sensing images, stretches northeastward from somewhere near Jiujiang, approximately along the Lower Yangtze-Wanjiang section of the Hukou-Pengze-Chizhou-Tongling line to somewhere near Wuhu, extending about 300 km (Fig. 6). This fault zone displays remarkable linearity in remote-sensing images. The Wanjiang section of Yangtze River is also obviously distributed along this fault zone. Some M3-5 earthquakes have also been recorded along this fault. Based on the focal mechanism solution results of the area, it is a dextral strike-slip fault with some Quaternary activity. Regionally, this fault zone is connected to the Xingzi-Jing’an fault to its southwest in Jiangxi, which marks the northwestern boundary of the Poyanghu basin. The entire fault zone forms a left-step en-echelon alignment with the southern section of the Tanlu fault zone. Hence, it may be a branch fault of the southern section of the Tanlu fault zone.

Figure  6.  The remote sensing image feature of the main Quaternary faults along the Wanjiang.

F1–the Lujiang-Wuxue section of Tanlu fault zone; F2–Anqing fault zone (or the Yanjiang fault); F3–Wanjiang fault zone; F4–the eastern section of Xiangfan-Guangji fault zone.

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4.1.2.2   NW trending faults: Wuxi-Suqian fault zone

Previous findings show a number of NW-NWW active faults in the Yangtze River delta area (Zhu JA et al., 1984; Wang EC et al., 2008; Fang DW et al., 1992), among which the most geologically or topographically significant active fault should be the Wuxi-Suqian fault running across the Yangtze River delta urban agglomeration (Fig. 7).

Figure  7.  Remote sensing image features of Wuxi-Suqian fault zone.

F1-1–Suqian fault; F2–Tanlu fault; F1-2–Yangzhou-Wuxi fault.

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Overall, the Wuxi-Suqian fault trending NW. It is sinistral strike-slip activity, with distinguishable normal fault components as well. This is characterized by landform highland in the southwest and faulting in the northeast in the Yangzhou-Wuxi-Suzhou section. Earlier researchers assumed that this fault originates in Pixian and runs across the Tanlu fault zone, past Suqian, Hongzehu, Gaoyou, Zhenjiang, and Changzhou to somewhere south of Wuxi, extending about 300 km (Wang EC et al., 2008). However, when the authors evaluated the geological and remote-sensing image data, the authors discovered that given the remarkable linearity of this fault, its trace could start at least from somewhere near Liaocheng of Shandong Province, running southeastward past Dongpinghu, Dushanhu, Shaoyanghu, Weishanhu, Luomahu, Suqian, Hongzehu, Gaoyouhu, Shaobohu, Yangzhou, East Zhenjiang, Changzhou, Wuxi, Suzhou, and Jiaxing, across the Hangzhou Bay and into the East China Sea from somewhere between Ningbo and Zhoushan, extending about 900 km. Furthermore, remote-sensing and digital elevation model (DEM) images also show that starting from somewhere near Liaocheng of Shandong Province towards the northwest, this fault can en echelon distribution with the NW-trending basin-mountain boundary at the eastern piedmont of the Taihangshan Mountains to the west of Shijiazhuang city. This potentially signifies that this fault must have run across the Hebei plain into the eastern piedmont of the Taihangshan Mountains. If that is the case, its total length would be around 1200 km, and it may be a regional active fault zone similarly active to, but even larger than, the Zhangjiakou-Penglai fault zone in North China. Strike variation and local discontinuity are indicative of the significant segmentation of this fault zone. On this basis, the authors can divide it into a Liaocheng-Suqian fault, Hongzehu-Yangzhou fault, Wuxi-Suzhou fault, and Jiashan-Zhoushan section. It is very likely that the Gaoyouhu and Hongzehu lakes, which occur along faults between Yangzhou and Suqian, are associated with the local tensional faulting or tectonic subsidence at the left-step en-echelon alignment of the secondary faults within this fault zone.

Whether in terms of geology, geomorphology, or seismology, the neotectonic activity has been quite obvious along the Wuxi-Suqian fault zone, as evidenced by:

(i) The obvious regional geological-topographic boundary lines. Geomorphologically, the landform is obviously higher on the west side of the fault than on the east on its east is a plain-subsidence area; on its west is a low and gentle hills area. Geologically, the most important characteristic is the differential formation distribution and Quaternary formation thickness between the two sides. To its southwest, pre-Paleogene bedrock are widely exposed, and Neogene and Early Pleistocene deposits are also widespread. On its northeast, the area is covered by Holocene formations and thick Late Pleistocene formations beneath them; the Quaternary thickness also increases promptly from a few to a dozen meters on the west side of the fault, to tens or hundreds of meters on the east side.

(ii) Distinct contribution made by the fault to the distribution or development of the present-day lakes and streams. For example, the lakes are linearly distributed along the fault. The most typical are the five larger ones: The Taihu, Gaoyouhu, Hongzehu, Luomahu, and Weishanhu lakes. On this basis, this fault is also called the “Five Lakes fault”. After the Yangtze River runs eastward across the fault along the Yangzhou-Zhenjiang line, its flow direction suddenly turns southeastward, possibly as a result of the fault activity during the Quaternary. This fault zone is also seen as controlling the distribution of the coastline in the southeast of the Hangzhou Bay.

(iii) Distinguishable dense earthquake distribution along the fault, and relatively frequent moderate to strong seismic activity in history. For example, the 925 CE northwest Xuzhou M5¾ earthquake, 999 CE Changzhou M5½ earthquake, 1624 CE Yangzhou M6.0 earthquake, and the 1913 Zhenjiang M5.5 earthquake are all possibly associated with the present activity of this fault zone.

Besides, there are many Neogene volcanic craters and Triassic rock salts on the west side of this fault zone, which are absent in the east. This signifies differential Cenozoic geological evolution between the sides of the fault, suggesting that this fault also served as a boundary structure between different fault blocks during the Cenozoic. The fault is dextrally faulted by the Tanlu fault zone where it joins the latter, suggesting that the latter has been more active afterwards. Given that this fault is large in size with remarkable neotectonic activity, and that it runs across a number of populous and economically developed cities (including the South Jiangsu modernization demonstration area) in the Yangtze River delta, potential external seismogenic ability and risks of the fault have to be specifically addressed during urban planning or when planning major infrastructure construction for major cities along or near this fault.

4.2   Seismic activity along the YREB

Earthquakes are the manifestation of fault activity induced by crustal deformation processes, and therefore correspond with regional tectonic activity and active fault development. The seismic activity of the YREB varies largely from one section to another. Throughout its recorded history, the YREB has suffered 1623 Ms $\geqslant$ 4.0 earthquakes, including 837 Ms $\geqslant$ 5.0 earthquakes, 196 Ms > 6.0 earthquakes, 43 Ms $\geqslant$ 7.0 earthquakes, and two Ms $\geqslant$ 8.0 earthquakes.

4.2.1   Seismic activity in the western YREB

The Sichuan-Yunnan area in the western YREB is the most seismically active area in Southwest China with high frequency and large seismic magnitude (Fig. 8). According to the historical and instrument records of earthquakes, there are at least 184 earthquakes with M $\geqslant$ 6, and 43 with M $\geqslant$ 7, including two with magnitude M $\geqslant$ 8, that is the Songming (Yunnan Province) M8 earthquake, and the May 12^th, 2008 Wenchuan (Sichuan Province) earthquake with Ms = 8.0. Most the earthquakes with magnitude M $\geqslant$ 7 mainly occurred along the main boundary fault zones of active blocks, such as the Ganzi-Xianshuihe-Xiaojiang fault zone, Longmenshan fault zone, Litang fault zone, East Yulongxueshan fault zone, Chenghai-Binchuan-Dali fault zone, Nantinghe fault zone, Wanding fault zone, and Gengma-Lancang-Jinghong fault zone.

Figure  8.  The history destructive earthquake distribution the Yangtze River Economic Belt (M ≥5.0).

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4.2.2   Seismic activity in the central-eastern YREB

4.2.2.1   Historical and instrument-recorded seismic activity in the central-eastern YREB

Overall, the seismic activity in the central-eastern YREB is relatively low in frequency and small in magnitude. Statistics indicate that, as far as has been recorded, no earthquake with M $\geqslant$ 7.0 has ever taken place; destructive earthquakes are mostly M6.0–6.9; only 12 earthquakes with magnitude 6.0 $\leqslant$ M < 7.0 have been recorded (Table 3). The earthquakes are also very unevenly distributed, mostly occurring to the north of the Yangtze River (Fig. 8). The seismic activity is weaker to the south as a whole, except for Hunan Province. The largest earthquake is the 1631 CE Changde M6¾ earthquake with maximum seismic intensity area of IX. The earthquake that triggered the most catastrophic geohazards is the 1856 CE Qianjiang M6¼ earthquake (formerly known as the Daluba earthquake). The consequent Xiaonanhai earthquake-induced landslide produced a 5 km long dammed lake. The landslide dam measures 1170 m in length and 67.5 m in height, totaling about 60×10⁶ m³ in area.

Table  3.  Main parameters of typical destructive earthquakes in the central-eastern Yangtze River Economic Belt.

Earthquake	Time/(Year-Month-Date)	Magnitude	Macro-epicenter	Max. Intensity	Direction of Long Axis	Seismogenic Fault
NW Fangxian, Hubei	788-02-12	6½	Leigutai, NW Zhushan	Ⅷ	NWW	Baofeng-Fangxian section of Baokang-Fangxian fault zone
Haozhou earthquake, Anhui	1481-03-18	6.0	S Haozhou	—	—	Wohe fault
Caoxian earthquake, Anhui	1585-03-06	5¾	Caoxian, Anhui	Ⅶ	NNE	The southern section of Tanlu fault
Yangzhou earthquake, Jiangsu	1624-02-10	6¼	Yangzhou, Jiangsu	Ⅷ	NE	Junction between Wuxi-Suqian fault zone and Chenjiabao-Xiaohai fault or northern Maodong fault
Changde earthquake, Hunan	1631-08-14	6¾	Changde	Ⅸ	NEE	Taiyangshan fault
NE Huoshan earthquake, Anhui	1652-03-23	6.0	Junction between NE Huoshanand Lu’an	Ⅷ	NW	Junction between Meishan-Longhekou fault and Luoerling-Tudiling (?)
Fengtai earthquake, Anhui	1831-09-28	6¼	NE Fengtai	Ⅷ	NW	Junction between Guzhen-Fengtai and Liufu fault
Qianjiang earthquake, Chongqing	1856-06-10	6¼	Xiaonanhai (formerly Daluba, Xianfeng, Hubei), Qianjian, Chongqing	Ⅷ	NNW	Enshi-Xianfeng fault zone (?)
Qiannan earthquake, Guizhou	1875-06-08	6¼	Luodian	Ⅷ	NEE	Junction between Yadu-Ziyun deep fault or Kaiyuan-Pingtang concealed deep fault
Huichang earthquake, Jiangxi	1806.01.11	6.0	Xiangxiang Town, 40 km south of Huichang	Ⅷ	NNW	Xunwu-Ruijin section of Heyuan-Shaowu fault zone
Zhenjiang earthquake, Jiangsu	1913-04-03	5.5	Zhenjiang	Ⅶ	NW	Wuxi-Suqian fault zone (?)
Huoshan earthquake, Anhui	1917-01-24	6¼	S Huoshan	Ⅷ	NNE	Junction between Tongbai-Mozitan fault and Luoerling-Tudiling fault
Macheng earthquake, Hubei	1932-04-06	6.0	Huangtugang, Macheng	Ⅷ	NNE	Macheng-Tuanfeng fault
Liyang earthquake, Jiangsu	1974-04-22	5.5	Liyang	Ⅶ	NE	Maodong fault or Jintan-Nandu fault
Jiujiang earthquake, Jiangxi	2005-11-26	5.7	Ruichang, Jiangxi	Ⅶ	NE	Tianjialong-Xixinqiao fault section of Tonggu-Wuning-Ruichang fault
Notes: Seismic data mentioned above are quoted from literature (Department of earthquake damage prevention State Seismological Bureau, 1995; Lou BT, 1996); the seismogenic faults are inferred from available isoseismic data and known regional active faults.

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To be associated with active faulting, the seismic activity along the YREB is also distinctly regionalized (Fig. 8), as reflected by the zoning along the NW-trending structure belt and the frequent occurrence at the intersections between the NW and NNE-trending faults. Regional moderate to strong earthquakes takes place, or are concentrated along or on the sides of the fault lines, with obvious rules in terms of the tectonic positions of their occurrence (Zhu JA et al., 1984; Wang EC et al., 2008). These special seismogenic tectonic positions include: (1) The boundary zones between different tectonic units larger the tectonic units, more frequent the high-intensity earthquakes on the boundaries; (2) Cenozoic or Quaternary basin-mountain boundary or transition, as this is often where basin-controlling faults are developed; (3) at the special stress-concentrated locations such as the intersections between faults of different strikes, the ends or strike deflections of regional active faults.

Based on the zoning of regional destructive earthquakes, nine zones can be identified:

(i) The NW-trending Ankang-Zhushan-Fangxian-Zhongxiang-Wuhan zone, where the largest earthquake in history was the 788 CE Ankang M6½ earthquake.

(ii) The NW-trending Funiushan-Tongbaishan-Dabieshan zone and the Nanyang-Xinyang-Macheng zone, where the largest earthquakes were the 46 CE Nanyang M6½ earthquake and the 1932 Macheng M6 earthquake.

(iii) The NW-trending Huangchuan-Shangcheng-Huoshan zone along Dabieshan Mountains structure belt, where the largest earthquakes were the 1652 CE M6 earthquake and the 1917 M6¼ earthquake, both in Huoshan.

(iv) The NNW-trending Xuchang-Huainan zone, where the largest earthquakes were the 1820 CE Xuchang M6 earthquake and the 1831 CE Fengtai M6¼ earthquake.

(v) The NNE-trending Tanlu-Jiujiang zone, corresponding to the southern section of the Tanlu fault, where the largest earthquake was the 1585-03-06 CE Lujiang (or south Caoxian, Anhui Province) M5¾ earthquake.

(vi) The NNE-trending Yangzhou-Liyang zone near Nanjing of Jiangsu Province, where the largest earthquakes were the 1624 CE Yangzhou M6¼ earthquake and the 1979 Liyang M6.0 earthquake.

(vii) The NW-trending South Yellow Sea offshore zone from the Yangtze Estuary of Shanghai to somewhere near Yancheng of Jiangsu Province, where the strong earthquakes include the 1852 CE M6¾, 1853 CE M6¾, 1967 M6½, 1984 M6.4 and 1996 M6.0 earthquakes. The 1996 M6.1 earthquake took place near Shanghai and caused the destruction of some buildings in the city, with strong vibrations felt in the city.

(viii) The NE-trending Wulingshan zone in Hunan Province, where the largest earthquake was the 1631 CE M6¾ Changde earthquake.

(ix) The Liupanshui fault zone in Northwest Guizhou Province, where the largest earthquake was around M5.5.

4.2.2.2   Paleoearthquake phenomena remains in the central-eastern YREB

In the central-eastern YREB, the absolute majority of the faults are much less active than those up around the Qinghai-Tibet plateau. Accordingly, most of the faults are less likely to trigger earthquakes and have lower earthquake frequencies. The maximum earthquake magnitudes recorded in history are equal to or less than M6.8 for all the regions along the river Chongqing, Hubei, Hunan, Jiangxi, Anhui, Jiangsu, Zhejiang, and Shanghai. Statistical and empirical data also demonstrate that an earthquake in Mainland China must be at least as large as M6.8 if it is to induce any surface rupture (Deng QD et al., 1992). That is, majority of the earthquakes in the central-eastern YREB will not induce surface rupture. This, however, doesn’t necessarily mean that they will not leave behind surface remains, since, besides coseismic surface rupture remains, earthquakes can also produce secondary phenomena such as soil liquefaction, ground fissures, collapses and landslides, all of which serve as important geological evidence for tracing regional paleoearthquake events. In fact, earlier researchers have also noticed some paleoearthquake-related geological remains in their geological survey of the central-eastern YREB. At the beginning of the 1980s, for example, Ding H et al. (1982) spotted some groupings of disordered large trees with several neoliths and earthenware, as well as signs related to tectonic deformation such as liquefaction folds, crumples and local dislocations, amid the Holocene sandy clayey beds across dozens of sand ore pits manually extracted on the banks of the Shahe River in Liyang of Jiangsu province, and, believing that these special deposits must be seismogenic deposits with piedmont landslide accumulation characteristics, named them “piedmont landslide accumulations”. Based on the era of the cultural remains contained in these “piedmont landslide accumulations” and historical records, there should have been two M7.0 paleoearthquakes in 123 CE and 250 CE. Recently, Chen LD et al. (2014) and Qi X et al. (2015) discovered some suspected paleoearthquake wedges or ground fissures possibly related to paleoearthquake activity from the Quaternary formations of Yangluo and Jiujiang during a geo-environmental survey and zoning research of the Middle Yangtze urban agglomeration. Chen LD et al. (2014) discovered some suspected paleoearthquake wedge structures centrally distributed in the Lower Pleistocene Yangluo gravel layers somewhere in the Wangmushan Mountains of Yangluo of Wuhan city, and suggested that they must be records of paleoearthquake activity in the Wangmushan fault, which is a part of the NW-trending Xiangfan-Guangji fault zone, and runs somewhere across Yangluo, during Mid-Pleistocene; Qi X et al. (2015) observed several ground fissures in some Quaternary formations in Jiujiang. They correlated the ground fissures to the regional fault distribution after measuring their macro and micro characteristics and suggested that the typical ground fissure occurrences during the Quaternary and maybe the surface responses of regional fault activity and paleoearthquake events. All these findings reinforce the importance of investigating the paleoearthquake remains in the central-eastern YREB, to gain a more comprehensive understanding of the earthquake potential and strong earthquake rules of the main active faults in this area.

In the geological reconnaissance along the Chongqing-Nanjing section of the Yangtze River’s trunk stream in 2015, the authors also spotted some noteworthy paleoearthquake remains (Fig. 8), including earthquake wedges, faulted formations, fault wedges and other typical paleoearthquake marks that are commonly seen in active faults in many places (Li P et al., 1982). Following is a brief description of these earthquake remains:

(i) Suspected paleoearthquake wedges in the Sanxia Airport area of Yichang city, Hubei Province

On the NW Muyushan roadside about 7.2 km from the north of Sanxia Airport, Yichang City (30°37.205′N, 111°29.05′E, 116 m ASL), a 1.0–1.5 m thick alluvial gravel bed was found overlaying a Paleogene-Neogene red sandstone formation in unconformity, and was wedged down into the underlying sandstone formation at the contact surface between the gravel and the sandstone layers (Fig. 9a). It looked like a seismic wedge, probably formed when gravel filled up the ground fissures induced by paleoearthquakes during the accumulation of the gravel bed; also, as this spot is 28–30 km away from Yuan’an and Wuduhe faults on the northwest side of the Jianghan basin, it is highly probable that it is subject to the strong earthquake activity along these fault zones. Besides, as the terrain there is relatively mild, any earthquake that could induce ground fissures in an already lithified formation in a similar topographic context must obviously be one with considerable intensity, or in a VII-VIII intensity area produced by an earthquake. As no strong earthquake has ever been recorded in Yichang, further investigation of this spot will help better understand the earthquake-geological environment of this area.

Figure  9.  Typical geological remains of paleoearthquake along the central-eastern Yangtze River Economic Belt.

a–suspected earthquake wedges between the Quaternary formation and the underlying red beds near Sanxia Airport, Yichang; b–paleoearthquake faults in the Late Cretaceous formations of Hongshigu Valley, Dahengshan Mountains, Mingguang City; c–seismic faults dislocating the Late Cretaceous formations in Guabushan Mountains, Liuhe, Nanjing City; d–paleoearthquake fault wedges in the Pliocene formations on the west of the Jiucaishan Mountains, east piedmont of the Maoshan Mountains, Jiangsu Province.

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(ii) Seismic faults in Hongshigu Geopark of Mingguang city, Anhui Province

In Hongshigu Geopark of the Dahengshan Mountains, Mingguang city, Anhui Province (32°36.599′N, 118°01.067′E, 109 m ASL), a Late Eocene basalt sequence with K-Ar age of 37.6 Ma (Chen DG et al., 1988) was found to overlie in unconformity over a Late Cretaceous red fine sandstone formation that occurred at 30–40°∠12–15° with extremely developed oblique beddings. Many parts of the underlying Cretaceous red sandstone formation were vertically dislocated by NNE nearly vertical, like knife-cut small faults, which appear to be normal faulting (Fig. 9b). As this area is close to the Anhui section of the Tanlu fault zone, further examination is necessary to determine whether it is associated with paleoearthquake activity along the Tanlu fault zone.

(iii) Seismic faults in Guabushan Volcano and Stone Forest Park of Nanjing city, Jiangsu Province

In Guabushan Volcano and Stone Forest Park of Liuhe district, Nanjing city, there is a basalt sequence featuring extremely developed columnar joints the Lower Pleistocene Jianshan formation (Q₁j) with K-Ar age at about 9.4 Ma suggesting that it is a Late Miocene basalt formation. Outside this park (32°14.906′N, 118°53.700′E, 41 m ASL), an Upper Miocene Huanggang formation (N₁h) consisting of yellow or red tuffaceous sandstone or gritstone interbedded with basalt, is seen to underlie this basalt formation. Regionally, the basalt interbeds in this formation date back to 10–12 Ma (Zhang XY et al., 2003). The Huanggang formation is underlain by a purplish red Upper Cretaceous Chishan (K₂c) mudstone and sandy mudstone that occur at 300°–360°∠32°–35°, and a brownish green mafic dike; between the purplish red sand/mudstone and the dike is a group of positive flower-like, strike-slip brittle faults with normal fault components. The main faults occur at 30°∠78° with clear and fairly fresh fault planes, resembling typical seismic faults (Fig. 9c). The top of this seismic fault is already overburdened by subsequent grayish yellow Xiashu loess, since the fault is not seen to have obviously dislocated the Huanggang formation. Hence, the most recent activity of the faults should be in the Mid-Miocene, and the faults has remained inactive since Xiashu loess deposition.

(iv) Paleoearthquake wedges west of the Jiucaishan Mountain of Jiangsu Province

On the west of the Jiucaishan mountains in Liyang County, at the eastern piedmont of the Maoshan Mountains (31°40′04.7″N, 119°19′12.3″E, 100 m ASL), a group of 19°∠56° normal faults, 268°∠62° microfracture zones and nearly N-S-trending normal faults potentially associated with paleoearthquakes were observed across the Pliocene brick-red limestone breccia formation exposed by manual excavation (Fig. 9d), where the fault fissures were filled with grayish yellow sandy soil, which is possibly equivalent to Xiashu soil, making up earthquake wedge-like structures. Combining this with the 1970 M6.0 earthquake in Liyang at the east piedmont of the Maoshan Mountains, the authors suggest that these suspected paleoearthquake wedges are indicative of a larger-magnitude paleoearthquake in this area, which coincides with the previous findings of the “piedmont landslide accumulations” (Ding H et al., 1982).

As discussed above, the paleoearthquake remains discovered so far are mostly located in Late Cretaceous to Early Quaternary formations. This, on the one hand, suggests high-intensity earthquake events in the central-eastern YREB during the Paleogene-Neogene but their frequency was possibly quite low on the whole; on the other hand, it also signals the possibility of large earthquakes that could result in significant surface disruption or surface rupture, though further examination is needed to determine the possible tectonic positions of these potential large earthquake exposures during the late Quaternary.

5.   Discussion

5.1   Characteristics of the active tectonic systems along the YREB

Different from the neotectonic position of the central-eastern YREB, the western YREB, typically Sichuan-Yunnan region, which is presently located on the southeastern margin of the Qinghai-Tibet plateau active orogen, is mainly subject to the intense continent-continent collision between the Indian and Eurasian plates, featuring intense crustal deformation, while in the central-eastern YREB, which is in an area subject to the boundaries of both the eastern and western plates of Mainland China. The central-eastern YREB, however, as it is quite a long distance from both boundaries, in addition to the plates themselves being quite rigid, the present-day crustal deformation there is weaker on the whole. The different plate boundary conditions have accounted for the distinct differences in the active tectonic system framework between the two areas.

5.1.1   Tectonic framework of the western YREB the Sichuan-Yunnan arc-shaped rotational-shear active tectonic systems

In the western YREB, the crustal movement in and around Sichuan-Yunnan since the Pliocene and during Late Quaternary appears to be represented by the clockwise rotation of the Sichuan-Yunnan block around the Eastern Himalayan syntaxis. This unique active tectonic framework is termed by us as the “Sichuan-Yunnan arc rotational-shear active tectonic system” (Wu ZH et al., 2012, 2015), and is defined as an “active tectonic system made up of an arc fault block, typically Sichuan-Yunnan, sandwiched between the arc Yushu-Xianshuihe-Xiaojiang-Dien Bien Phu fault system and the nearly N-S-trending Sagaing dextral strike-slip fault system, together with the active zones within this fault block featuring rotation, shear, and extension”. The principal part of this tectonic system is mainly controlled by two first-order tectonic boundary zones (the Sagaing fault system and the Yushu-Xianshuihe-Xiaojiang-Dien Bien Phu fault system), and one second-order tectonic zone (the Litang-Dali-Ruili arc structure belt). These three large boundary structure belt divide this active tectonic system into an inner and an outer part, i.e. the Sichuan-Yunnan outer arc zone and the western Yunnan inner arc zone. This tectonic system framework has obviously controlled or affected the macroseismic activity in and around Sichuan-Yunnan (Wu et al., 2015). As Yunnan Province and southwest Sichuan Province in YREB are mostly located in this highly active tectonic system, their historical earthquakes all feature high frequency and high intensity.

5.1.2   Chessboard-like active fault system framework of the central-eastern YREB

Macroscopically, the seven NE-NNE and seven NW-NWW regional structure belt (the “7-longitudinal, 7-horizontal” pattern) obviously make up a typical “chessboard-like tectonic system” in the description of the geomechanical tectonic system (Lee SG, 1973), or a networked tectonic system or an X-type fault system. In the chessboard active tectonic system framework of the central-eastern YREB, the intersections between the regional NNE-NE faults and NW-NWW faults are the most susceptible to destructive earthquakes. From geomechanical observations, these are places where tectonic stresses are more likely to concentrate and are therefore seismogenic tectonic positions of a tectonic system where faults are more likely to become active and trigger earthquakes (Lee SG, 1973). This tectonic framework also affects the direction of the Yangtze River’s trunk streams. A typical example is the unique SE-NE-SE (W-shaped) repeated deflection of the Yangtze River after it has flowed out of the Three Gorges from Yichang, as the main river channel formed by following the route of the NW and the NE structure belts of the chessboard tectonic system.

Earlier researchers suggested that the chessboard tectonics of the central-eastern YREB should be the latest crustal fault system still under development the recent crustal rupture network. This assertion is based on observations that the present-day earthquakes often appear to take place in a NW or NE-trending, fairly regular network or conjugate, and are believed to be controlled by two corresponding tectonic lines, with strong earthquakes being more likely to take place at or near the intersection between these two structure lines (Ding GY et al., 1979; Zhang WY et al., 1984). In the central-eastern YREB, the most remarkable earthquake zones or structure lines are the NW-trending Wuhan-Jiujiang and Heze-Yangzhou zones, and the NE-trending Linyi-Huangchuan and Nanjing-Tongling zones (Ding GY et al., 1979). The former actually correspond with the regional Xiangfan-Guangji fault zone and Wuxi-Suqian fault zone; the latter is associated with the Tanlu fault zone and the Wanjiang fault or Jiangnan fault zone. Hence, the authors suggest that the chessboard fault system framework of this area must be the reactivation of old structural belts under the newest tectonic stresses. It is true that due to the significant influences of the giant NE-trending structural belts in East China produced by the Yanshanian movement, the NE-NNE structural belts in this area are much larger, more continuous, and more active, while most of the NW-NWW faults are smaller, less continuous, and less active, perhaps except for the Xiangfan-Guangji and Wuxi-Suqian faults. The present-day plate boundary conditions also indicate that the early NE-NNE structural belts in this region are more likely to become active again, as the main constraints for the tectonic stress environment of the middle-lower reaches of the YREB are the nearly E-W compression exerted on the Yangtze Plate by the eastward extrusion of materials from the Qinghai-Tibet plateau, after the northward subduction and continent-continent collision of the Indian Plate with the Eurasian Plate in the west, the sub-horizontal compression exerted to East China by the westward high-angle subduction of the West Pacific Plate in the east, and possibly also the vertical forces caused by the upwelling of deep materials. Furthermore, nearly N-S dextral shearing also exists between the Indian and the South China plates. Superposition of all these forces has obviously benefited the dextral strike-slip movement of the regional NE-NNE faults. The chessboard tectonics featuring two groups of faults with different development degrees is of a simple shear type, probably attributable to the regional dextral shearing between the Indian and South China plates.

In the western part of the YREB, fault activity is generally strong, and strong seismic activity mainly occurs in the Late Quaternary active faults, especially the Holocene active faults. In the central and eastern regions, most of the regions are relatively stable, and the Late Quaternary active faults are not well developed. The average interval for most of the fault repetitive activities is very long, and may be tens of thousands or even hundreds of thousands of years. Active fault studies should therefore focus on a longer time scale, at least throughout the Quaternary and even into the Pliocene and Miocene. However, the Tanlu fault zone is an exception the 1668 CE great Tancheng M8½ earthquake, the largest seismic event recorded in history in eastern China, occurred along the fault. It is a huge NNE trending right-lateral strike-slip active fault of the Late Quaternary, but has a low sliding speed (about 0.7 mm/a) and long recurrence interval of earthquake (probably more than 10000) (Li K et al, 2019).

5.2   Future seismic risks in the YREB

5.2.1   Future strong seismic risks in the western YREB Sichuan-Yunnan

Regionally, the Sichuan-Yunnan arc rotational-shear tectonic system represents the middle section of the Qinghai-Tibet-Yunnan-Myanmar-Indonesia “inverted S” rotational-shear active tectonic system (i.e. the “eta-shaped” tectonic system in geomechanics). The authors’ recent investigation of the M ≥ 6.8 historical earthquakes throughout the Qinghai-Tibet-Yunnan-Myanmar-Indonesia “inverted S” rotational-shear active tectonic system revealed strong spatial-temporal linkage between the strong earthquake events in the Sichuan-Yunnan arc rotational-shear system deformation zone and those in the western section (i.e. inside the Qinghai-Tibet plateau, equivalent to the head of the “eta-shaped” tectonic system) and the eastern section (mainly the Sumatra Island arc zone, equivalent to the tail of the “eta-shaped” tectonic system) of the Qinghai-Tibet-Yunnan-Myanmar-Indonesia “inverted S” rotational-shear active tectonic system. That is, large earthquakes take place in the center (approximately on the scale of a few months, a few years or a dozen years) not long after a large earthquake sequence takes place in the head and the tail (Wu ZH et al., 2014a). This linkage of large earthquake events is actually a typical response to the earthquake control of an active tectonic system, as well as an indication of the close kinematic and dynamic connections of the tectonic activity among the components of a tectonic system. In the most recent large earthquake sequence since 1997–2000, however, Sichuan-Yunnan, the central part of this tectonic system, has remained exceptionally seismically quiet period for more than a decade, which could signal notably higher risks of large earthquakes in this area in future. Based on the three criteria for determining large earthquake risks along an active fault zone, including elapsed time, seismic empty area and the chain reaction of strong earthquakes, and according to the latest analysis on the seismic empty areas and seismic b values in Southwest China (Liu YH et al., 2014, 2015), the authors further determined that Southwest China’s Sichuan-Yunnan is home to at least ten active fault zones with high large earthquake risks (Wu ZH et al., 2014b). These include: The Anninghe and Qiaojia sections in the center and the Chengjiang-Jianshui at the southern end of the Xianshuihe-Xiaojiang fault zone, the northwestern Yunnan’s Heqing-Songgui graben of the Litang-Dali-Ruili arc structure belt, the Qina-Binchuan section of the Chenghai-Binchuan fault zone, the Wanding and Nantinghe fault zones, the southeastern branch (Jinghong section) of the Lancang-Jinghong fault zone, and the Yuanmo and Baoshan grabens inside active blocks. Furthermore, according to the Late Quaternary activity of the faults and their contributions to the present-day crustal deformation, the authors assume that the Qiaojia section of the Xiaojiang fault zone, the Wanding fault zone, the middle section of the Chenghai-Binchuan fault zone, the Jinghong section of the Lancang-Jinghong fault zone, and the Baoshan graben are more likely to suffer strong earthquake events in future.

5.2.2   Seismic activity and future strong earthquake risks in central-eastern YREB

As active faults and tectonic systems have obviously constrained medium-strong earthquakes in the central-eastern YREB, under the nearly E-W-compressing present-day tectonic stresses, locations having the above-mentioned special geotectonic conditions obviously have the potential of suffering repeated medium-strong earthquakes, though the time interval may vary from a thousand to ten thousand years depending on the intensity of the fault activity. Historical earthquake recurrence theory assumes that a place having suffered medium-strong earthquakes in history can still suffer earthquakes of similar intensity in future. Tectonic seismic analogy theory predicts that locations having similar geotectonic conditions are likely to suffer earthquakes of similar intensities. On these bases, historical earthquake regions in the central-eastern YREB are likely to suffer repeated strong earthquakes. These include Changde, Macheng, Huoshan, Yangzhou and Liyang, and other locations having similar geotectonic conditions to strong earthquake areas, such as Jingzhou, Yueyang, Xiangyin, Wuxi and Suzhou. Besides, there are also some buried Quaternary fault zones with different strikes and sizes in northern Jiangsu Province and the Yangtze River delta, where potential risks of M6.0 or larger destructive earthquakes have to be addressed.

Following is a brief analysis of the future seismic risks in the central-eastern YREB and some of the outstanding concerns that need to be addressed according to the fault and seismic activity and critical infrastructures in this area.

5.2.2.1. Outstanding concerning some issues relating to future seismic risks

(i) The most recent activity and potential seismogenic ability of the main fault or structure belts in the area have to be further assessed.

(ii) When the global plate movement is entering a new active stage, it is well worth paying close attention to the current seismic risks in this area, especially the Suzhou-Wanjiang section of the Tanlu fault zone where major/great earthquakes may take place in future.

(iii) There is a possibility that the largest earthquake will exceed M6.9, i.e. be of M7 magnitude or larger, and that the seismic intensity may become higher than VIII in the central-eastern YREB.

(iv) Reservoir earthquakes: The area is home to the largest reservoir in China the Three Gorges reservoir and many minor ones, such as the Danjiangkou and Huoshan reservoirs. Earthquakes near reservoirs or reservoir-induced earthquakes can both, threaten the safety of the dams and lead to a succession of derivative disasters.

(v) Earthquakes near the Yangtze River can threaten the safety of the flood control levees and trigger secondary disasters along the Yangtze River.

(vi) Seismic safety of off-shore projects along the southern reaches of the Yellow Sea.

(vii) Potential threats to the projected inland nuclear power stations from potential regional strong earthquake activity.

5.2.2.2. Possibility of M$\geqslant$7 earthquakes in future

(i) This is the most outstanding problem currently facing the Mid-Lower Yangtze River economic belt. Studies indicate that, as of 2015, a giant seismic empty area (Ms $\geqslant$ 6) had existed for 105 years since it formed during the interlude 1910–1984 from the North China plains to North Jiangsu’s Huaihe plains; its emptying range, emptying time and average seismic magnitude are all similar to the empty area before the 1668 CE Tancheng M8.5 earthquake. It is slightly smaller than the 1668 earthquake empty area, which covered Hebei, Henan, Shandong, Huanghai, Anhui, Jiangsu, and Shanghai. Its long axis strikes NNE, coinciding with that of the central southern section of the Tanlu fault. This is the largest empty area discovered in East China since 1668 (Fig. 10), and so far, the most serious earthquake risk phenomenon in East China.

Figure  10. 

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(ii) Anomalies at the seismic system precursor station. The precursor anomalies involve Jiangsu, Shandong, and Anhui, in the central southern section of the Tanlu fault zone, mostly at the Jiangsu-Anhui junction, with more than 30 precursors. They have developed continuously since 2010.

(iii) Two possible styles of energy release of the seismic empty area: Release in clusters, groups or series of a few about M7 quakes, or in one about M8 large quake.

The 1668 CE M8.5 earthquake along the middle section of the Tanlu fault zone took place in southern Shandong Province, near North Jiangsu Province, with epicenter intensity of XII. The current precursor anomalies are mainly detected in Jiangsu-Anhui. From the seismic empty areas and the macroseismic recurrence intervals, it is very likely that future large earthquakes will take place inside Jiangsu-Anhui.

5.2.2.3. Earthquake-induced secondary disasters in the Yangtze River delta

(i) Amplification by ground soil. From the historical earthquake record, despite the modest earthquake magnitude in the Yangtze River delta, the shallow seismic focuses still result in considerable epicenter intensities. The ground soil in this area is composed of loose delta deposits. The weak ground soil will obviously amplify the seismic vibration, and lead to heterogeneous settlement of the ground and sand-soil liquefaction, potentially causing damage to infrastructure and buildings and exacerbating earthquake consequences.

(ii) Far-field effects cannot be underestimated. The soft ground soil and high rises in the area will vibrate strongly with the long-duration seismic fluctuation of any M8 earthquake in the neighboring areas of Taiwan, East China Sea, North China, Northwest China and Southwest China, causing severe damage to high rise structures. In the neighboring South Yellow Sea, earthquakes larger than M6 and even M7 have already taken place, directly threatening Shanghai and other areas; the seismic potential in the southern section of the Tanlu fault zone can also affect this area and is therefore well worth paying close attention.

5.3   Crustal stability along the YREB

In the western YREB, especially the area south of the Shangri La to Xichang line, which has the densest active faults in Mainland China, numerous active faults have cut this block into many micro-fault blocks. It also appears to have the strongest macroseismic activity in Mainland China. Overall, the western YREB area has strong crustal activity but has the worst crustal stability in the YREB and even in China. The central-eastern YREB, as a whole, has weaker present-day crustal activity and better crustal stability.

(i) In terms of geotectonics and tectonic systems, the Yangtze and South China blocks, as a relatively consolidated whole, have undergone long and stable deposition after collision in the Late Proterozoic. However, the Caledonian (mainly Silurian), Indosinian and Yanshanian regional orogenic processes (Pan GT et al., 2009; Shu LS, 2006) during this period have resulted in the development of numerous fault structures of varying sizes and cutting depths across the Yangtze-South China Block to the east of the Sichuan basin, even though the entire block has maintained fairly good integrity and relatively rigid lithospheric properties. Consequently, the crust or upper crust of the block has been cut into many microblocks, which could cause the tectonic stresses in the crust to be too dispersed across the block. Furthermore, the “chessboard tectonic system” of the central-eastern YREB itself also signifies that most faults in this area are weak active. In other words, the deformation intensity is too small to cause significant strike-slip dislocation between different strikes of faults. Hence, the geotectonics and present-day active fault system framework demonstrate that, except for the Tanlu fault zone which cuts off the lithosphere and makes up the boundaries between different tectonic domains, it is impossible for most of the other faults to breed high-intensity earthquakes larger than M7.0. This is the basic tectonic explanation for the limited neotectonic deformation in the central-eastern YREB.

(ii) According to the fault block tectonics theory or the active block concept, the boundary of a relatively rigid block or basin is regionally the tectonic position where stresses are more likely to concentrate and cause centralized deformation or trigger earthquakes. The central-eastern YREB is mainly covered with medium-low mountains or hills. There are no large basins or complete rigid blocks other than minor basins, which include the Jianghan-Dongting basin, Poyanghu basin, Hefei basin, and Subei basin. The peripheries of these basins are home to regionally more significant active faults and have also suffered more strong earthquakes in history than the rest of the area. Hence the crustal stability of these places is more of a concern than that of the remaining region.

(iii) In terms of the action conditions of plate boundaries, the majority of the western YREB (Sichuan-Yunnan), as part of the eastern Qinghai-Tibet plateau deformation zone (corresponding with the central southern section of the North-South tectonic zone), features intense deformation and frequent strong earthquakes during neotectonics. In the central-eastern YREB region, which is farther away from the plate boundaries to its east and west, and which has the North-South tectonic zone to its west as a deformation buffer and the South Yellow Sea and East China Sea to the east as marginal sea basin deformation buffers, the plate forces were already weakened when they were transferred there. Also, given the relatively rigid lithosphere and fairly fragmented upper crust of this area, relatively strong structural deformation is obviously impossible.

These geotectonic and plate boundary conditions lead to the conclusion that most of the faults in the central-eastern YREB are the reactivation of old structure belts, and that a substantive majority of them exhibit very low rates of activity featuring slow deformation. Hence, individual faults have low seismic frequency and modest seismic intensity. Although some of the faults have the potential of breeding M6 or M7 earthquakes, the earthquake probability is very low.

Overall, determining fault activity is the most critical constraint for the investigation and assessment of the active tectonics and regional crustal stability of the YREB. However, studying in this respect is challenged by a range of real problems, including: Severely insufficient coverage of regional active fault survey and investigation degree of main active faults; the particular geology, geography, and social conditions of the central-eastern YREB, which are themselves a great challenge to practical fault activity determination efforts; lack of knowledge about the geotectonic origins of typical historical strong earthquakes, preventing accurate clarification of the main active faults in the area; shortage of reliable Quaternary formation sequence and age scales, potentially causing the dating of the most recent fault activity to become distorted; limited availability of funds for research in this field, which limits the application and development of new technologies and new methods. Furthermore, the active faults and seismic risks of low rates fault-activity areas are also of great concern. In investigating urban active faults, special attention should be paid to the limitations of the “upper faulted point” method for determining fault activity when they are used for Quaternary plains or Quaternary-covered areas, as well as the limitations of historical earthquake records when they are applied to assess urban seismic risks and the consequent underestimation of seismic risks.

6.   Conclusions

(i) There are main 165 active faults in the YREB, including 79 in the western YREB and 86 in the central-eastern YREB. Among them, there are seven noteworthy regional active faults zones in the central-eastern YREB having more significant earthquake potential in future: The NE-trending Qianjiang-Jianshi fault zone; Changde-Jingzhou fault zone; Yueyang-Wuhan fault zone; the Suqian-Feidong-Huangmei section of the Tanlu fault zone; the Anqing fault zone; Wanjiang fault zone; and the NW-trending Wuxi-Suqian fault zone.

(ii) The main active fault zones in the western YREB, together with the neighboring regional active faults, make up an arc fault block region comprised primarily of Sichuan-Yunnan, and a “Sichuan-Yunnan arc rotational-shear active tectonic system” strong deformation region featuring rotation, shear, and extensional deformation. The active faults in the central-eastern YREB, with seven NE-NNE and seven NW-NWW active faults (the “7-longitudinal, 7-horizontal” pattern), macroscopically make up a typical “chessboard tectonic system” moderate-weak deformation region.

(iii) All of the M7–M8 earthquakes and the absolute majority of the 6.0 $\leqslant$ M < 7.0 earthquakes recorded within the YREB historically took place in and to the west of the North-South structure belt; no M $\geqslant$ 7.0 earthquakes, other than 12 of magnitudes 6.0 $\leqslant$ M < 7.0, have ever taken place in the central-east region (Chongqing-Shanghai). The active tectonic system framework has obviously controlled the regional seismic activity. Active fault zones represent the main locations of moderate to strong earthquakes. In the central and eastern YREB, however, the intersections between different strikes of faults are often important locations for destructive earthquakes.

(iv) In the eastern section of the YREB, the seismic activity is moderate both in intensity and frequency. No M7 or larger earthquakes have ever been recorded. Nor is there much possibility for large earthquakes in future. This is good for the seismic safety of this area, since direct earthquake hazards will not be serious. Secondary hazards, however, are still a big problem there and must be seriously addressed. Effective steps must be taken to minimize potential damage. Furthermore, as the Yangtze River delta lies over unconsolidated Quaternary depositional formations, ground motion is strongly amplified, making the seismic intensity exceptionally large. Tall buildings and far-field resonance are both negative factors. Hence anti-seismic reinforcement will be necessary for infrastructure, and new technologies such as vibration isolation and early warning must be used.

(v) The southwestern Sichuan-Yunnan block in the western YREB and its neighboring area have remained exceptionally seismic quiescence over the past years, however, they contain at least ten active fault zones having high macroseismic risks in future. These include: The Anninghe and Qiaojia section in the center and the Chengjiang-Jianshui at the southern end of the Xianshuihe-Xiaojiang fault zone, the northwestern Yunnan’s Heqing-Songgui graben of the Litang-Dali-Ruili arc structure belt, the Qina-Binchuan section of the Chenghai-Binchuan fault zone, the Wanding and Nantinghe fault zones, the southeastern branch (Jinghong section) of the Lancang- Jinghong fault zone, and the Yuanmou and Baoshan grabens inside blocks.

In the central-eastern YREB, more attention must be paid to the possibility of repeated strong earthquakes in the historical destructive earthquake areas, such as Changde, Macheng, Huoshan, Yangzhou, and Liyang, and locations having similar geotectonic conditions to strong earthquake areas, such Jingzhou, Yueyang, Xiangyin, Wuxi, and Suzhou. In northern Jiangsu and the Yangtze River delta, there are some buried Quaternary fault zones of varying strikes and sizes. The potential risks of M6.0 or larger destructive earthquakes should also be addressed. Besides, given the weak fault activity and low historical strong earthquake frequency in the central-eastern YREB, it is necessary to conduct archaeological earthquake and paleo-earthquake surveys to complement the limited historical earthquake record. Particularly, more attention must be paid to the geological remains preserved in the Cenozoic formations such as seismic wedges, seismic faults and fault wedges, which provide scientific basis for a more comprehensive assessment of the regional seismic risks.

Acknowledgment

The authors want to extend gratitude to research fellow Chang-xing Long of the Institute of Geomechanics, Chinese Academy of Geological Sciences for helpful discussions, to associate research fellow Jian-hua Li of the same institute for providing useful information, and to undergraduate student Kai Li of Capital Normal University for drawing some of the maps herein. This research is funded by the China Geological Survey project (DD20160268).