Seismogenic Structure around the Epicenter of the May 12,2008 Wenchuan Earthquake from Micro-seismic Tomography

A three-dimensional local-scale P-velocity model down to 25 km depth around the main shock epicenter region was constructed using 83821 event-to-receiver seismic rays from 5856 aftershocks recorded by a newly deployed temporary seismic network. Checkerboard tests show that our tomographic model has lateral and vertical resolution of -2 km. The high-resolution P-velocity model revealed interesting structures in the seismogenic layer： （1） The Guanxian-Anxian fault, Yingxiu-Beichuan fault and Wenchuan-Maoxian fault of the Longmen Shan fault zone are well delineated by sharp upper crustal velocity changes; （2） The Pengguan massif has generally higher velocity than its surrounding areas, and may extend down to at least -10 km from the surface; （3） A sharp lateral velocity variation beneath the Wenchuan-Maoxian fault may indicate that the Pengguan massif＇s western boundary and/or the Wenchuan-Maoxian fault is vertical, and the hypocenter of the Wenchuan earthquake possibly located at the conjunction point of the NW dipping Yingxiu-Beichuan and Guanxian-Anxian faults, and vertical Wenchuan-Maoxian fault; （4） Vicinity along the Yingxiu- Beichuan fault is characterized by very low velocity and low seismicity at shallow depths, possibly due to high content of porosity and fractures; （5） Two blocks of low-velocity anomaly are respectively imaged in the hanging wall and foot wall of the Guanxian-Anxian fault with a -7 km offset with -5 km vertical component.

Relocation and seismogenic structure of the 1998 Zhangbei-Shangyi earthquake sequence

On January 10, 1998, an earthquake of ML=6.2 occurred in the border region between the Zhangbei County and Shangyi County of Hebei Province. This earthquake has been the most significant event occurred in the northern China in the recent years. Historical seismicity in the Zhangbei-Shangyi region was very low. In the epicentral area no active fault capable of generating a moderate earthquake like this event was found. The earthquake locations of the main shock and its aftershocks of the Zhangbei-Shangyi earthquake sequence given by several agencies and authors were diverse and the resulted hypocentral distribution revealed no any dominant horizontal lineation. To study the seismogenic structure of the Zhangbei-Shangyi earthquake, in this paper the main shock and its aftershocks with ML3.0 of the Zhangbei-Shangyi earthquake sequence were relocated using the master event relative relocation algorithm. The relocated results show that the epicentral location of the main shock was 41.145癗, 114.462癊, which was located 4 km to the NE of the macro-epicenter of the main shock. The relocated focal depth of the main shock was 15 km. The hypocenters of the aftershocks distributed in a nearly vertical N20E-striking plane and its vicinity. The relocated results of the Zhangbei-Shangyi earthquake sequence clearly indicated that the seismogenic structure of this event was a nearly N-S- to NNE-SSW-striking fault with right-lateral and reverse slip, and that the occurrence of this event was associated with the horizontal and ENE-oriented compressive tectonic stress, which was compatible with the tectonic stress field in the northern China.

Spatio-temporal characteristics of aftershocks and seismogenic structure of the 2011 Mw9.0 Tohoku earthquake,Japan

The Tohoku megathrust earthquake, which occurred on March 11, 2011 and had an epicenter that was 70 km east of Tohoku, Japan, resulted in an estimated ten′s of billions of dollars in damage and a death toll of more than 15 thousand lives, yet few studies have documented key spatio-temporal seismogenic characteristics. Specifically, the temporal decay of aftershock activity, the number of strong aftershocks (with magnitudes greater than or equal to 7.0), the magnitude of the greatest aftershock, and area of possible aftershocks. Forecasted results from this study are based on Gutenberg-Richter’s relation, Bath’s law, Omori’s law, and Well’s relation of rupture scale utilizing the magnitude and statistical parameters of earthquakes in USA and China (Landers, Northridge, Hector Mine, San Simeon and Wenchuan earthquakes). The number of strong aftershocks, the parameters of Gutenberg-Richter’s relation, and the modified form of Omori’s law are confirmed based on the aftershock sequence data from the Mw9.0 Tohoku earthquake. Moreover, for a large earthquake, the seismogenic structure could be a fault, a fault system, or an intersection of several faults. The seismogenic structure of the earthquake suggests that the event occurred on a thrust fault near the Japan trench within the overriding plate that subsequently triggered three or more active faults producing large aftershocks.

Study of the Seismogenic Structure of the October 12^(th),2019 MS5.2 Beiliu Earthquake,Guangxi,China

On October 12th,2019,a MS5.2 earthquake occurred in Beiliu City,Guangxi Zhuang Autonomous Region,China,with a focal depth of 10 km. The epicenter is located in the junction of Guangxi and Guangdong where the moderate-strong earthquakes are relatively active. The highest intensity of this earthquake is estimated up to Ⅵ besides the isoseismic line showed an ellipse shape with a long axis trend in the NW direction.The aftershocks are not evenly distributed. The parameters of the focal mechanism solutions are: strike 346°,dip 85°,rake 19° for the nodal planeⅠ,and strike 254°,dip 71°,rake 175° for the nodal planeⅡ. The type of the coseismic fault is strikeslip. After analyzing these results above and the active faults near the epicenter,we get that the nodal planeⅠ is interpreted as the coseismic rupture plane and the BamaBobai Fault is a seismogenic structure of MS5.2 Beiliu earthquake.

Examination of Historical Data of the 1125 Lanzhou M 7.0 Earthquake and Analysis of Seismogenic Structure

Detailed examination of historical data of earthquakes and field investigations of loess landslide caused by the earthquake and tracing of active faults in Lanzhou area indicate that the Yijitanpu town, one of six towns of Jincheng city, was devastated by the 1125 Lanzhou earthquake. The citly is now located in the Vinylon Factory south of Hekou (River Mouth) in the Xigu district of Lanzhou city. We delermined that the six old towns mentioned in historical records lie in an area stretching from the south of Xigu district to Hekou in Lanzhou. This is consistent with the distribution of loess landslides caused by the earthquake, the extension of Holocene active faults, and the distribution of traces of the seismic rupture zone. A comprehensive analysis shows that the seismogenic structure for the 1125 Lanzhou M 7.0 earthquake should be the Xianshuigou fault segment at the western termination of the north-border active fault zone of the Maxianshan Mountains which are located in south of Lanzhou city with the distance of only 4 km.

Research on the Historical Data of the 1585 A. D. South Chaoxian Earthquake and Its Seismogenic Structure

In 1585,a MS5 3/4 earthquake occurred in the south of Chaoxian county,Anhui Province. The parameters of this earthquake were reported differently in various versions of earthquake catalogues. According to detailed textual research on the historic records of this earthquake,the epicenter location of the earthquake was further confirmed by means of seismo-geological field investigations in the Chaohu-Tongling region along the western Yangtze River valleys. Shallow seismic prospecting and drilling methods were applied in studying the buried fault. The possibility of the existence of seismogenic faults and fault activity in the western Yangtze River area were analyzed in depth,and the causative tectonic background of the 1585 MS5 3/4 south Chaoxian earthquake was studied. The results of this study indicate that the Yanjiaqiao-Fengshahu fault,which was active in the early to mid-Pleistocene,is possibly the causative structure of this earthquake. To identifying the seismogenic structure of the 1585 south Chaoxian earthquake will help gain more knowledge about the tectonic background of moderate and small earthquake activity in Eastern China.

The Seismogenic Structure and Deformation Mechanism of the Lushan(MW 6.6) Earthquake, Sichuan, China

On April 20 th, 2013, an earthquake of magnitude MW 6.6 occurred at Lushan of Sichuan on the southern segment of the Longmenshan fault zone, with no typical coseismic surface rupture. This work plotted an isoseismal map of the earthquake after repositioning over 400 post–earthquake macro–damage survey points from peak ground acceleration（PGA） data recorded by the Sichuan Digital Strong Earthquake Network. This map indicates that the Lushan earthquake has a damage intensity of IX on the Liedu scale, and that the meizoseismal area displays an oblate ellipsoid shape, with its longitudinal axis in the NE direction. No obvious directivity was detected. Furthermore, the repositioning results of 3323 early aftershocks, seismic reflection profiles and focal mechanism solutions suggests that the major seismogenic structure of the earthquake was the Dayi Fault, which partly defines the eastern Mengshan Mountain. This earthquake resulted from the thrusting of the Dayi Fault, and caused shortening of the southern segment of the Longmenshan in the NW–SE direction. Coseismal rupture was also produced in the deep of the Xinkaidian Fault. Based on the above seismogenic model and the presentation of coseismic surface deformation, it is speculated that there is a risk of more major earthquakes occurring in this region.

Seismic intensity investigation of the 2001 M=6.0 Yajiang-Kangding earthquake and discussion on its background of seismogenic structures

An M=6.0 earthquake occurred on February 23, 2001 in the western Sichuan Province, China. The macro seismic epicenter situated in the high mountain-narrow valley region between Yajiang and Kangding counties. According to field investigation in the region, the intensity of epicentral area reached VIII and the areas with intensity VIII, VII and VI are 180 km2, 1 472 km2 and 3 998 km2, respectively. The isoseismals are generally in elliptic shape with major axis trending near N-S direction. The earthquake destroyed many buildings and produced some phenomena of ground failure and mountainous disasters in the area with intensity VIII. This event may be resulted from long-term activities of the Litang fault and Yunongxi fault, two main faults in the western Sichuan. The movements between the main faults made the crust stress adjusted and concentrated, and finally the earthquake on a secondary fault in the block released a quite large energy.

Discussion on moment tensor solution and seismogenic structure of Ruichang-Yangxin earthquake on 10 September 2011

Ruichang-Yangxin earthquake is another moderate earthquake in Yangxin-Jiujiang area since 2005 Jiujiang-Ruichang M5.7 earthquake. In order to more understand the seismic activities in this area, we study the moment tensor solution and the seismogenic structure of the Ruichang-Yangxin earthquake. Precise earthquake relocation shows that the mainshock occurred on the southwestern part of the NE-trending fault and aftershocks are distributed not only along the NE-trending fault but also along its conjugated NW-trending fault. By comprehensive analysis on the earthquake distribution, characteristics of isoseismal curve, focal mechanism, and regional structure characteristics, it is inferred that this earthquake is caused by the NE-trending Tanlu fault. In addition, it has close relationship with the conjugated NW-trending fault as well. Many researches have shown that the junction area is the earthquake-prone area, and should be paid more attention to. And our study also proves this viewpoint.

Seismogenic structure of the 2016 Ms6.4 Menyuan earthquake and its effect on the Tianzhu seismic gap

On January 21, 2016, a strong earthquake with a magnitude of Ms6.4 occurred at Menyuan, Qinghai Province of China. In almost the same region, there was another strong earthquake happened in 1986, with similar magnitude and focal mechanism. Based on comprehensive analysis of regional active faults, focal mechanism solutions, precise locations of aftershocks, as well as GPS crustal deformation, we inferred that the Lenglongiing active fault dips NE rather than SW as suggested by previous studies. Considering the facts that the 2016 and i986 Ms6.4 Menyuan earthquakes are closely located with similar focal mechanisms, both of the quakes are on the north side of the Lenglongling Fault and adjacent to the fault, and the fault is dipping NE direction, we suggest that the fault should be the seismogenic structure of the two events. The Lenglongling Fault, as the western segment of the well-known Tianzhu seismic gap in the Qilian-Haiyuan active fault system, is in a relatively active state with frequent earthquakes in recent years, implying a high level of strain accumulation and a high potential of major event. It is also possible that the Lengiongiing Fault and its adjacent fault, the Jinqianghe Fault in the Tianzhu seismic gap, are rupturing simultaneously in the future.

Focal mechanism and seismogenic structure of the Shiqu MS4.4 earthquake

The focal mechanism solution of the Shiqu MS 4.4 earthquake occurred on May 16th,2017 in Sichuan Province is studied by the gCAP method using the waveform data from the regional seismic networks in Sichuan,Qinghai,Tibet and Gansu provinces.The strike/dip/dipping angle of the first nodal plane are 214/80/167and those of the second nodal plane are 306/77/10,the optimal centroid depth is 7.30.6 km and the moment magnitude is MW 4.5.Furthermore,the study investigates the robustness of the results against the error of crustal velocity structure,location,data quality and difference of seismic parameters,subsequently obtaining a stable resolved focal mechanism.According to the geological structure in the seismogenic area,spatial distribution of aftershock sequenceof the regional tectonic stress field,and the focal mechanism of the main shock,we suggest that the Shiqu earthquake is induced by a left-lateral strike-slip mechanism and the second nodal plane is inferred to be the seismogenic fault,consistent with the geometry of the Changshagongma fault which is the secondary fault of the northwest part of the Xianshuihe fault zone.

Application of Planar Seismogenic Structure to the Seismotectonic Method——A Case Study in the Dayao-Yao'an Area

In seismic hazard analysis of nuclear power plant of China there is a need to identify both seismogenic structures and seismotectonic zones. In past practice,the identification of the seismogenic structures was often based on the surface active faults and characterization of linear seismic source. In a situation which shows quite strong non-random seismic activity and lacks surface active faults,it is difficult to evaluate the seismic hazard reasonably. Taking seismogenic structures in the Dayao-Yao'an area as a case study in this paper,we discuss the need and the possibility to apply the planar seismogenic structure to the seismotectonic method. We suggest that the planar seismogenic structure should be considered when applying the seismotectonic method to the seismic risk assessment of nuclear engineering in future.

Determination of the Seismogenic Structure of the 1861 East of Pulandian M6.0 earthquake

The 1861 M6. 0 earthquake occurring in the east of Pulandian is another strong earthquake with M ≥ 6. 0 besides the 1975 Haicheng MT. 3 earthquake in the Liaodong peninsula. Through repeated investigations, the epicenter of the 1861 earthquake was located at Gupao, a village east of Pulandian. Based on the analyses of damage survey and precise location of modern instrumental earthquake data, the activity and seismic risk of the Jinzhou fault, Pulandian bay fault and the NW-trending Pulandian fault were analyzed. And by comparing the deep seismogenic environment between Pulandian and Haicheng, it is found that, as a neogenic active fault, the NW-trending fault, conjugated with the Jinzhou fault, has a higher seismic risk. The NW-trending fault is the seismogenic structure of the 1861 M6. 0 earthquake. And the Jinzhou fault, as a major fault in the Liaodong Peninsula, has controlled the seismicity of the region. The Pulandian bay fault is relatively inactive, with weak seismicity, and unrelated to the earthquake.

Analysis on the Focal Mechanism and Seismogenic Structures of the Yao’an earthquake with M_S6. 0,20091

Using the joint inversion method with the amplitude ratio of P-wave,SV-wave and SHwaves,this paper calculates the focal mechanisms of the aftershock sequence of the Yaoan earthquake with MS6. 0. According to the spatial distribution of earthquake sequence,the author analyzes the characteristics of the stress field and seismogenic fault. The result shows that:( 1) the seismogenic fault of the Yaoan earthquake is a vertical right-lateral strike-slip fault,striking NWW-SEE. The result is reliable and consistent with the nodal planes of the Harvard CMT solution and also in accord with the predominant direction of aftershocks.( 2) The predominant direction of principal compressive stress,NWW-SEE is consistent with the regional tectonic stress,and some aftershocks are different from the main shock. The stress field of the main shock is controlled by the regional tectonic stress field,indicating the diversity and complexity in the seismic area.( 3) By comprehensively analyzing the distribution of the earthquake sequence,focal mechanism and fault structure in the seismic area,it is found that the Maweijing fault is the seismogenic fault of the Yaoan earthquake.

Textual Research on the Historical Data of the 1573 AD Minxian Earthquake in Gansu Province and Discussion on Its Seismogenic Structure

According to the detailed study of the historical earthquake records and causative structure of the Minxian M6 1/2 earthquake in 1573 A.D., we have found that the most grievous disaster area lies nearby the Minxian county seat （Minzhou county at that time ）. So, we have identified the extremely seismic area of the 1573 A.D. The Minxian M6 1/2 earthquake was located in Minxian city, the intensity of the meizoseismal region is Ⅷ - Ⅳ, the epicenter is 34.4°N, 104.0°E, the location precision is Ⅱ and the deviation of location is less than or equal to 25km. Tectonically, this area lies in the transition region of stress transfer and structural transform between the east Kunlun fault and the northern margin of the west Qiuling fault. The differential activity of the Lintan-Dangchang fault zone is obvious, and only parts of the segment put up Holocene activity. There are landslides and rock bursts of different sizes in the meizoseismal region. By integrated analysis, we conclude that the Minxian-Dangchang segment of the Lintan-Dangchang fault is the seismogenic structure of the 1573 A.D. M6 1/2 Minxian earthquake, in Gansu Province.

Re-discussion on the Jiaochang Arcuate Structure, Sichuan Province, and the Seismogenic Structure for Diexi Earthquake in 1933

The Jiaochang arcuate structure is one of the numerous arcuate structural belts in Sichuan. The present paper gives a further argument about the characteristics of that arcuate structure and the new activity of the Songpinggou fault and affirms that the Songpinggou fault is an active fault in the Holocene epoch. The Diexi M 7.5 earthquake took place in 1933 on the west wing of that arcuate structure, near the apex of the arc. Many authors have given quite different opinions about the genetic structure of that earthquake. The authors have made on the spot investigations time and again over recent years. Besides this, the authors have also further studied the shape of intensity contour lines, the distribution characteristics of ground surface seismic hazards, the left lateral dislocation of buildings along the Songpinggou fault, the NW trending ground fissures that developed on the ground surface after earthquake, and so on. On this basis, it is still considered that the seismogenic fault of the 1933 Diexi M 7.5 earthquake was the Songpinggou fault on the west wing of the Jiaochang arcuate structure.

Application of Characteristics of Seismogenic Structures in the Determination of Parameter of Potential Seismic Source Areas

The characteristics of seismogenic structures are an important basis for delineating the potential seismic source areas and determining the annual occurrence rate of earthquakes. The potential seismic source area does not only have the intension that “this area has the possibility for destructive earthquakes to occur in the future" but also means that earthquakes of high magnitude interval have the characteristics of similar recurrence. When determining the seismic activity parameters of a statistical unit, some active tectonic blocks in the unit may have different background earthquakes. In order to better reflect the heterogeneity in space of seismic activities, it is necessary to divide the potential seismic source areas into three orders. By analyzing the recurrence characteristics of earthquakes of high magnitude interval in the potential source area and calculating the occurrence probability of earthquakes of high magnitude interval in the potential seismic source area in the time window for prediction, the average annual occurrence rate of earthquakes can be obtained by the method of probability equivalent conversion in the time window for prediction. This would be helpful for considering the recurrence characteristics of strong earthquakes in potential source areas within the framework of seismic risk analysis of China. Besides, the insufficient frequency of characteristic earthquakes of the next high magnitude interval in the potential source area and the heterogeneity of strong earthquakes on seismogenic structures are analyzed to see their application in seismic risk analysis.

Seismogenic tectonics of the Qian-Gorlos earthquake in Jilin Province,China

The Qian-Gorlos earthquake, which occurred in the Songliao basin in Jilin Province in 1119 AD, was the largest earthquake to occur in NE China before the 1975 Haicheng earthquake. Based on historical records and surface geological investigations, it has been suggested previously that the earthquake epicenter was in the Longkeng area. However, other workers have considered the epicenter to be in the Halamaodu area based on the landslides and faults found in this region. No seismogenic structure has yet been found in either of these two regions.We tried to detect active faults in the urban areas of Songyuan City, where the historical earthquake was probably located. One of the aims of this work was to clarify the seismogenic structure so that the seismic risk in the city could be more accurately evaluated. The area was investigated and analyzed using information from remote sensing and topographic surveys, seismic data from petroleum exploration, shallow seismic profiles, exploratory geological trenches on fault outcrops, and borehole data. The geophysical data did not reveal any evidence of faults cutting through Cretaceous or later strata under the Longkeng scarp, which has been suggested to be structural evidence of the Qian-Gorlos earthquake. The continuous fault surfaces on the back edge of terraces in theHalamaodu area stretch for [3.5 km and were probably formed by tectonic activity. However, results from shallow seismic profiles showed that the faults did not extend downward, with the corresponding deep structure being identified as a gentle kink band. A new reverse fault was found to the west of the two suggested epicenters, which presented as a curvilinear fault extending to the west, and was formed by two groups of NE- and NW-trending faults intersecting the Gudian fault. Three-dimensional seismic and shallow seismic data from petroleum exploration revealed its distinct spatial distribution and showed that the fault may cut through Late Quaternary strata. Exploration boreholes and later geomorphological studies provided further proof of this. Based on these results and analysis,the Gudian fault was confirmed as having been an active fault since the Late Quaternary, with the possibility of earthquakes of magnitude [7 in the future. The QianGorlos earthquake was most probably the result of breakage on one or two sections of this 66-km-long fault.

Characteristics of ground cracks caused by the M_(S) 6.4 Yangbi earthquake and the corresponding tectonic significance

An M_(S)6.4 earthquake occurred near Yangbi County, Dali Bai Autonomous Prefecture, Yunnan Province, at 21:48on May 21, 2021. The earthquake location is characterized by complex geological structures, with multiple active faults distributed around the epicenter that is located at the west edge of the Sichuan-Yunnan rhombic block(25.67°N, 99.87°E). A total of 42 ground cracks are found by earthquake field investigations. The cracks are mainly concentrated in the Ⅷ degree area on the west side of the Yangbi River. Among these, 9 coseismic tectonic ground cracks generated by shear fractures are found in three villages(i.e., Akechang, Meijia-Lijia, and Huajiazhuang), which are distributed along the strike of the northwest-trending linear folds, showing the tectonic characteristics of right-lateral tension or left-stepping cracks. The structural attribute of ground cracks sustains the kinematic properties of the Weixi-Qiaohou fault, namely right-lateral strike-slip.

Interpretation of the west segment of the coastal fault zone in the coastal region of South China based on the gravity data

By systemic processing, comprehensive analysis, and interpretation of gravity data, we confirmed the existence of the west segment of the coastal fault zone(west of Yangjiang to Beibu Bay) in the coastal region of South China. This showed an apparent high gravity gradient in the NEE direction, and worse linearity and less compactness than that in the Pearl River month. This also revealed a relatively large curvature and a complicated gravity structure. In the finding images processed by the gravity data system, each fault was well reflected and primarily characterized by isolines or thick black stripes with a cutting depth greater than 30 km. Though mutually cut by NW-trending and NE-trending faults, the apparent NEE stripe-shaped structure of the west segment of the coastal fault zone remained unchanged,with good continuity and an activity strength higher than that of NW and NE-trending faults. Moreover,we determined that the west segment of the coastal fault zone is the major seismogenic structure responsible for strong earthquakes in the coastal region in the border area of Guangdong, Guangxi, and Hainan.