On June 1, 2022, a M_S6.1 earthquake occurred in Lushan, Sichuan Province, western China, which is approximately 10km from the Lushan M_S7.0 event on April 20, 2013. To understand if the earthquake has the same seismogenic structure as the Lushan M_S7.0, we relocated the event in the Lushan area using the multi-stage locating method based on the seismic phase arrival data of the Sichuan Seismic Network from April 20, 2013, to July 1, 2022. A total of 6992 M_L≥1.0 earthquakes were acquired, with a relative locating error of 0.5km and 0.7km in the horizontal and vertical directions, respectively, with a travel time residual(RMS)of 0.18s. The results show that the M_S6.1 event is located at 102.943°E, 30.382°N with an initial-rupture focal depth of 15.6km, lying on the NW side of the 2013 Lushan M_S7.0 event. The sub-surface rupture length of the long and short axis is 10 and 8km, measured from the dense aftershock area in NE-SW and NW-SE directions, respectively. The NE-SW profile in the Lushan area shows that the depth of Lushan M_S7.0 earthquake in 2013 was about 15km, similar to that of Lushan M_S6.1 and M_S4.5 on June 1, 2022. The M_S6.1 earthquake sequence, located at the NE end of the long axis, shows no evidence to break through the rupture termination point of the Lushan M_S7.0 earthquake and enters the Dayi seismic gap, which is bounded by the 2008 Wenchuan M_S8.0 and 2013 Lushan M_S7.0 aftershock regions. The short-axis profile shows that the M_S6.1 earthquake sequence occurred on a new back-thrust fault in the pre-existing seismogenic structure of the 2013 Lushan M_S7.0. The new structure dips SE and ruptures in a slight arc protruding into the NW, parallel to the northern segment of the seismogenic structure of the 2013 Lushan M_S7.0 earthquake with a horizontal distance of about 5km. The new and old structures connect at the detachment base to the main segment of the 2013 Lushan M_S7.0 earthquake.

We also inverted the focal mechanism of the Lushan M_S6.1 earthquake using the CAP(Cut and Paste)method. The result indicates that the centroid depth of the M_W5.7 main event is 14km which is very close to the initial-ruptured depth of 15km calculated by the phase arrival times. The best double couple parameters are 221°/40°/105° for nodal plane Ⅰ and 22°/52°/78° for nodal plane Ⅱ. The parameters are in order of the strike, dip, and rake angles. Combined with the realization of the NE-striking, SE-dipping seismogenic structure characteristics determined by the accurate locating of the earthquake sequence, it can be quickly confirmed that the nodal plane Ⅱ is the fault plane.

Based on the accurate locating results, focal mechanism solutions, and geodynamic background of the focal area, it is inferred that the seismogenic structure of the Lushan M_S6.1 earthquake is induced by the thrust dislocation of a NE-SW trending and SE inclining thrust fault in the southern section of Longmenshan fault zone. Finally, we discussed the relationship between M_S7.0 and M_S6.1 in the Lushan area. The two could be considered a unique sequence: the mainshock and the maximum aftershock, respectively, regarding spatial relationship and tectonic correlation. However, the time interval of these two earthquakes significantly overextends the statistical relationship between the principal earthquake and the maximum aftershock. Furthermore, considering the effects of the Coulomb stress change produced by the earthquakes repeated at the end of the Dayi gap, Lushan earthquake further enhanced the stress level in the Dayi seismic gap located in its northern segment.

On February 3, 2020, an earthquake with a magnitude M_S5.1 occurred in Qingbaijiang District, Chengdu City, Sichuan Province. The epicenter is located in the north segment of the Longquan Shan fault zone in the western Sichuan Basin. This fault zone locates in the forebulge of the foreland thrust belt of the Longmen Shan fault zone in the southeast margin of Tibetan plateau and is the east boundary of the western Sichuan foreland basin at the same time, so it has special tectonic significance. There are two branch faults in the north segment of Longquan Shan fault zone, which are distributed on the east and west sides, respectively, and the epicenter distance is almost similar to the two faults. At present, the seismogenic fault, earthquake genesis and dynamic source of the earthquake are not clear. As this earthquake is a moderate earthquake event, it is usually very uncertain to interpret it with structural geological or seismological data alone. Therefore, this study attempts to carry out a comprehensive study on the Qingbaijiang M_S5.1 earthquake by performing cross fusion of multi-disciplinary data, adopting the multi-constraint method from geophysics, seismology and geodesy, and combining with structural geology and fault related fold theory. We collected three seismic reflection profiles located in the north segment of the fault zone to reveal the basic structural characteristics underground. The detachment layer in the middle-lower Triassic Jialingjiang-Leikoupo formation is developed at the depth of 4~6km below the anticline, and two obvious opposite thrust faults are developed on the two wings of the anticline, which are breakthrough fault-propagation fold deformation. The east branch thrust fault gradually rises from the detachment layer of Leikoupo formation to the surface, and the west branch thrust fault is exposed on the surface and connected with the detachment layer downward. The waveform data recorded by 14 fixed stations within 150km from the epicenter of Sichuan seismic network are studied and collected. The focal depth, focal mechanism and moment magnitude of the earthquake are obtained by using CAP waveform inversion method. The focal depth is 5km, indicating that the earthquake is related to shallow fault activity, the focal mechanism is 18°/32°/100° for nodal plane I and 186°/59°/84° for nodal plane Ⅱ, the moment magnitude is 4.64. Using the travel time data of P and S seismic phases, the Qingbaijiang earthquake sequence is relocated by HypoSAT location method and double difference location method. It is concluded that the epicenter position of the main earthquake is 30.73°N and 104.48°E. From February 4 to June 26, 2020, a total of 61 aftershock events were relocated, with magnitude 0≤M_L≤3.0 and depth ranging from near surface to 15km. The 61 aftershocks spread about 5km in the NW-SE direction and have conjugate distribution in NW and NE directions, which may be related to the small thrust fault developed on the east branch of Longquan Shan Fault. Aftershocks have a good linear distribution in NE direction, which is closer to the east branch of the north segment of Longquan Shan fault zone, and the distribution direction is also consistent with the fault strike. On the seismic reflection profile, the aftershock projection is densely distributed along the east branch fault. The occurrence of the east branch fault is consistent with the focal mechanism nodal plane I, which is a low angle thrust fault dipping to NW. The InSAR coseismic deformation field near the epicenter is extracted by using the Sentinel data of orbit 55 and orbit 62 collected from ESA, including 8 single view complex images of orbit 55 and orbit 62, respectively. The surface deformation caused by this earthquake is in the middle of two thrust faults, and the maximum coseismic deformation can reach 4cm. The deformation caused by the earthquake is uplifting in the northwest and depressing in the southeast of the epicenter. The largest depression is located between the epicenter and the east branch fault. The thrust activity of the east branch fault is more in line with the above surface deformation characteristics. In this study, the seismotectonics of the 2020 Qingbaijiang M_S5.1 earthquake is analyzed in detail using multi-disciplinary and multi-constraint method. The east branch fault in the north segment of the fault zone is determined as the seismogenic fault, and the possible seismic dynamic background is discussed. This result provides a scientific basis for fault activity analysis and seismic risk assessment in Longquan Shan area and has a great significance for further exploring the expansion and growth of Longmen Shan in the southeast margin of Tibetan plateau toward Sichuan Basin.

Based on the focal mechanism solutions of 2 600 M_L≥3.0 earthquakes in Sichuan and Yunnan area from January 2000 to March 2017, the focal mechanism quantitative classification and stress field inversion are carried out for the sub blocks and fault zones with relatively dense focal mechanisms. Using the focal mechanism solutions of 727 M_L≥4.0 earthquakes from January 1970 to March 2017, the regional stress tensor damping method is used to inverse the spatial distribution of principal compressive stress in Sichuan and Yunnan area before and after Wenchuan M_S8.0 and Lushan M_S7.0 earthquakes, and the temporal and spatial evolution characteristics of current stress field are discussed.
The focal mechanisms are distributed mainly in Longmenshan fault zone, Xianshuihe-Anninghe-Zemuhe-Xiaojiang fault zone, Mabian-Yanjin fault zone, Lijiang-Xiaojinhe fault zone, the central Yunnan block, the west Yunnan block and the southwest Yunnan block in Sichuan and Yunnan area. The focal mechanism is mainly strike slip type in Sichuan and Yunnan area, but there are local differences. The Longmenshan fault zone is dominated by thrust type earthquakes, while in the Mabian-Yanjin fault zone, there are relatively more strike slip and thrust type earthquakes. The types of earthquakes in Sichuan Basin are complex, and there is no obvious dominant type. In general, the focal mechanisms of the Longmenshan fault zone and Sichuan Basin earthquakes are affected by strong earthquake and other factors, and the focal mechanism types have good inheritance in Sichuan and Yunnan area.
The stress field in Sichuan and Yunnan area has obvious subarea characteristics, and it rotates clockwise from north to south. The compressive stress in Longmenshan fault zone and Sichuan Basin shows nearly EW direction. It shows NWW direction in the eastern boundary of Sichuan and Yunnan rhombic block and NNW direction in the inner part of rhombic, while it shows NNE direction in the western and southern Yunnan blocks. The principal compressive stress in Sichuan is more complex than that in Yunnan. The principal compressive stress direction in Sichuan experiences EW-NW-EW rotation from west to east, the dip angle is steep in the west and slow in the east, and the stress regime also experiences the transition from normal faulting to strike-slip to thrust. The principal compressive stress direction in Yunnan is NNE in the west and NNW in the east, forming an inverted “V” shape in space, the stress regime is mainly strike-slip and the dip angle is horizontal.
Before and after the Wenchuan M_S8.0 and Lushan M_S7.0 strong earthquakes, the stress field in the Longmenshan fault zone changed greatly, followed by the Sichuan Basin and its surrounding areas, and there was no obvious change in other areas of Sichuan and Yunnan. The stress field in the Longmenshan fault zone experienced a complete transformation process from basic stress field to variable stress field to basic stress field.

Based on the statistical results of 86 earthquakes with magnitude≥5.0 in the middle section of the north-south seismic belt and its surrounding regions since 1973, the types of earthquake sequences and the spatial distribution characteristics have been studied. Main conclusions are drawn as follows: 1)The sequence types of moderate and strong earthquakes in the study area are dominated by mainshock-aftershock sequence type(MAT), followed by multiple main-shock type(MMT)and least the isolated earthquake type(IET)sequence. In the same sequence type, with the increase of earthquake magnitude, the proportion of the MAT sequence increased, while the number of MMT and IET gradually decreased, M≥7 earthquakes are mainly of MAT, and there are no IET earthquakes. Among the different rupture types, the MAT earthquakes are the most in the thrust-type, while the MMT earthquakes are more likely to occur in the strike-slip and the normal-fault earthquakes. 2)There is a relatively good linear relationship between the mainshock-aftershock sequence type earthquakes and the maximum aftershock magnitude of the MAT and MMT sequences; the largest aftershock of most earthquakes mostly occurred in 15 days after the mainshock, the largest aftershock of MAT mainly occurred within 3 days after the mainshock, the largest aftershock of MMT earthquakes mainly occurred within 12 days after the mainshock, and the largest aftershock of IET earthquakes mostly occurred on the day of the earthquake. 3)The spatial distribution of seismic sequence shows that the MAT earthquake distribution range is relatively wide, the MMT earthquakes are mainly concentrated in Batang-Litang, Mabian-Zhaotong, Songpan area in Sichuan Province and Yunlong, Yao 'an, Longling and nearby areas in northwest Yunnan Province. IET earthquakes are more likely to occur in Ganzi-Yushu fault zone, the northwestern segment of Xianshuihe fault zone and in Sichuan Basin. 4)The distribution of seismic sequence types in the middle section of the north-south seismic belt and its adjacent areas may be related to the geological structure, historical seismic activity and the crustal stress in this region. The distribution of seismic sequence types also reflects the tectonic movement and dynamic environment in this region.

On August 8, 2017, a strong earthquake of M7.0 occurred in Jiuzhaigou County, Aba Prefecture, northern Sichuan. The earthquake occurred on a branch fault at the southern end of the eastern section of the East Kunlun fault zone. In the northwest of the aftershock area is the Maqu-Maqin seismic gap, which is in a locking state under high stress. Destructive earthquakes are frequent along the southeast direction of the aftershocks area. In Songpan-Pingwu area, only 50~80km away from the Jiuzhaigou earthquake, two M7.2 earthquakes and one M6.7 earthquake occurred from August 16 to 23, 1976. Therefore, the Jiuzhaigou earthquake was an earthquake that occurred at the transition part between the historical earthquake fracture gap and the neotectonic active area. Compared with other M7.0 earthquakes, there are few moderate-strong aftershocks following this Jiuzhaigou earthquake, and the maximum magnitude of aftershocks is much smaller than the main shock. There is no surface rupture zone discovered corresponding to the M7.0 earthquake. In order to understand the feature of source structure and the tectonic environment of the source region, we calculate the parameters of the initial earthquake catalogue by Loc3D based on the digital waveform data recorded by Sichuan seismic network and seismic phase data collected by the China Earthquake Networks Center. Smaller events in the sequence are relocated using double-difference algorithm; source mechanism solutions and centroid depths of 29 earthquakes with M_L≥3.4 are obtained by CAP method. Moreover, the source spectrum of 186 earthquakes with 2.0≤M_L≤5.5 is restored and the spatial distribution of source stress drop along faults is obtained. According to the relocations and focal mechanism results, the Jiuzhaigou M7.0 earthquake is a high-angle left-lateral strike-slip event. The earthquake sequence mainly extends along the NW-SE direction, with the dominant focal depth of 4~18km. There are few shallow earthquakes and few earthquakes with depth greater than 20km. The relocation results show that the distribution of aftershocks is bounded by the M7.0 main shock, which shows obvious segmental characteristics in space, and the aftershock area is divided into NW segment and SE segment. The NW segment is about 16km long and 12km wide, with scattered and less earthquakes, the dominant focal depth is 4~12km, the source stress drop is large, and the type of focal mechanism is complicated. The SE segment is about 20km long and 8km wide, with concentrated earthquakes, the dominant depth is 4~12km, most moderate-strong earthquakes occurred in the depth between 11~14km. Aftershock activity extends eastward from the start point of the M7.0 main earthquake. The middle-late-stage aftershocks are released intensively on this segment, most of them are strike-slip earthquakes. The stress drop of the aftershock sequence gradually decreases with time. Principal stress axis distribution also shows segmentation characteristics. On the NW segment, the dominant azimuth of P axis is about 91.39°, the average elevation angle is about 20.80°, the dominant azimuth of T axis is NE-SW, and the average elevation angle is about 58.44°. On the SE segment, the dominant azimuth of P axis is about 103.66°, the average elevation angle is about 19.03°, the dominant azimuth of T axis is NNE-SSW, and the average elevation angle is about 15.44°. According to the fault profile inferred from the focal mechanism solution, the main controlling structure in the source area is in NW-SE direction, which may be a concealed fault or the north extension of Huya Fault. The northwest end of the fault is limited to the horsetail structure at the east end of the East Kunlun Fault, and the SE extension requires clear seismic geological evidence. The dip angle of the NW segment of the seismogenic fault is about 65°, which may be a reverse fault striking NNW and dipping NE. According to the basic characteristics of inverse fault ruptures, the rupture often extends short along the strike, the rupture length is often disproportionate to the magnitude of the earthquake, and it is not easy to form a rupture zone on the surface. The dip angle of the SE segment of the seismogenic fault is about 82°, which may be a strike-slip fault that strikes NW and dips SW. The fault plane solution shows significant change on the north and south sides of the main earthquake, and turns gradually from compressional thrust to strike-slip movement, with a certain degree of rotation.

Small earthquakes have been recorded in Yibin area, Sichuan Province since 1970, the frequency and intensity of seismicity have shown an increasing trend in recent ten years, and the earthquakes are distributed mainly in Changning, Gongxian and Junlian areas. Based on the seismic data from January 2008 to May 2015 recorded by Sichuan and Yunnan regional networks and Yibin local network, seismicity analysis, precise location and velocity structure inversion for earthquakes in Yibin area are carried out, the three-dimensional spatial distribution of seismic activity and the velocity structure at different depths in this region are investigated, trying to analyze the seismic activity law and seismogenic mechanism in Yibin area.
The earthquake relocation result shows that the spatial cluster distribution of earthquakes is more obvious in Yinbin area, the earthquakes are concentrated in Changning-Gongxian and Gongxian-Junlian regions. The seismic activity presents two dominant directions of NW and NE in Changning-Gongxian region, and shows asymmetric conjugate distribution, the long axes of NW-trending and NE-trending seismic concentration area are about 30km and 12km respectively, and the short axes are about 5km. There is a seismic sparse segment near Gongxian, the frequency and intensity of seismicity in the southeast side are obviously higher than that in the northwest side, and the earthquakes with larger magnitude are relatively deep, the focal depth is gradually shallower with the distance away from Gongxian. Seismic activity is sparse in the west and dense in the east in Gongxian-Junlian region, the predominant direction of earthquakes in the seismic dense area of the eastern segment is NE. Seismic activity extends in opposite direction in the easternmost part of the two earthquake concentrated area.
The P-wave velocity structure at different depths in the study area is obtained using joint inversion method of source and velocity structure. In view of the predominant focal depth in this region, this paper mainly analyzes the velocity structure of the upper crust within 10km. Within this study area, the P-wave velocity of earthquake concentration areas is relatively high within 10km of the predominant focal depth, especially in the northwest of Gongxian and eastern Junlian area, the P-wave velocity on the southeast of Gongxian increases gradually with depth, especially at 6km depth. These high-velocity zones are generally related to brittle and hard rocks, where the stress is often concentrated.
Comparing earthquake distribution and velocity structure, seismic activity in this area mainly occurs in high-low velocity transition areas, the inhomogeneity of velocity structure may be one of the factors controlling earthquake distribution. The transition zone of high and low velocity anomalies is not only the place where stress concentrates, but also the place where the medium is relatively fragile, such environment has the medium condition of accumulating a large amount of strain energy and is prone to fracture and release stress.

The Daliangshan sub-block is a boundary region among the Bayan Har block, the Sichuan-Yunnan block and the South China block. It hosts four major fault systems:The southwest to south trending Xianshuihe-Zemuhe Fault zone in the west, the Longmenshan fault zone is the northern boundary, the Zhaotong-Lianfeng fault zone in the south, and the NS-trending Mabian-Yanjin fault zone in the east. This study focused on focal mechanisms and the regional stress field of the Daliangshan sub-block to help understand the earthquake preparation process, tectonic deformation and seismic stress interaction in this area. We collected broadband waveform records from the Sichuan Seismic Network and used multiple 1-D velocity models to determine the focal mechanisms of moderate and large earthquakes(M_L ≥ 3.5)in the Daliangshan sub-block by using the CAP method. Results for 276 earthquakes from Jan 2010 to Aug 2016 show that the earthquakes are dominated by strike-slip and trust faulting, very few events have normal faulting and the mixed type. We then derived the regional distribution of the stress field through a damp linear inversion(DRSSI)using the focal mechanisms obtained in this study. Inversion results for the spatial pattern of the stress field in the block suggest that the entire region is predominantly under strike-slip and trust faulting regimes, largely consistent with the focal mechanisms. The direction of maximum compression axes is NW-NWW, and part of the area is slightly rotated, which is consistent with the GPS velocity field. Combining geodynamic background, this work suggests that because the Sichuan-Yunnan block is moving to SE and the Tibetan plateau to SE-E along major strike-slip faults, the stress field of the Daliangshan sub-block and its adjacent regions is controlled jointly by the Bayan Har block, the Sichuan-Yunnan block and the South China block.

On 23 September 2016, two earthquakes with magnitude of M4.9 and M5.1 occurred successively near Litang city in Sichuan Province, southwestern China. These two events are located between two large-scale fault zones, i.e., the Jinshajiang and Litang faults, in the northwest of the Sichuan-Yuannan active block, eastern Tibetan plateau. Based on the phase data and waveform data from the Sichuan regional seismic network, the M4.9 and M5.0 mainshocks and 390 aftershocks have been relocated using the multi-step locating method, and the focal mechanism solutions and centroid depths for the two mainshocks were calculated by the CAP waveform inversion method. From the spatial distribution of the relocated aftershocks and fault plane solutions of the two mainshocks, combining with the seismic intensity map and tectonic setting, we suggested that the two earthquakes were generated by the E-W trending northward dipping Hagala fault. The nodal plane consistent with the strike and dip of the Hagala fault is interpreted as the coseismic rupture plane with a dip angle of 44° for both the M4.9 and M5.1 earthquakes. And we inferred that the M4.9 and M5.1 earthquakes may be resulted from the nearly E-W striking Hagala normal faulting in the upper crust between the Litang and Batang regions due to the continuous eastward extrusion of the material of the Qiangtang block in the west.

The Oct.1,2014 M5.0 Yuexi earthquake occurred on the Daliang Shan fault zone where only several historical moderate earthquakes were recorded.Based on the waveform data from Sichuan regional seismic network,we calculated the focal mechanism solution and centroid depth of the M5.0 Yuexi earthquake by CAP (Cut and Paste) waveform inversion method,and preliminarily analyzed the seismogenic structure.We also calculated the apparent stress values of the M5.0 earthquake and other 14 M_L≥4.0 events along the Shimian-Qiaojia fault segment of the eastern boundary of the Sichuan-Yunnan block.The result indicates that the parameters of the focal mechanism solution are with a strike of 256°,dip of 62°,and slip of 167° for the nodal plane Ⅰ,and strike of 352°,dip of 79°,and slip of 29° for the nodal plane Ⅱ.The azimuth of the P axis is 121° with dip angle of 11°,the azimuth of T axis is 217° with dip angle of 28°,and the centroid depth is about 11km,and moment magnitude is M_W5.1.According to the focal mechanism solution and the fault geometry near the epicenter,we infer that the seismogenic fault is a branch fault,i.e.,the Puxiong Fault,along the central segment of the Daliang Shan fault zone.Thus,the nodal plane Ⅱ was interpreted as the coseismic rupture plane.The M5.0 Yuexi earthquake is a strike-slip faulting event with an oblique component.The above findings reveal the M5.0 Yuexi earthquake resulted from the left-lateral strike-slip faulting of the NNW Dalang Shan fault zone under the nearly horizontal principal compressive stress regime in an NWW-SEE direction.The apparent stress value of the Yuexi earthquake is 0.99MPa,higher than those of the M_L ≥ 4.0 earthquakes along the eastern boundary of the Sichuan-Yunnan block since 2008 Wenchuan M8.0 earthquake,implying a relatively high stress level on the seismogenic area and greater potential for the moderate and strong earthquake occurrence.It may also reflect the current increasing stress level of the entire area along the eastern boundary,and therefore,posing the risk of strong earthquakes there.

Based on focal mechanism solutions of Wenchuan M≥4.0 and Lushan M≥3.0 aftershocks, using inversion method of stress field to analyze the spatial distribution characteristic of compressive stress (S₁) and stress tensor variance of Wenchuan and Lushan aftershock zones, the relation between spatial and temporal distribution of stress tensor variance and strong aftershock activity are studied. The results show that (1) The orientations of compressive stress (S₁) are complex in Wenchuan and Lushan aftershock zones, there exists obvious regional difference spatially, the S₁ orientations all present disorder feature near the main shock rupture zone, and the stress tensor variances are obviously higher. (2)Along Wenchuan aftershock zone from southwest to northeast, the compressive stress orientation gradually changes from EW to NW-SE, finally, it presents near EW direction at north segment of aftershock zone, and the stress type is all thrust; the S₁ orientation presents NEE direction in Lixian branch of Wenchuan aftershock zone, the stress type shows strike-slip type, the compressive stress presents a near horizontal feature in whole aftershock zone. (3)The compressive stress orientation presents NW direction in Lushan aftershock zone, the dip angle is near horizontal, the stress type is thrust near the main shock and strike-slip and thrust in other areas. (4)The spatial and temporal distribution of stress tensor variance has certain indicative significance for occurrence of strong aftershock, most later-phase strong aftershocks of Wenchuan and Lushan earthquake occurred in the low-value area of stress tensor variance and its fringe areas, the origin time of Wenchaun strong aftershocks are also at the time frame of low value of stress tensor variance.

Based on seismic data of the regional network of the last 34 years,we have analyzed current faulting behaviors of major fault zones in Mabian area,southern Sichuan,and preliminarily identified the risky fault-segments on which potential strong and large earthquakes may occur in future,with the method combining the spatial distribution of b-values with activity background of historical strong earthquakes and current seismicity.Our results mainly show:(1)The spatial distribution of b values displays significant heterogeneity in the study area,which reflects the spatial difference of cumulative stress level along various fault zones and segments in the area;(2)Three anomalously low b-value areas with different sizes exist on Mabian-Yanjin Fault zone,these anomalies can be identified as asperities under relatively high cumulated stress levels,in which,two asperities,located at north of Mabian county and Lidian town in western Muchuan county,and near Yanjin at the south end of the fault zone,respectively,may be the potential seismogenic sources of large earthquakes in Mabian area in the near future,and the third asperity with a small size located at southern Suijiang may be the potential strong-earthquake source;(3)An asperity at south-western segment of Longquanshan Fault may be the site of potential moderate to strong earthquakes;and(4)the asperity on the segment between Huangmu town in Hanyuan county and Longchi town in Emeishan city on Jinkouhe-Meigu Fault has potential for moderate to strong earthquakes.

The M_S 8.0 Wenchuan earthquake and its 2216 aftershocks were relocated using the double difference algorithm.The horizontal and vertical errors of the 2061 relocated hypocenters are approximately 1～2km and 2～3km,respectively.The epicenter of main shock is approximately 31.00°N,103.38°E,the focal depth is about 13km and the seismogenic structure is the central fault of Longmenshan Fault zone.The total length of spatial distribution of aftershocks along the strike of the fault is about 330km and the predominance distribution of focal depth is 3～20km,which shows obviously the characteristic of segmented activity.The seismicity of the southern part mainly concentrates on the central fault of Longmenshan Fault zone,and some earthquakes occurred on the range-front and range-back faults;the dip of the three faults seems to become gentler gradually from west to east,forming imbricate ruptures.The central fault and Pingwu-Qingchuan Fault of the Longmenshan Fault zone are involved in the seismogenic process,and the seismic rupture is both of thrust napping and right-lateral strike-slipping.

In this paper,we make an effort to study the feasibility to assess magnitudes of maximum potential earthquakes in sub-areas of moderately and weakly active faults in eastern China mainland by using parameters of frequency-magnitude relationship,and develop the corresponding methodology.Data suggest that frequency-magnitude relationships of fault sub-areas in eastern China mainland accord with the characteristic earthquake model,meaning that for a fault sub-area,the ratio a/b of the constants,which is also called as the maximum intercept magnitude of the exponential portion(i.e.the G-R relationship)of the frequency-magnitude relationship,is obviously less than that of the characteristic magnitudes M_C.In order to make the ratio a/b be usable in indirectly assessing the magnitudes of maximum potential earthquakes in fault sub-areas,we develop a method to establish long-duration frequency-magnitude relationships for fault sub-areas by combining data of both historical and modern seismicity.In this method,event's numbers from the two sources of data are all normalized to a duration of t=500years.We then calculate a_t/b values of the normalized G-R relationships for 130 fault sub-areas.Our analyses reveal that maximum magnitudes,M_max,of earthquakes occurring and recorded in the studied fault sub-areas are positively correlative with sizes of a_t/b values,and with the increase of a_t/b values the upper-limits of the maximum magnitudes,M_max,show the feature of monotonously rising and relatively smooth variation.Based on such feature we develop three empirical formulae of the relations between the upper-limits M_mu of the maximum magnitudes,and a_t/b values,for the three regions,i.e.North China,Central and East China,and South China and the southeastern coastal area,respectively,and take them as empirical models to estimate magnitudes of maximum potential earthquakes in fault sub-areas.By using the newly developed method and empirical models we estimate magnitudes of maximum potential earthquakes in several fault sub-areas.Our research also suggests that several types of the abnormal seismicity,such as the swarms of moderate and small size earthquakes,aftershocks and triggered earthquake sequences,and artificially induced seismicity,as well as the determination of minimum complete magnitudes,have influences to the calculation results of a_t/b values,and that severely influenced a_t/b values are overestimated and show deflecting to the right on M_max-a_t/b diagrams.The empirical models and method developed in this study can be applied to the assessment of magnitudes of maximum potential earthquakes for sub-areas of moderately and weakly active faults in eastern China mainland.

Estimation of sizes of potential earthquakes is required in long-term seismic hazard assessment.Such estimation is usually related to a single active fault segment with specific scale,and can be carried out by using an empirical relationship between magnitude and rupture-scale of earthquake.This type of empirical relationships published already for China mainland is established on data of surface ruptures along seismogenic active faults in West China.While,the special active tectonic environment in the region of North China makes it impossible to establish the same type of empirical relationships that are suitable for the region by using the surface-rupture data.Therefore,for North China,we should make an effort to develop the empirical relationships between earthquake magnitude and(sub-surface)source rupture-scale.In the latest 40 years,4 major earthquakes of magnitudes ≥7.0,7 strong ones of magnitudes 6.0 to 6.9 and tens of moderate ones(including aftershock events)of magnitudes 5.0 to 5.9,occurred in North China.For a part of these earthquakes,data or distribution patterns of aftershock sequences with good or high precision locations are available,and for many of these earthquakes,studies on source rupture processes and parameters have been carried out.These make it possible to establish preliminarily the empirical relationships between magnitude and rupture-scale for North China.For this purpose,from data of earthquakes that occurred in North China since 1965,we systematically collected and compiled the relevant rupture-scale parameters obtained from methods of analyzing seismic wave spectrum,coseismic crustal deformation and aftershock distribution,or available from published researches,including rupture length L,downdip rupture width W,and rupture area A(A=L×W).We also re-determined the rupture-scale parameters for part of these earthquakes basing on aftershock distributions.In order to deal with uncertainties in the source rupture-scale parameters due to the use of various methods,and to take the main factors influencing the uncertainties into account,we broungt forward 8 criterions for determining the reliable parameters through further analyses and identification.With synthetic analyses basing on these criterions,we obtained reliable rupture-lengths of 34 earthquakes and reliable rupture-areas of 20 of the 34 earhtquakes.Focal mechanism data suggest that the absolute majority of the 34 ruptures are of strike-slip type.By using the least-square method,we further established two regression relationships between surface-wave magnitude M_S and rupture-length L,and between magnitude M_S and rupture-area A for seismogenic active faults.They are M_S=3.821+1.860lg(L)and M_S= 4.134+0.954lg(A),respectively.A comparative analysis with previous empirical relationships of the same type suggests that the two new empirical relationships developed in this research are suitable very well for estimation of sizes of potential earthquakes on seismogenic active strike-slip faults in North China and in the urban area of Beijing and its surroundings.