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.

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.

The small earthquakes in and around Pubugou reservoir area are relocated by the double-difference method,and the earthquake properties of two cluster regions near water area are also analyzed.The 1834 earthquakes in the study area from 14 October 2006 to 31 December 2011 are relocated, and we have gotten 1708 earthquake relocation results eventually,amounting to 93.1% of the total. After the relocation,the RMS average value of arrival time residual decreases from 0.93s to 0.21s,the average error is 0.5km in EW direction,0.6km in NS direction,and 2.9km in vertical direction.The epicenter distribution is more concentrated,and the dominant distribution is also more obvious,the cluster region of small earthquakes distributes in the middle part of reservoir area,near to and downstream of the dam.The average focal depth is 9.2km. Earthquakes with focal depths less than 10km amount to 59% of relocation results,and those within 15km amount to 72%,the deepest focal depth is 33.1km.According to spatial distribution of relocation results,we divide the study area into four main regions (A,B,C and D) to discuss respectively.The evolutions of earthquake activities with time show that the variation of earthquake activities in region C and D is relativity larger than that in region A and B; the earthquakes of region C and D are highly centralized,and the focal depth is obviously shallow. With respect to the origin time of earthquakes,the earthquakes in region A and B occurred randomly in 24 hours of a day,which is accord with the time characteristic of natural earthquakes,while the occurrence time of earthquakes in region C and D had obvious regularity,most of them occurred in daytime,and focused on pre- and post 12'clock at noon and 18'clock in the afternoon.The results of field survey show that the seismic cluster area in region C is in accord with the construction sites of Ya'an-Xichang expressway and new Hanyuan county,and the cluster in region D is in accord with the quarry of Kaergou and Jialiega which are all near the reservoir dam. By synthesis analysis,we think that small earthquakes of region C and D may not be tectonic earthquakes; moreover,most of them occurred before November 2009,a time before the impoundment of Pubugou reservoir. So the earthquakes are not related with the impoundment either,but possibly the explosion ones caused by all kinds of construction.

The precisely located earthquake catalogue is important to seismicity, seismic tomography and crustal stress inversion studies. It also has great application value in rapid report of an earthquake that just occurred. By considering the arrival time uncertainty, and the constraints on station elevation and seismic depth, we propose a relatively accurate method to estimate hypocentral location and its uncertainty based on inversion theory. Our method can combine the arrival times of Pg wave, Sg wave, Pn wave and Sn wave in hypocenter location, so it increases the location accuracy by involving more data; and it can be also used in local and regional earthquake location simultaneously. In order to test our location method, we located earthquakes by using the simulated data with different uncertainty of Pg,Sg,Pn,Sn arrivals. The result shows that the location determined by using our method is more accurate than that by using other method. We apply it to earthquakes occurring in the period from 2001 to 2008 in Sichuan area, and obtained a more clustered hypocentral distribution convergent to the fault zones. The result provides a solid foundation for studies of seismicity, geometry of the active faults and seismic tomography in Sichuan region. It is also helpful to study the seismicity precursors before the Wenchuan earthquake.

Based on waveform data recorded by Zipingpu Reservoir Digital Seismic Network and Chendu Digital Seismic Network from August 2004 to May 2008 before Wenchuan MS 8.0 earthquake,and using ratio of amplitude,we calculated the focal mechanism parameters of 486 earthquakes with magnitude above 1.6 in Zipingpu area,and analyzed the temporal-spatial variation characteristic of focal mechanisms and stress.Results show:from 2006 to 2008,there were seldom thrust-type earthquakes,and strike-slip earthquakes increased in Zipingpu reservoir water field.The ratio of thrust earthquake was high,and strike-slip earthquakes' was low in areas far away from water field.Since the impoundment of the reservoir in October,2005,the thrust earthquakes in area A reduced significantly.The seismicity did not decrease in areas B and C,but showed a trend of increase.The strike-slip faulting earthquakes increased significantly in area A,but in areas B and C,they did not show an increasing trend.Under the background of increased thrust earthquakes,strike-slip earthquakes decreased in area C.The azimuth of mean principal compression stress fields deflected and disturbed to a certain extent before the M_S 8.0 Wenchuan earthquake in all areas.From the analysis of temporal variation of mean principal compression stress in all A,B and C areas,the variation of stress field of the whole study area is similar to that of Sichuan-Qinghai block,i.e.deflection and disturbance of mean stress field occurred in 2006.The variations in area A,B and C were similar,but deflection and disturbance in areas B and C were more distinct in scope.Analyses on the ratio between the thrust,strike-slip and normal earthquakes and on the temporal variation of stress fields of areas A,B and C show that,after the impoundment,the relative change of ratio of earthquake types of area A is the smallest,that of C the biggest,and that of area B is between A and C.This may be because that area C is right in the water area of the reservoir,B is farther,and A is the farthest from reservoir waters.

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.