It is important to study the characteristics of the tectonic stress field studies which could provide a deeper understanding of the internal stress environment of the crust. It can provide useful assistance for exploring the relationship between the tectonic stress field and earthquake development. At the same time, it plays an important role in understanding block interactions, fault movement, tectonic deformation, and revealing the dynamic mechanical processes of the continent. The focal mechanism solutions contain abundant information reflecting the stress field.

In this paper, using the broadband records from 128 permanent and temporary regional stations from the Chinese National Seismic Network(CNSN)deployed in the Sichuan-Yunnan Province and its adjacent, we determined the focal mechanisms of 3 951 earthquakes by the cut-and-paste(CAP)method and the HASH method. The friction coefficient and stress properties of the main active fault and characteristics of the tectonic stress field in this area are analyzed by using two different methods which are the damped inversion method(STASI)and iterative joint inversion method from focal mechanisms.

The results of the focal mechanisms show that: there are 2 512 strike-slip earthquakes in the study area, accounting for 63.58% of all earthquakes; there are 818 normal fault type and normal strike-slip type earthquakes, accounting for 20.70% of all earthquakes; there are 621 reverse strike slip and reverse thrust earthquakes, accounting for 15.72% of all earthquakes. The most of earthquakes in the study area are distributed in active fault zones, the strike of the fault plane is consistent with the orientation of active fault zones. It revealed predominantly strike-slip faulting characteristics of earthquakes in the Eastern Boundary of the Sichuan-Yunnan Block, while the reverse thrust of earthquakes is mainly concentrated in the Longmenshan fault zone, as well as the NW trending Mabian-Yanjin Fault and the NE trending of Ludian-Zhaotong and Lianfeng faults which lied on the eastern boundary of the Sichuan-Yunnan block. Overall, the characteristics of the source mechanism are consistent with the regional tectonic background.

Results of the stress field inversion confirmed main active fault in the Eastern Boundary of the Sichuan-Yunnan Block is under a strike-slip stress regime, maximum and minimum compressional stress axes are nearly horizontal. The maximum compressional axes are primarily oriented in NW-SE and NWW-SEE direction, and they experience a clockwise rotation from north to south. Against the strike-slip background, normal faulting stress regimes and reverse faulting stress can be seen in the regional areas. The most prominent is the Daliangshan fault zone, which has obvious differences from the overall characteristics of the stress field with the eastern boundary of the Sichuan Yunnan Block. The maximum horizontal principal stress in the northern section shows a nearly EW direction, with a strike-slip type stress property, and the NW-SE direction in the southern section, with a thrust type stress property. The distribution characteristics of the stress field are consistent with the fault type of sinistral strike-slip and thrust on the eastern boundary of the Sichuan Yunnan block

The shape ratio R-value varies significantly, the R-value in the Sanchakou area is relatively high, with obvious extrusion characteristics, the R-values of the Xianshuihe fault zone, Anninghe fault zone and Xiaojiang fault zone are all between 0.25-0.5, showing NE-SW compression and NW-SE tension, and the tensile stress may be much less than the compressive stress(strike-slip type). The R values of the northern segment of the Daliangshan fault zone, the southern segment of the Anninghe fault zone, and Zemuhe fault zone are all between 0.5-1, showing NW-SE compression and NE-SW tension, and the compressive stress is greater than the tensile stress. To sum up, the current stress characteristics of the eastern boundary of the Sichuan Yunnan rhombic block are shear strain and local compression or tension.

There are different friction coefficients of the main faults in the study area: The Anninghe fault zone is 0.60, the Xianshuihe and Zemuhe fault zones are 0.80, the Xiaojiang fault zone is 0.75 and northern and southern sections of the Daliangshan fault zone are 0.65 and 0.85. The friction coefficients of the Xianshuihe Fault, the southern section of the Daliangshan Fault, and the Zemuhe Fault are above 0.75. The high friction coefficients of these fault zones may be because they are strike-slip faults, and the friction coefficients themselves are relatively high. The southern section of the Xiaojiang fault zone may be related to the development of fault gouges in the fault zone.

The M_S7.1 earthquake in Wushi, Xinjiang on January 23, 2024, represents the largest earthquake in the Tianshan seismic belt since the 1992 Suusamyr M_S7.3 earthquake in Kyrgyzstan. Preliminary precise aftershock localization and initial field investigations indicate an NE-trending aftershock zone with a length of 62km that is concentrated at the mountain-basin transition area. This event produced geological hazards, including slope instability, rockfalls, rolling stones, and ground fissures, primarily within a 30-kilometer radius around the epicenter. The epicenter, located approximately 7 kilometers north of the precise positioning in this study, witnessed a rapid decrease in geological hazards such as collapses, with no discernible fresh activity observed on the steep fault scarp along the mountainfront. Consequently, it is inferred that the causative fault for this main shock may be an NW-dipping reverse fault, with potential rupture not reaching the surface.

Moreover, a surface rupture zone with a general trend of N60°E, extending approximately 2 kilometers, and displaying a maximum vertical offset of 1m, was identified on the western side of the micro-epicenter at the Qialemati River. This rupture zone predominantly follows the pre-existing fault scarp on higher geomorphic surfaces, indicating that it is not new. Its characteristics are mainly controlled by a southeast-dipping reverse fault, opposite in dip to the causative fault of the main shock. The scale of this 2-kilometer-long surface rupture zone is notably smaller than the aftershock zone of the Wushi M_S7.1 earthquake. Further investigation is warranted to elucidate whether or not the M_S5.7 aftershock and the relationship between the SE-dipping reverse fault responsible for the surface rupture and the NW-dipping causative fault of the main shock produced it.

From March 8^th to 29^th, 1966, five earthquakes(M≥6)occurred in the Xingtai area, with the M_S6.8 earthquake on March 8^th and the M_S7.2 earthquake on March 22^nd being the most severely damaged. The Xingtai earthquake resulted in over 8 000 deaths and the economic losses up to 1 billion yuan. The Xingtai earthquake has opened the scientific practice of earthquake prediction in China and is a milestone in the development of earthquake science in China.

Based on previous research results, there is a deep fault beneath the Xingtai earthquake area, which is the energy source of earthquakes, while there is a relatively independent fault system in the shallow part, which is generally recognized by scholars. However, the divergence regarding the seismogenic structure of the Xingtai earthquake mainly focuses on the unclear coupling relationship between the deep and shallow structural systems in the seismic area. The structural relationship between deep seismic faults and the shallow Xinhe Fault system requires new evidence to determine. In addition, previous scholars have proposed the viewpoint of “Newly generated Fault”, which can better explain the rupture characteristics of the Xingtai earthquake, but it still needs to be supported by the inversion results of the seismic rupture process based on the three-dimensional crustal fine structure. There are many small earthquakes in the Xingtai area. Deep structural information can be obtained using small earthquake data. Especially after 2009, the significant improvement in earthquake positioning accuracy in North China has made it possible to obtain new insights into deep structures. By locating small earthquakes, the spatial distribution and motion characteristics of faults are characterized, and seismic travel time tomography reveals the deep crustal velocity structure characteristics of the earthquake area. Combining previous geophysical exploration results, conducting deep and shallow structural analysis is of great significance for studying the spatial distribution, motion characteristics, and coupling relationship between deep and shallow structural systems of the fault system in the study area. The continuous aftershocks after the 1966 M_S7.2 earthquake in Xingtai, Hebei Province, have provided favorable conditions for conducting studies on deep tectonic structures in the region.

In this paper, based on the observation data of the Hebei seismostation from 1991 to 2021, we obtained the precise position results of 9 644 earthquakes in Xingtai and its neighboring area using the double-difference positioning method, and depicted the spatial patterns of deep ruptures. Based on the observation data of the North China Mobile Seismic Array from 2006 to 2008, 38 578 P-wave arrivals were used to obtain high-resolution travel time tomography results in the study area. This study shows that there are strong lateral heterogeneities in the velocity structure of the crust in the study area, with obvious low-velocity anomalies in the upper crust and high-velocity anomalies in the middle and lower crusts between the Xinhe Fault and the Yuanshi Fault, and the Xingtai earthquake is located at the junction of the high- and low-velocity anomalies, which has the medium conditions for accumulating large amounts of strain energy and is prone to rupture and stress release. The general trend of the dense zone of small earthquakes in the Xingtai earthquake area is relatively consistent with that of the eastern boundary of the high- and low-velocity anomalies. It is assumed that the deep and shallow fractures spreading along the eastern boundary of the high- and low-velocity bodies have been connected up and down and that the boundary of the anomalies is also a part where velocity changes are relatively strong and easily lead to seismic rupture; the results of various seismic and geological surveys have revealed that a deep major rupture that cuts through the entire crust exists beneath the Xingtai earthquake zone, with SE tendency and the upper breakpoint located near Dongwang, and the Xingtai earthquake prompted the deep and shallow pre-existing ruptures to connect from top to bottom.

The September 5, 2022, M6.8 Luding earthquake occurred along the southeastern segment of the Xianshuihe fault zone. Tectonics around the epicenter area is complicated and several faults had been recognized. Focal mechanisms of the main shock and inversions from earthquake data suggest that the earthquake occurred on a northwest-trending, steeply dipping strike-slip fault, which is consistent with the strike and slip of the Xianshuihe fault zone. We conducted a field investigation along the fault sections on both sides of the epicenter immediately after the earthquake. NW-trending fractures that were recognized as surface ruptures during the earthquake, and heavy landslides along the fault section between Ertaizi-Aiguocun village were observed during the field investigations. There are no surface ruptures developed along the fault sections north of the epicenter and south of Aiguocun village. Thus it can be concluded that there is a 15.5km-long surface rupture zone developed along the Moxi Fault(the section between Ertaizi and Aiguo village). The surface rupture zone trends northwest and shows a left-lateral strike slip, which is consistent with the strike and motion constrained by the focal mechanism. The coseismic displacements were measured to 20~30cm. Field observations, focal fault plane, distribution of the aftershocks, GNSS, and InSAR observation data suggest that the seismogenic structure associated with the M6.8 Luding earthquake is the Moxi Fault that belongs to the southeastern segment of the Xianshuihe fault zone. Slip along the segment south of the epicenter generated this earthquake, and also triggered slip along a northeast-trending fault and the northwestern section of the Moxi Fault in the epicenter. So, the M6.8 Luding earthquake is an event that is nucleated on the section south of the epicenter and then triggered an activity of the whole fault segment.

With the continuous increasing density of the seismic network and the improvement of the seismograph observation capability, the number of observed seismic events has increased dramatically and the location accuracy has been continuously improved. Therefore, obtaining fault geometry and its parameters from massive seismic data has become an essential method for seismogenic structure research. At present, in the research of obtaining faults and their parameters based on seismic data, there are two main methods of selecting data: One is to select seismic data empirically based on the understanding of fault structures and the spatial distribution of seismic data, and then fit the fault plane from these data. However, it depends on prior information, i.e. the knowledge of existing fault structures and the linear distribution of earthquakes, and it is difficult to process relatively poor linear trends. The other is based on the spatial clustering of seismic data, which adopts unsupervised clustering technology in machine learning to select data. This method avoids the dependence on experience and is more suitable for fault segment data obtained from massive seismic data. Fault parameters can be inversed by fault segment data to determine the fault structure and give its quantitative parameters. However, the current clustering technique for obtaining fault parameters has some limitations, such as the selection of the optimal parameters being difficult, data with different densities being dealt with by the same parameters, and poor method generality. In order to automatically identify faults and obtain fault parameters based on the spatial distribution of earthquakes, and avoid the aforementioned limitations, a new method based on the improved DBSCAN algorithm is presented in this study.
The method proposed in this study uses the k-average nearest neighbor method(K-ANN)and the mathematical expectation method to generate the candidate sets of eps and minPts threshold parameters, which are selected as optimal parameters based on the density hierarchy stability. Considering the spatial density differences of seismic events on different faults and the same fault, this study performs layer-by-layer density clustering from high density to low density. First, the above steps achieve the automatic selection of optimal parameters for clustering and identifying fault segments. Secondly, the fault parameters of the identified fault segments are calculated by the combination of the simulated annealing(SA)global search method and the local search method of Gaussian Newton(GN). Then, the adjacent similar fault segments are merged. Finally, the faults and their parameters are obtained.
The reliability of the automatic fault identification method was verified by synthetic data and the double-difference location catalog of Tangshan area, China. The following results were obtained: Ⅰ. The improved DBSCAN algorithm can automatically identify the fault segments, which is verified by the application of synthetic data and the double-difference location data of the Tangshan area. Ⅱ. Based on the double-difference location data of the Tangshan area, eight fault segments were identified using the improved DBSCAN algorithm. The specific names of the 8 faults are as follows: Douhe fault segment, Weishan-Fengnan fault segment, Luanxian-Laoting fault segment, Lulong fault segment, Xujialou-Wangxizhuang fault segment, Luanxian fault north segment, Leizhuang fault segment, and Chenguantun fault segment, and their strike and dip angle are 229.1°, 230.4°, 132.2°, 31.7°, 191.3°, 31°, 229.5°, 84.9°, and 51.6°, 88.4°, 89.3°, 88.6°, 88.4°, 88.2°, 73.8° and 85.4°, respectively. The parameters of the first five faults are mostly consistent with those of previous research results. The last three faults are the newly identified faults in this study based on the seismic catalog, and the parameters of two of them have been confirmed by previous research results or focal mechanism parameters on the faults.
In a word, the improved DBSCAN algorithm in this study can realize fault segment automatic identification, but there are still some problems that need to be improved urgently. In the follow-up research, we will continue to improve the automatic fault identification method and increase its ability of automatic fault identification so as to provide more accurate fault data for related research.

The Tienshan orogenic belt is one of the most active intracontinental orogenic belts in the world. Studying the deep crust-mantle structure in this area is of great significance for understanding the deep dynamics of the Tienshan orogen. The distribution of fixed seismic stations in the Tianshan orogenic belt is sparse. The low resolution of the existing tomographic results in the Tienshan orogenic belt has affected the in-depth understanding of the deep dynamics of the Tienshan orogenic belt. In this paper, the observation data of 52 mobile seismic stations in the Xinjiang Seismic Network and the 11 new seismic stations in the Tienshan area for one-year observations are used. The seismic ambient noise tomography method is used to obtain the Rayleigh surface wave velocity distribution image in the range of 10~50s beneath the Chinese Tienshan and its adjacent areas (41°~48° N, 79°~91° E). The joint inversion of surface wave and receiver function reveals the S-wave velocity structure of the crust and uppermost mantle and the crustal thickness below the station beneath the Chinese Tienshan area(41°~46° N, 79°~91° E). The use of observation data from mobile stations and new fixed seismic stations has improved the resolution of surface wave phase velocity imaging and S-wave velocity structure models in the study area.
The results show that there are many obvious low-velocity layers in the crust near the basin-bearing zone in the northern Tienshan Mountains and the southern Tienshan Mountains. There are significant differences in the structural characteristics and distribution range of the low-velocity zone in the northern margin and the southern margin. Combining previous research results on artificial seismic profiles, receiver function profiles, teleseismic tomography, and continental subduction simulation experiments, it is speculated that the subduction of the Tarim Basin and the Junggar Basin to the Tienshan orogenic belt mainly occurs in the middle of the Chinese Tienshan orogenic belt, and the subduction of the southern margin of the Tienshan Mountains is larger than that of the northern margin, and the subduction of the eastern crust is not obvious or in the early subduction stage. There are many low-velocity layers in the inner crust of the Tienshan orogenic belt, and most of them correspond to the strong uplifting areas that are currently occurring. The thickness of the crust below the Tienshan orogenic belt is between 55km and 63km. The thickness of the crust(about 63km)is the largest near the BLT seismic station in the Bazhou region of Xinjiang. The average crustal thickness of the Tarim Basin is about 45km, and that of the Junggar Basin is 47km. The S-wave velocity structure obtained in this study can provide a new deep basis for the study of the segmentation of the Tienshan orogenic belt and the difference of the basin-mountain coupling type.

The M_S7.0 Jiuzhaigou earthquake in Sichuan Province of 8 August 2017 triggered a large number of landslides. A comprehensive and objective panorama of these landslides is of great significance for understanding the mechanism, intensity, spatial pattern and law of these coseismic landslides, recovery and reconstruction of earthquake affected area, as well as prevention and mitigation of landslide hazard. In this paper, we use the trinity method of space, sky and earth to create a panorama of the landslides triggered by this event. There are 4 roads in the distribution area of the coseismic landslides. The Jinglinghai-Xiamo and Jiudaoguai-Jiuzhaitiantang road sections register the most serious coseismic landslides. The landslides are mainly of moderate-and small-scales, and also with a few large landslides and avalanches. A detailed visual interpretation of the coseismic landslides is performed in two areas of Wuhuahai(11.84km²) and Zharusi-Shangsizhai village(47.07km²), respectively. The results show the overall intensity of landsliding(1088 landslides, a total area 1.514km²) in the Wuhuahai area is much higher than those in the Zharusi-Shangsizhai village area(528 landslides, a total area 0.415km²). On the basis of a scene of post-earthquake Geoeye -1 satellite images, we delineate more than 4 800 coseismic landslides with a total occupation area 9.6km². The spatial pattern of these landslides is well related with the locations of the inferred seismogenic fault and aftershocks. Widely distributed earthquake-affected weakened slopes, residual loose materials staying at high-position slopes and in valleys have greater possibilities to fail again and generate new landslides or debris flows under the conditions of strong aftershocks or heavy rainfalls in the future. Geological hazard from these events will become one of the most serious problems in the recovery and reconstruction of the earthquake-affected area which should receive much attention.

Differently from the existing studies, about 210 days of the original seismic recordings since the Ludian M_S6.5 earthquake are collected from almost all of the nearby stations, and a velocity model and a non-linear location technique are specially selected, in order to relocate the sources of the earthquake sequences. What is more, the same model as used in determining the absolute locations is adopted as the DD technique is used to determine their relative locations. Then the strikes and dips of the seismogenic faults are estimated by linearly fitting the source locations, and finally a new explanation is proposed for the sequence formation. It is shown that the sequence may be divided into 4 sub-areas spatially, each of which corresponds to a nearly vertical fault with but different dimensions and striking azimuths, and that two of them are relatively larger and linked with each other, being the main faults of the sequence, and two others are relatively smaller and separated away from the main faults. These 4 faults, together with the local existing faults, form a radiating-shaped structure reflecting the complicated tectonics, which is very likely to be related with the density variation in lower crust.

The M_W6.6 Arketao earthquake,which occurred at 14:24:30 UTC 25 November 2016 was the largest earthquake to strike the sparsely inhabited Muji Basin of the Kongur extension system in the eastern Pamir since the M 7 1895 Tashkurgan earthquake.The preliminary field work,sentinel-1A radar interferometry,and relocated hypocenters of earthquake sequences show that the earthquake consists of at least two sub-events and ruptured at least 77km long of the active Muji dextral-slip fault,and the rupture from this right-lateral earthquake propagated mostly unilaterally to the east and up-dip.Tectonic surface rupture with dextral slip of up to 20cm was observed on two tens-meter long segments near the CENC epicenter and 32.6km to the east along the Muji Fault,the later was along a previously existing strand of the Holocene Muji fault scarps.Focal mechanisms are consistent with right-lateral motion along a plane striking 107°,dipping 76° to the south,with a rake of 174°.This plane is compatible with the observed tectonic surface rupture.More than 388 aftershocks were detected and located using a double-difference technique.The mainshock is relocated at the Muji Fault with a depth of 9.3km.The relocated hypocenters of the 2016 Arketao earthquake sequence showed a more than 85km long,less than 8km wide,and 5~13km deep,NWW trending streak of seismicity to the south of the Muji Fault.The focal mechanism and mapping of the surface rupture helped to document the south-dipping fault plane of the mainshock.The listric Muji Fault is outlined by the well-resolved south-dipping streak of seismicity.The 2016 Arketao M_W6.6 and 2015 Murghob M_W7.2 earthquakes highlight the importance role of strike-slip faulting in accommodating both east-west extensional and north-south compressional forces in the Pamir interior,and demonstrate that the present-day stress and deformation patterns in the northern Pamir plateau are dominant by east-west extension in the shallow upper crust.

Yingjiang area is located in the China-Burma border,the Sudian-Xima arc tectonic belt,which lies in the collision zone between the Indian and Eurasian plates.The Yingjiang earthquake occurring on May 30th,2014 is the only event above M_S6.0 in this region since seismicity can be recorded.In this study,we relocated the Yingjiang M_S5.6 and M_S6.1 earthquake sequences by using the double-difference method.The results show that two main shocks are located in the east of the Kachang-Dazhuzhai Fault,the northern segment of the Sudian-Xima Fault.Compared with the Yingjiang M_S5.6 earthquake,the Yingjiang M_S6.1 earthquake is nearer to the Kachang-Dazhuzhai Fault.The aftershocks of the two earthquakes are distributed along the strike direction of the Kachang-Dazhuzhai Fault (NNE).The rupture zone of the main shock of Yingjiang M_S6.1 earthquake extends northward approximately 5km.The aftershocks of two earthquakes are mainly located in the eastern side of the Kachang-Dazhuzhai Fault with a significant asymmetry along the fault,which differ from the characteristics of the aftershock distribution of the strike-slip earthquake.It may indicate that the Yingjiang earthquakes are conjugate rupture earthquakes.The non-double-couple components are relatively high in the moment tensor.We speculate that the Yingjiang earthquakes are related to the fractured zone caused by the long-term seismic activity and heat effect in the deep between Kachang-Dazhuzhai Fault and its neighboring secondary faults.Aftershock distribution of the Yingjiang M_S6.1 earthquake on the southern area crosses a secondary fault on the right of the Kachang-Dazhuzhai Fault,suggesting that the coseismic rupture of the secondary fault may be triggered by the dynamic stress of the main shock.

The studies of earthquake stress transfer and its influence on regional seismicity have found that earthquake occurrences are highly interactive and correlated rather than isolated and random in traditional point in recently years. A lot of phenomena in earthquake observations such as aftershock distribution, stress shadow, earthquake interaction and migration were well explained based on the theory of earthquake stress interaction. It is important that understanding the process of earthquake interaction could give an insight into the physical mechanism of earthquake cycle, and could help us assess the seismic hazard in future.It has long been recognized that regional stress accumulated by tectonic motion is released when earthquake occurs. When earthquakes occur, the accumulated stress does not vanish completely, but is redistributed through the process of stress transfer, and then the redistributed stress may trigger potential earthquakes. The increment of Coulomb failure stress loading in the certain regions may improve the seismic activities. By contrast, the decrement of Coulomb failure stress in the areas of stress shadow where the stress on faults may unload could lead to the decrement of seismic activities.On August 3, 2014, an M_S6.5 earthquake occurred in Zhaotong-Ludian region, Yunnan Province, China, killing and injuring hundreds of people. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settlement so as to avoid future disasters. Based on the elastic dislocation theory and multi-layered lithospheric model, we calculate the co-stress changes caused by the Zhaotong-Ludian earthquakes to discuss its influences on aftershock distribution and surrounding faults. It is shown that the Coulomb stress changes based on the rupture in the NNW direction can explain better the aftershock distribution. It indicates that the NNW direction may represent the real rupture. The aftershocks mainly distribute in the regions with increased stress along main rupture and west to the rupture. In other regions with increased stress, the distributions of aftershock are rare which may indicate the low tectonic stress accumulation in these regions. The stress accumulation and corresponding seismic hazard on the southern part of Zhaotong Fault, Qiaojia segment of Zemuhe-Xiaojiang Fault and northeastern part of Lianfeng Fault are further increased by the Zhaotong-Ludian earthquake. We should pay special attention to the southern part of Zhaotong Fault where seismic activity is very high in recently years and the increment of Coulomb failure stress in this area is more than 0.1bar(0.1bar is the threshold of earthquake triggering). In order to make a more objective and comprehensive discussion, we calculate the sensitivity of the parameters such as effective coefficient of friction, the calculated depth and multilayered crustal model.

Relocated earthquake catalogue by double difference(DD)algorithm, while significantly improving the location precision, suffers from the degeneration of catalogue completeness. One of the questions subject to discussion is whether such DD-relocated catalogue is really an improvement, or otherwise a drawback, when calculating the spatially dependent statistical parameters of seismicity. In such statistical calculation, catalogue completeness is one of the key issues determining the quality of the result. Investigating this problem, this paper carries out a case study on the August 3, 2014, Ludian, Yunnan, M_S6.5 earthquake sequence. Aftershocks within 40 days since the mainshock are analyzed using the routine catalogue provided by the national seismic network and a DD-relocated catalogue. The Gutenberg-Richter b-value, as well as its spatial distribution, from both catalogues, are calculated and compared. Results show that the degeneration of catalogue completeness of the DD-relocated catalogue depends on the clustering property of the earthquakes to much extent. Degeneration of catalogue completeness occurs at the margin of an earthquake cluster. Distribution of b-values based on the DD-catalogue provides clues to the source properties of the Ludian earthquake.

We integrated two-month phase data recorded by Yunnan Seismic Network, Zhaotong Seismic Network, Qiaojia Seismic Array and temporal stations deployed around the Ludian earthquake source region and relocated the aftershock sequence of the Ludian earthquake. The locations of 1 750 aftershocks were determined using double-difference location algorithm. The relocation result shows that the aftershock distribution has two predominant directions, to the southeast and southwest, and shows itself as an asymmetric conjugate shape. The lengths of the two aftershock strips are about 16km. The angle between the two strips is about 100°. Aftershock distribution shows that the seismogenic fault of the Ludian earthquake is a high-angle strike-slip fault. The mainshock is located at the middle at southwest of the two aftershock strips. Early aftershocks are distributed mainly along the NW-SE direction, perpendicular to the Zhaotong-Ludian Fault. The aftershocks located to the southwest of the mainshock may be triggered by the mainshock. According to the aftershock distribution and its relations with neighboring faults, focal mechanism of the mainshock, the long axis orientation of seismic intensity map, and distribution of landslides, we speculate that the seismogenic fault is the Baogunao-Xiaohe Fault. There are significant differences not only in seismic activity, deep velocity structure, but also the block movement direction and rate on both sides of the Baogunao-Xiaohe Fault. The northward expansion of aftershock activity may be blocked by the high-velocity anomaly zone located on the north side of the Baogunao-Xiaohe Fault.

Using the absolute relocation method, we relocate 1593 earthquakes from the seismic waveform data recorded by the dense temporary seismic array from August 2011 to August 2012, which is deployed in the southern segment of the north-south seismic belt. Then seismic traveltime tomography method is applied to obtain the 3-D crustal P-wave velocity structure in the western Yunnan area. The inversion results indicate that high-velocity anomaly extends to the middle crust from the surface in the Panzhihua area, with the high velocity on its west merging into a large-scale high-velocity block. We speculate that the high-velocity block plays a certain impediment to southward escaping of the Tibetan plateau materials, which caused the rapid uplift of the northern sub-block of the Sichuan-Yunnan active block. In the south of the Jinshajiang-Red River Fault, low-velocity anomaly exsists in the lower crust of the Tengchong block and Baoshan block. Since it is located in the subduction boundary between the Indian plate and Eurasian plate, we consider that the low-velocity anomaly may be caused by the high temperature derived from the upper mantle due to the Indian plate's eastward subduction. Jinshajiang-Red River Fault is the southern boundary of the Sichuan-Yunnan block. The high-velocity crustal structure and the weak seismic activities in the middle segment of the Red River Fault(Midu to Red River areas)imply that this region is locked currently. However, as the boundary between the blocks, it should be one of the key monitoring areas in the future.

The crustal and upper mantle S-wave velocity structures in Changbaishan volcanic region were obtained from surface wave tomography and teleseismic receiver function modeling.In Changbaishan region,the S-wave velocity shows a thin lithosphere,thick asthenosphere with relatively low S-wave velocity in upper mantle,which indicates the high temperature volcano system at least extends to asthenosphere. There exist distinct low velocity layers in the crust of the volcano area.Beneath WQD station near to the caldera,the low velocity layer at 8km depth is 20km thick with the lowest S-wave velocity about 2.2km/s.Beneath the EDO station located 50km north of Tianchi caldera,no obvious low velocity layer was detected in the crust.The average crustal V_P/V_S near the caldera is higher than those obtained in surrounding area.In the volcanic region,the thickness of crustal low velocity layer is greater and the lowest velocity is more obvious with the distance shorter to the caldera.It indicates the existence of the high temperature material or magma reservoir in the crust near the Tianchi caldera. The receiver functions and inversion result from different back azimuths at CBS permanent seismic station shows that the thickness of near surface low velocity layer and Moho depth change with directions. The near surface low velocity layer is obviously thicker in south direction.The Moho depth shows slightly rising in the direction of the caldera located.We consider that the special near surface velocity structure is the main cause of relatively lower prominent frequency of volcanic earthquake waveform recorded by CBS station.The slightly rising of Moho depth beneath Tianchi caldera indicates there is a material exchanging channel between magma reservoir and upper mantle.

By analyzing seismic data recorded by temporary seismic network deployed at Changbaishan Tianchi volcanic region in the summers of 2002,2003 and 2005,we find one type of events whose spectra appear to be special.The station-averaged spectrum of each event consists of a series of evenly-spaced narrow peaks,the amplitudes of peaks change gradually with the frequency,the shape of spectrum is quite similar to harmonic signal in time domain.We called such event as harmonic-spectral event.During the three summers,38 harmonic-spectral events were detected,and most of them occurred in seismic swarms.Analyses show that the harmonic spectral characteristics of these events are most likely associated with seismic sources,which might be caused by interaction with crack walls and pressure disturbances within magmatic or hydrothermal fluids filled inside of cracks at special excitation conditions,during the rock rupture processes.We suggest that the apparent increasing of seismicity and emergence of harmonic events in Changbaishan Tianchi volcanic region are associated with deep magmatic intrusion activities.