An Assessment of the Seismicity of the Bursa Region from a Temporary Seismic Network

A temporary earthquake station network of 11 seismological recorders was operated in the Bursa region, south of the Marmara Sea in the northwest of Turkey, which is located at the southern strand of the North Anatolian Fault Zone (NAFZ). We located 384 earthquakes out of a total of 582 recorded events that span the study area between 28.50–30.00°E longitudes and 39.75–40.75°N latitudes. The depth of most events was found to be less than 29 km, and the magnitude interval ranges were between 0.3 ≤ M_L ≤ 5.4, with RMS less than or equal to 0.2. Seismic activities were concentrated southeast of Uludag Mountain (UM), in the Kestel-Igdir area and along the Gemlik Fault (GF). In the study, we computed 10 focal mechanisms from temporary and permanents networks. The predominant feature of the computed focal mechanisms is the relatively widespread near horizontal northwest-southeast (NW–SE) T-axis orientation. These fault planes have been used to obtain the orientation and shape factor (R, magnitude stress ratio) of the principal stress tensors (σ₁, σ₂, σ₃). The resulting stress tensors reveal σ₁ closer to the vertical (oriented NE–SW) and σ₂, σ₃ horizontal with R = 0.5. These results confirm that Bursa and its vicinity could be defined by an extensional regime showing a primarily normal to oblique-slip motion character. It differs from what might be expected from the stress tensor inversion for the NAFZ. Different fault patterns related to structural heterogeneity from the north to the south in the study area caused a change in the stress regime from strike-slip to normal faulting.     

Stress Patterns in Northern Iraq and Surrounding Regions from Formal Stress Inversion of Earthquake Focal Mechanism Solutions

The collision zone between the Arabian and Eurasian plates is one of the most seismically active regions. Northern Iraq represents the northeastern part of the Arabian plate that has a suture zone with the Turkish and Iranian plates called the Bitlis–Zagros suture zone. The orientations of the principal stress axes can be estimated by the formal stress inversion of focal mechanism solutions. The waveform moment tensor inversion method was used to derive a focal mechanism solution of 65 earthquakes with magnitudes range from 3.5 to 5.66 in the study area. From focal mechanism solutions, the direction of slip and the orientations of the moment stress axes (P, N, and T) on the causative fault surface during an earthquake were determined. The dataset of the moment stress axes have been used to infer the regional principal stress axes (σ ₁, σ ₂, and σ ₃) by the formal stress inversion method. Two inversion methods, which are the new right dihedron and the rotational optimization methods, were used. The results show that six stress regime categories exist in the study area. However, the most common tectonic regimes are the strike-slip faulting (43.94 %), unspecified oblique faulting (27.27 %), and thrust faulting (13.64 %) regimes. In most cases, the strike-slip movement on the fault surfaces consists of left-lateral (sinistral) movement. The normal faulting is located in one small area and is due to a local tensional stress regime that develops in areas of strike-slip displacements as pull-apart basins. The directions of the horizontal stress axes show that the compressional stress regime at the Bitlis–Zagros suture zone has two directions. One is perpendicular to the suture zone near the Iraq–Iran border and the second is parallel in places as well as perpendicular in others to the suture zone near the Iraq–Turkey border. In addition, the principal stress axes in the Sinjar area near the Iraq–Syria border have a E–W direction. These results are compatible with the tectonic setting of the Arabian–Eurasian continental collision zone and the anticlockwise rotation of the Arabian plate that is evidently responsible for the strike-slip displacements on fault surfaces.     

Seismicity constraints on stress regimes along Sinai subplate boundaries

The relative movement between African, Arabian and Eurasian plates has significantly controlled the tectonic process of Sinai subplate region, although its kinematics and precise boundaries are still doubtful. The respective subplate bounded on both sides by the Aqaba-Dead Sea transform fault to the east and the Gulf of Suez, the only defined part, to the west. Seismicity parameters, moment magnitude relation and fault plane solutions were combined to determine the active tectonics along the aforementioned boundaries. Seven shallow seismogenic zones were defined by the heterogeneity in stress field orientations. Along the eastern boundary, the average fault plane solution obtained from the moment tensor summation (MTS) reveals a sinistral strike-slip faulting mechanism. The corresponding seismic strain rate tensor showed that the present tectonic stress producing earthquakes along the boundary is dominated by both NW-SE compression and NE-SW dilatation. Towards the north, the average focal mechanism showed a normal faulting mechanism of N185°E compression and an N94°E extension in the Carmel Fairi seismic zone. On the other hand, the active crustal deformation along the western boundary (Gulf of Suez region) showed a prevailing tensional stress regime of NE to ENE orientations; producing an average fault plane solution of normal faulting mechanism. The seismic strain rate tensor reveals a dominant stress regime of N58°E extension and N145°E compression in consistence with the general tectonic nature in northeastern Africa. Finally, the extensional to strike-slip stress regimes obtained in the present study emphasize that the deformation accommodated along the Sinai subplate boundaries are in consistence with the kinematics models along the plate boundaries representing the northern extremity part of the Red Sea region.     

Earthquake focal mechanisms and stress inversion in the Irpinia Region (southern Italy)

The goal of this study was to estimate the stress field acting in the Irpinia Region, an area of southern Italy that has been struck in the past by destructive earthquakes and that is now characterized by low to moderate seismicity. The dataset are records of 2,352 aftershocks following the last strong event: the 23 November 1980 earthquake (M 6.9). The earthquakes were recorded at seven seismic stations, on average, and have been located using a three-dimensional (3D) P-wave velocity model and a probabilistic, non-linear, global search technique. The use of a 3D velocity model yielded a more stable estimation of take-off angles, a crucial parameter for focal mechanism computation. The earthquake focal mechanisms were computed from the P-wave first-motion polarity data using the FPFIT algorithm. Fault plane solutions show mostly normal component faulting (pure normal fault and normal fault with a strike-slip component). Only some fault plane solutions show strike-slip and reverse faulting. The stress field is estimated using the method proposed by Michael (J Geophys Res 92:357–368, 1987a) by inverting selected focal mechanisms, and the results show that the Irpinia Region is subjected to a NE–SW extension with horizontal σ ₃ (plunge 0°, trend 230°) and subvertical σ ₁ (plunge 80°, trend 320°), in agreement with the results derived from other stress indicators.     

The October 11, 1999 and November 08, 2006 Beni Suef Earthquakes

Two felt moderate-sized earthquakes with local magnitudes of 4.9 on October 11, 1999 and 4.3 on November 08, 2006 occurred southeast of Beni Suef and Cairo cities. Being well recorded by the digital Egyptian National Seismic Network (ENSN) and some regional broadband stations, they provided us with a unique opportunity to study the tectonic process and present-day stress field acting on the northern part of the Eastern Desert of Egypt. In this study, we analyze the main shocks of these earthquakes and present 15 well recorded aftershocks (0.9 ≤ ML ≤ 3.3) which have small errors on both horizontal and vertical axes. The relocation analysis using the double difference algorithm clearly reveals a NW trending fault for the 1999 earthquake. The spatial distribution of its aftershocks indicates a propagation of rupture from the SW towards the NW along a fault length ~5 km dipping nearly ~40°SW. We also determined the focal mechanisms of the two main shocks by two methods (polarities and amplitudes ratios of P, S_V and S_H and regional waveform inversion). Our results indicate a normal faulting mechanism with a slight shear component for the first event, while pure normal faulting for the second one. The spatial distribution of the 1999 aftershocks sequence along with the retrieved focal mechanism confirmed the NW plane as the true fault plane. While for the 2006 event, the few aftershocks do not reveal any fault geometry; its focal mechanism indicated a pure normal fault nearly trending WNW-ESE that corresponds more likely to the extension of the 1999 earthquake fault. The seismicity distribution between the two earthquake sequences reveals a noticeable gap that may be a site of a future event. The NNE-SSW extensional stress indicated by the mechanisms of these events is in agreement with the regional stress field and the rifting of the northern Red Sea in its northern branches (Gulf of Suez and Gulf of Aqaba). The source parameters (seismic moment, moment magnitude, fault radius, stress drop and displacement across the fault) were also estimated and compared based on both the regional waveform inversion and the displacement spectra and interpreted in the context of the tectonic setting. The obtained results imply a reactivation of the pre-exiting NW-SE faults as a result of extensional deformation from the northern Red Sea-Gulf of Suez rifts.     

Modern tectonic stress field in the Chinese mainland inverted from focal mechanism solutions

IntroductionTheinversionapproachofregionalstressfielddevelopedinrecent10to20yearsprovidesausefultoolforstudyingthemeanstressinagivenregion(Angeller,1979;Ellsworth,1981,Xu,Ge,1984).Becauseitusesmultitudinousfaultsinsteadofsinglefault,itcanremovetheinhomogeneityoflocalmediumsoastorevealtheregionalstressinformation.Besides,thismethodproducesaRvalue,whichisdefinedby(O-2--q)/(q--q),andmaydescribestherelativemagnitUdeofintermediateprincipalstress,whereq,acand%arethemaximum,theintermediateandthemi…     

EARTHQUAKE FOCAL MECANISMS IN THE DALIANGSHAN SUB-BLOCK AND ADJACENT AREAS AND CHARACTERISTICS OF THE REGIONAL STRESS FIELD

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.     

滇西北地区现今构造应力状态及其与地震的关系

基于搜集到的2000—2018年滇西北地区M_S≥3.0地震的震源机制解,运用线性叠加反演法进行应力场反演,分析了滇西北地区现今的构造应力状态,进一步探讨了应力张量方差的时空分布与地震活动的关系。结果显示:① 滇西北地区地震的震源机制解类型复杂,主要以走滑型（46%）为主,正断型（27%）次之;② 研究区的构造应力场具有整体的一致性和局部的非均匀性,呈现为NNW向挤压和ENE向拉张的走滑型应力结构,说明研究区受到来自NNW向的水平挤压作用,对该地区上地壳运动和断裂活动起主导作用;③ 滇西北地区的应力张量方差大都小于0.2,除北部一些地区外,应力场基本处于均匀状态。根据应力张量方差随时间的变化和后续地震可知,中强地震大都发生在应力张量方差值低于0.2的情形,且主要发生在应力张量方差减小即震源机制解趋于一致的过程中。空间上这些地震基本都发生在应力张量方差的低值分布区及其边缘,这一结果有助于判定发震地点和了解区域应力集中增强过程。      

Stress tensor and focal mechanisms in the Dead Sea basin

We use the recorded seismicity, confined to the Dead Sea basin and its boundaries, by the Dead Sea Integrated Research (DESIRE) portable seismic network and the Israel and Jordan permanent seismic networks for studying the mechanisms of earthquakes in the Dead Sea basin. The observed seismicity in the Dead Sea basin is divided into nine regions according to the spatial distribution of the earthquakes and the known tectonic features. The large number of recording stations and the adequate station distribution allowed the reliable determinations of 494 earthquake focal mechanisms. For each region, based on the inversion of the observed polarities of the earthquakes, we determine the focal mechanisms and the associated stress tensor. For 159 earthquakes, out of the 494 focal mechanisms, we could determine compatible fault planes. On the eastern side, the focal mechanisms are mainly strike-slip mechanism with nodal planes in the N-S and E-W directions. The azimuths of the stress axes are well constrained presenting minimal variability in the inversion of the data, which is in agreement with the Eastern Boundary fault on the east side of the Dead Sea basin and what we had expected from the regional geodynamics. However, larger variabilities of the azimuthal and dip angles are observed on the western side of the basin. Due to the wider range of azimuths of the fault planes, we observe the switching of σ₁ and σ₂ or the switching of σ₂ and σ₃ as major horizontal stress directions. This observed switching of stress axes allows having dip-slip and normal mechanisms in a region that is dominated by strike-slip motion.     

南北地震带震源机制解与构造应力场特征

南北地震带作为中国大陆地应力场一级分区的边界,其构造应力场的研究对理解大陆强震机理、构造变形和地震应力的相互作用具有重要意义.本文收集南北地震带1970—2014年的震源机制解819条,按照全球应力图的分类标准对震源机制解进行分类,发现其空间分布特征与地质构造活动性质比较吻合.P轴水平投影指示了活动块体的运动方向,T轴水平投影在川滇块体及邻近地区空间差异特征最为突出,存在顺时针旋转的趋势.南北地震带的最大水平主应力方向具有明显的分区特征,北段为NE向走滑类型的应力状态,中段为NEE—EW—NWW向的逆冲类型,南段为SE—SSE—NS—NNE向走滑和正断类型,在川滇块体的北部和西边界应力状态为EW—SE—SSE的正断层类型,表明来自印度板块的NNE或NE向的水平挤压应力和青藏高原物质东向滑移沿大型走滑断裂带向SE向平移的复合作用控制了南北地震带的岩石圈应力场.川滇块体西边界正断层类型应力状态范围与高分辨率地震学观测得到的中下地壳低速带范围基本吻合,青藏高原向东扩张的塑性物质流与横向边界(丽江—小金河断裂带)的弱化易于应变能的释放,在局部地区使NS向拉张的正断层向EW向拉张正断层转变.反演得到的应力状态基本上与各种类型地震的破裂方式比较吻合,也进一步验证反演结果的可靠性,可为地球动力学过程的模拟和活动断层滑动性质的厘定提供参考.     

Relative location and source mechanism of inland earthquakes in Northern Egypt

We have relocated 259 inland earthquakes in northern Egypt using the double-difference hypocenter technique. Among this dataset we are able to determine source mechanisms of 200 events using P-wave polarities and amplitude ratios as well. The studied earthquakes have been recorded by the Egyptian National Seismological Network from October 1997 to December 2006 with local magnitude (M_L) varies between 1.5 and 5.0. Three earthquake dislocations have been defined namely: Dahshour, southeast Beni-Suef, and Cairo-Suez district. Earthquake activities tend to occur in clusters along the first dislocation (Dahshour) however, relatively scattered along the second (southeast Beni-Suef) and the third (Cairo-Suez district) dislocations. At Dahshour dislocation three distinct clusters have been distinguished. Source mechanism solutions of Dahshour earthquakes displayed normal faulting with a strike-slip component to strike-slip faulting with a minor normal dip-slip component. Most of earthquake focal mechanism orientations are varying from NE-SW to NW-SE. The fault plane solutions of Beni-Suef earthquakes represented normal faulting with a strike-slip component. If the NNW-SSE striking plane has been chosen to be the actual fault plane, some solutions would indicated normal faulting with a sinistral strike-slip motion and other reflect normal faulting with a dextral strike-slip component. The fault plane solutions of Cairo-Suez district earthquakes are compatible with E-W to ENE-WSW striking normal fault with a dextral strike-slip motion.     

A new technique for moment tensor inversion with applications to the 2008 Wenchuan M S8.0 earthquake sequence

A MS8.0 earthquake occurred in Wenchuan County, Sichuan Province, China, on May 12, 2008, and subsequently, numerous aftershocks followed. We obtained the moment tensor solutions and source time functions (STFs) for the Wenchuan earthquake and its seven larger aftershocks (MS5.0~6.0) by a new technique of moment tensor inversion using the broadband and long-period seismic waveform data from the Global Seismic Network (GSN). Firstly, the theoretical background and technical flow of the new technique was briefly introduced, and an aftershock of the Wenchuan earthquake sequence was employed to illustrate the real procedure for inverting the moment tensor; secondly, the moment tensor solutions and STFs of the eight events, including the main shock, were presented, and finally, the interpretation of the results was made. The agreement of our results with the GCMT results indicates the new approach is efficient and feasible. By using this approach, not only the moment tensor solution can be obtained but also the STF can be retrieved; the inverted STFs indicate that the source rupture process may be com-plicated even for the moderate earthquakes. The inverted focal mechanisms of the Wenchuan earthquake sequence show that the most of the aftershocks occurred in the main faults of the Longmenshan fault zone with predomi-nantly thrustingwith minor right-lateral strike-slip component, but some of them may have occurred in the sub-faults with strike-slip faulting in the vicinity of the main faults.     

The Bullas (Murcia, SE Spain) earthquake, 29 January 2005

The intensity and focal mechanism of the Bullas earthquake of 29 January 2005 have been determined. The intensity assessment has been carried out using the EMS-98 scale and plotted on an intensity map. The focal mechanisms for the main shock and largest aftershock have been estimated from moment tensor inversion of body waves. The mechanisms obtained show left-lateral strike-slip faulting and foci at shallow depths, 3–4 km. These results are compared with those of the Bullas 2002 and Mula 1999 earthquakes in the same region.This paper has not been submitted elsewhere in identical or similar form, nor will it be during the first three months after its submission to Journal of Seismology.     

Stress studies in the Central Alborz by inversion of earthquake focal mechanism data

The Alborz is one of the most important seismotectonic provinces in Iran. Furthermore, emplacement of Tehran as a mega city in southern part of the Alborz intensifies the seismic vulnerability in this area. In this study, the focal mechanism data from teleseismic and local seismic networks are used for stress tensor inversion. The earthquake focal mechanisms in the Central Alborz are divided into several groups with respect to their location. Two different stress tensor inversions, linear and nonlinear, are used for obtaining the principal stress orientations. The results show spatial variations in tectonic stress field, consistent with fault orientations and faulting mechanisms. The maximum compressional stress directions obtained in this study are confirmed by fast S-wave polarization axes reported by a previous shear wave splitting study. The maximum horizontal stress directions are also compared with GPS strain rates. The results indicate a partitioning of deformation in the area due to regional stresses along preexisting faults.     

Late Cenozoic stress states around the Bolu Basin along the North Anatolian Fault, NW Turkey

This study defines the Late Cenozoic stress regimes acting around the Bolu Basin along the North Anatolian Fault in northwestern Turkey. The inferred regional stress regime, obtained from the inversion of measured fault-slip vectors as well as focal mechanism solutions, is significant and induces the right-lateral displacement of the North Anatolian Fault. The field observations have also revealed extensional structures in and around the Bolu Basin. These extensional structures can be interpreted as either a local effect of the regional transtensional stress regime or as the result of the interaction of the fault geometries of the dextral Duzce Fault and the southern escarpment of the North Anatolian Fault, bordering the Bolu Basin in the north and in the south, respectively.The inversion of slip vectors measured on fault planes indicates that a strike-slip stress regime with consistent NW- and NE-trending σ_Hmax(σ₁) and σ_Hmin(σ₃) axes is dominant. Stress ratio (R) values provided by inversion of slip vectors measured on both major and minor faults and field observations show significant variations of principal stress magnitudes within the strike-slip stress regime resulting in older transpression to younger transtension. These two stress states, producing dextral displacement along NAF, are coaxial with a consistent NE-trending σ₃ axis. The earthquake focal mechanism inversions confirm that the transtensional stress regime has continued into recent times, having identical horizontal stress axis directions, characterized by NW and NE-trending σ₁ and σ₃ axes, respectively. A locally consistent NE-trending extensional, normal faulting regime is also seen in the Bolu Basin. The stress-tensor change within the strike-slip stress regime can be explained by variations in horizontal stress magnitudes that probably occurred in Quaternary times as a result of the westward extrusion of the Anatolian block.     

Spatio-temporal variation and focal mechanism of the Wenchuan MS8.0 earthquake sequence

Based on abundant aftershock sequence data of the Wenchuan M_S8.0 earthquake on May 12, 2008, we studied the spatio-temporal variation process and segmentation rupture characteristic. Dense aftershocks distribute along Longmenshan central fault zone of NE direction and form a narrow strip with the length of 325 km and the depth between several and 40 km. The depth profile (section of NW direction) vertical to the strike of aftershock zone (NE direction) shows anisomerous wedgy distribution characteristic of aftershock concentrated regions; it is related to the force form of the Longmenshan nappe tectonic belt. The stronger aftershocks could be divided into northern segment and southern segment apparently and the focal depths of strong aftershocks in the 50 km area between northern segment and southern segment are shallower. It seems like 'to be going to rupture' segment. We also study focal mechanisms and segmentation of strong aftershocks. The principal compressive stress azimuth of aftershock area is WNW direction and the faulting types of aftershocks at southern and northern segment have the same proportion. Because aftershocks distribute on different secondary faults, their focal mechanisms present complex local tectonic stress field. The faulting of seven strong earthquakes on the Longmenshan central fault is mainly characterized by thrust with the component of right-lateral strike-slip. Meantime six strong aftershocks on the Longmenshan back-range fault and Qingchuan fault present strike-slip faulting. At last we discuss the complex segmentation rupture mechanism of the Wenchuan earthquake.     

Seismological and GPS constraints on Sinai sub-plate motion along the Suez rift

This work presents new seismological and Global Positioning System (GPS) results aimed at understanding the nature and rate of strain associated with the opening of the Suez rift that separates the Sinai sub-plate from the African plate. The Sinai sub-plate has played a significant role in the tectonic evolution of the northern Red Sea and the Eastern Mediterranean region. Most small, moderate and large earthquakes occur within belts associated with the geologically documented borders of this sub-plate including the Dead Sea fault (DSF) system in the east, the Cyprian arc (CA) in the north, and the Suez rift (SR) to the southwest. The DSF and CA are well defined; however, the SR is only partially defined. Earthquake foci distribution supports the idea that the SR is seismically active, and this earthquake activity cannot be ignored throughout the kinematics evaluation of northern Red Sea region. The earthquake activity is relatively higher in the southern part of the SR and gradually decreases northward. The high seismicity is mainly attributed to the presence of the Sinai triple junction. Earthquake focal mechanisms in the SR are dominated by oblique normal faulting with left-lateral strike-slip components on NW trending fault planes consistent with regional kinematics. The extensional semi-principal stress axes derived from fault plane solutions are oriented NNE-SSW in good agreement with the current stress field obtained from borehole breakouts along the SR as well as results from GPS surveying. Recent survey-mode GPS observations provide evidence for coherent northerly motion of the Sinai sub-plate that varies between 2 and 5 mm/yr. Moreover, strain analysis indicates that the southern SR is dominated by extension while its northern segment is characterized by constriction, inconsistent with earthquake focal mechanisms and regional tectonic models. The overall northward motion of the Sinai sub-plate indicates that slab-pull rather than ridge-push is the dominant force controlling regional kinematics. Based on the low rate of extension and lack of oceanic crust, the SR can be considered an incipient plate boundary between the Sinai sub-plate and the Nubian plate.     

Fracture characteristics of the 1997 Jiashi, Xinjiang, China, earthquake swarm inferred from source spectra

Broadband P and S waves source spectra of 12 MS5.0 earthquakes of the 1997 Jiashi, Xinjiang, China, earthquake swarm recorded at 13 GDSN stations have been analyzed. Rupture size and static stress drop of these earthquakes have been estimated through measuring the corner frequency of the source spectra. Direction of rupture propagation of the earthquake faulting has also been inferred from the azimuthal variation of the corner frequency. The main results are as follows: ①The rupture size of MS6.0 strong earthquakes is in the range of 10～20 km, while that of MS=5.0～5.5 earthquakes is 6～10 km.② The static stress drop of the swarm earthquakes is rather low, being of the order of 0.1 MPa. This implies that the deformation release rate in the source region may be low. ③ Stress drop of the earthquakes appears to be proportional to their seismic moment, and also to be dependent on their focal mechanism. The stress drop of normal faulting earthquakes is usually lower than that of strike-slip type earthquakes. ④ For each MS6.0 earthquake there exists an apparent azimuthal variation of the corner frequencies. Azimuthally variation pattern of corner frequencies of different earthquakes shows that the source rupture pattern of the Jiashi earthquake swarm is complex and no uniform rupture expanding direction exists.     

A new technique for moment tensor inversion with applications to the 2008 Wenchuan <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>8.0 earthquake sequence

A M_S8.0 earthquake occurred in Wenchuan County, Sichuan Province, China, on May 12, 2008, and subsequently, numerous aftershocks followed. We obtained the moment tensor solutions and source time functions (STFs) for the Wenchuan earthquake and its seven larger aftershocks (M_S5.0～6.0) by a new technique of moment tensor inversion using the broadband and long-period seismic waveform data from the Global Seismic Network (GSN). Firstly, the theoretical background and technical flow of the new technique was briefly introduced, and an aftershock of the Wenchuan earthquake sequence was employed to illustrate the real procedure for inverting the moment tensor; secondly, the moment tensor solutions and STFs of the eight events, including the main shock, were presented, and finally, the interpretation of the results was made. The agreement of our results with the GCMT results indicates the new approach is efficient and feasible. By using this approach, not only the moment tensor solution can be obtained but also the STF can be retrieved; the inverted STFs indicate that the source rupture process may be complicated even for the moderate earthquakes. The inverted focal mechanisms of the Wenchuan earthquake sequence show that the most of the aftershocks occurred in the main faults of the Longmenshan fault zone with predominantly thrustingwith minor right-lateral strike-slip component, but some of them may have occurred in the subfaults with strike-slip faulting in the vicinity of the main faults.     

CHARACTERISTICS OF FOCAL MECHANISMS AND STRESS FIELD IN THE EASTERN BOUNDARY OF SICHUAN-YUNNAN BLOCK AND ITS ADJACENT AREA

This study is devoted to a systematic analysis of the stress state of the eastern boundary area of Sichuan-Yunnan block based on focal mechanisms of 319 earthquakes with magnitudes between M3.0 and M6.9, occurring from January 2009 to May 2018. We firstly determined the mechanism solutions of 234 earthquakes by the CAP method, using the broadband waveforms recorded by Chinese regional permanent networks, and collected 85 centroid moment tensor solutions from the GCMT. Then we investigated the regional stress regime through a damp linear inversion. Our results show that:1)the focal mechanisms of moderate earthquakes are regionally specific with three principal types of focal mechanisms:the strike-slip faulting type, the thrust faulting type and the normal faulting type. The strike-slip faulting type is significant in the eastern boundary area of Sichuan-Yunnan block along the Xianshuihe-Xiaojiang Fault, the Daliangshan Fault, and the Zhaotong-Lianfeng Fault. The thrust faulting type and the combined thrust/strike-slip faulting type are significant along the Mabian-Yanjin Fault, Ebian-Yanfeng Fault and the eastern section of Lianfeng Fault; 2)The most robust feature of the regional stress regime is that, the azimuth of principal compressive stress axis rotates clockwise from NWW to NW along the eastern boundary of Sichuan-Yunnan Block, and the clockwise rotation angle is about 50 degrees. Meanwhile, the angels between the principal compressive axis and the trend of eastern boundary of Sichuan-Yunnan Block remain unchanged, which implies a stable coefficient of fault friction in the eastern boundary fault zone of Sichuan-Yunnan Block. The movement of the upper crust in the southeastern Tibetan plateau is a relatively rigid clockwise rotation. On the whole, the Xianshuihe-Xiaojiang Fault is a small arc on the earth, and its Euler pole axis is at(21°N, 88°E). The Daliangshan Fault is surrounded by the Anninghe-Zemuhe Fault, which formed a closed diamond shape. When the Sichuan-Yunnan block rotates clockwise, the Daliangshan Fault locates in the outer of the arc, while the Anninghe-Zemuhe Fault is in the inward of the arc, and from the mechanical point of view, left-lateral sliding movement is more likely to occur on the Daliangshan Fault. Our results can be the evidence for the study on the "cut-off" function of the Daliangshan Fault based on the stress field background; 3)The regional stress regime of the eastern boundary faults zone of the Sichuan-Yunnan Block is the same as the south section of the Dalianshan Fault, and the focal mechanism results also reveal that the Dalianshan Fault is keeping left-lateral strike-slip. There may be the same tectonic stress field that controls the earthquake activities in the southern section of Daliangshan Fault and Zhaotong-Lianfeng Fault. The regional stress regime of Zhaodong-Lianfeng Fault is also the same with the Sichuan-Yunnan Block, which implies that the control effect of the SE movement of the Sichuan-Yunnan block may extend to Weining.