Analyzing the focal mechanisms of main shocks and aftershocks of strong earthquakes is crucial for constraining the seismogenic fault geometry and their rupture evolution. In this study, we investigated the 2023 Jishishan Ms6.2 earthquake sequence by integrating near-field waveforms from a multiple-network seismic array. Using double-difference relocation and focal mechanism inversion, we determined precise hypocenter for 249 events (Ms1.0 similar to Ms6.2) and obtained 89 high-quality focal mechanism solutions for events ranging from Ms1.5 to 6.2. Kagan angle analyses were further used to quantify rupture direction variations. Our results show that the sequence is dominated by thrust mechanisms (64.19%). The main shock is a NW-striking thrust event (Mw 5.99; depth 10.5 km) with nodal plane parameters of strike 295 degrees, dip 52 degrees, and rake 50 degrees, indicating a source region under NE-SE trending compressional stress. Combined with aftershock relocation results, the seismogenic fault is interpreted as a NE -dipping thrust structure with a along-strike segmentation, distinct from the known Northern Laji Shan Fault - consistent with field observations. Temporal evolution analysis reveals those focal mechanisms near the main shock and in the Liuji Township area dominated by a pattern of thrust-dominated, followed by mixed-type faulting, and a return to thrust-dominated faulting. In contrast, the Datun area remained consistently thrust-dominated throughout the sequence. Kagan angle analysis confirms significant rupture heterogeneity between the main shock and aftershocks, with marked differences in rupture direction consistency between shallow and deep sections beneath the Datun area. This study demonstrates the utility of dense seismic networks in detailed focal mechanism research and provides new insights into the rupture processes of strong continental earthquakes.

Since the filling of the Baihetan Water Reservoir (BWR) in April 2021, a significant number of earthquakes have occurred, with varying levels of seismicity in different regions. Therefore, studying the seismogenic environment and seismic mechanism is crucial for understanding the variability of earthquakes in the BWR area. We utilized continuous waveform data recorded over three years following the filling of BWR, collected from 86 stations covering the BWR area and its surrounding areas, and measured 1545 high-quality Rayleigh-wave phase velocity dispersion curves. We then obtained the 3D S-wave velocity and azimuthal anisotropy models through direct inversion of the dispersion data. The new results provide insights into the seismogenic environment and earthquake mechanism along with their relationship with the fluids in the BWR area. The low VS anomaly in the shallow layers of the Jinsha River and its vicinity is attributed to the influence of lithology and fluid infiltration in the study area. The high VS anomaly along the Zhaotong-Ludian fault zone indicates that it may be the main channel for the intrusion of Emeishan basalt into the upper crust. The moderate and strong earthquakes, as well as the earthquakes in the source area of the Ms 6.5 Ludian earthquake, mostly occur along the boundaries between the high and low VS anomaly. This observation indicates the presence of the previously undetected faults on the northwest side of the Zhaotong-Ludian fault. Earthquakes in a Y-shaped distribution near Qiaojia predominantly occur in a low VS zone, suggesting that the seismicity in the BWR region is markedly influenced by fluid infiltration. The spatiotemporal variability of earthquakes may be influenced by regional lithology and tectonic conditions.

利用新疆、西藏区域台网记录的P波和S波到时数据,对2009年1月至2021年9月期间的青藏高原西北缘的地震事件开展了精定位;利用新疆区域台网的地震波形数据,对2021年9月4日皮山Ms5.4地震和9月5日叶城Ms5.4双震序列中的12次Ms≥3.5地震事件开展了震源机制解反演;综合精定位与震源机制解结果,分析了皮山Ms5.4和叶城Ms5.4双震序列的震源特征;结合本文作者已取得的于田地区强震活动性研究结果,讨论了青藏高原西北缘两个强震区内(皮山—叶城地区与于田地区)的强震相关性.研究结果表明,(1)皮山Ms5.4和叶城Ms5.4双震序列的发震断层为NW向的泽普断裂带,两次Ms5.4地震的质心深度为17 km左右.(2)皮山Ms5.4和叶城Ms5.4地震之间伴有NE-SW向的次级隐伏拉张断层破裂.(3)2015年皮山Ms6.5地震序列呈现出NW向的单侧破裂,其余震区与此次皮山Ms5.4和叶城Ms5.4双震序列余震区没有重叠,两者属于不同的地震序列.(4)近年来,青藏高原西北缘"盆山交接地带"上的于田地区和皮山—叶城地区的强震活动性特征、发震构造模式及局部的构造动力源均不同.以东经80.为界,在其东侧的于田地区,大部分地震沿着NE向大型断裂带分布,少数地震沿着近乎NS向或NNE-SSW向的条带展布;断层错动方式以正断型或走滑型为主.在其西侧皮山—叶城地区,大部分地震活动沿着NW向分布,少量地震沿着NNE向的条带展布;断层错动方式以逆冲错动为主.

The Menyuan area of Qinghai Province, located in the northeast margin of the QinghaiTibet Plateau, has frequent seismicity. Since 1986, three strong earthquakes with M-s >6 have occurred in the area. Studying the seismicity characteristics, seismogenic structure and seismogenic environment in this area can provide important information for the analysis of seismogenic mechanism and the assessment of seismic risk. Based on the earthquake catalogue and seismic phase data provided by the China earthquake Networks Center, we use the VP/Vs model consistencyconstrained double difference seismic tomography method to obtain the precise seismic locations since 2009 and the high-resolution 3D V-P, Vs and V-p/V-5 models in the Menyuan area. The focal mechanism solutions of the Menyuan M(s)6. 9 earthquake and aftershocks with M-s>4. 5 were calculated using the "Cut and Paste" method. In addition, the focal mechanism solutions of the historical moderate-strong earthquakes in Menyuan area were collected. The results show that the aftershock sequences of the 2013 Menyuan M(s)5. 3 earthquake and the 2016 Menyuan M(s)6. 4 earthquake are distributed along the Northern Lenglongling fault, the aftershock sequence of the 2022 Menyuan M(s)6. 9 earthquake is distributed along the Tuolaishan fault and the Lenglongling fault, and their distribution directions are consistent with the trends of nearby faults. The focal mechanism solutions of the 1986 Menyuan M(s)6. 4 earthquake, the 2013 Menyuan M(s)5. 3 earthquake and the 2016 Menyuan M(s)6. 4 earthquake are thrust type, while the focal mechanism solutions of the 2022 Menyuan M(s)6. 9 earthquake and its aftershocks are strike-slip type. Combined with the seismic locations, the seismogenic fault of Menyuan M(s)6. 9 earthquake is not the same as the previous three moderate-strong earthquakes. There is a corresponding relationship between the distribution of faults or the location of moderate-strong earthquakes and the 3D velocity structure in the study area. The media properties on both sides of the Tuolaishan fault and the Lenglongling fault are obviously different. The stress is accumulated at the edge of the rigid medium, resulting in frequent occurrence of moderate-strong earthquakes along the faults at the edge of the abnormal bodies in Menyuan area.

A moment magnitude (M-w) 5.0 earthquake hit Qiaojia, Yunnan, China on 18 May 2020. Its hypocentre is only approximately 20 km away from the Baihetan reservoir, the second largest hydropower station in China. The Baihetan Reservoir is located at the junction of multiple fault zones on the eastern boundary of the Sichuan-Yunnan rhombic block, an area with high background seismic activity. The Baihetan Reservoir was planned to be impounded in April 2021 and the M-w 5.0 earthquake occurred during its water-retaining. Thus, it is critical to investigate the seismogenesis of the Qiaojia M-w 5.0 main shock and evaluate the risk of inducing earthquakes near the Baihetan Reservoir after impoundment. In this study, we built a complete and accurate earthquake catalogue to analyse seismicity in the reservoir area before and after the M-w 5.0 Qiaojia earthquake. We adopted a machine learning-based seismic phase picker, PhaseNet, to automatically detect seismic picks from continuous raw seismic data. Seismic phase picks were associated and located using sequential earthquake association and location methods, including REAL, VELEST and hypoDD. We eventually obtained high-precision locations of 1640 earthquakes by the hypoDD. The distribution of earthquake locations indicates that a concealed fault nearly vertical to the surface accommodated the M-w 5.0 Qiaojia main shock. The majority of its aftershocks is located within a narrow depth range of 8-13 km, indicating that the stresses in the hypocentral area were concentrated near the hypocentre of the M-w 5.0 earthquake. Along with focal mechanism solutions, we suggested that the M-w 5.0 Qiaojia earthquake is more likely a tectonic earthquake. However, we cannot exclude the possibility that earthquakes could be induced after the impoundment of Baihetan Reservoir, because the identified concealed fault is located in the middle of many large fault zones and only 20 km away from the Baihetan Reservoir.

The 17 June 2019 Ms 6.0 Changning earthquake occurred on the southern margin of the Sichuan basin in China, which breaks the historical record for the largest earthquake in the Sichuan basin. Based on the abundant local seismic, strong motion, and Interferometric Synthetic Aperture Radar line of sight displacement data, we investigate the detailed source characteristics of this earthquake sequence. We determine focal mechanisms of 68 ML = 2:0 aftershocks with P wave first-motion polarities and S/P amplitude ratios. The triangle diagram of focal mechanisms shows that 82% of the aftershocks have thrust faulting mechanisms. The spatial distribution of aftershocks together with the determined focal mechanisms indicates that this earthquake sequence was mainly controlled by the southeast-northwest-extended faults. In addition, the revealed diversity of aftershock focal mechanisms implies that some small subsidiary faults with different geometries and motion features have likely been ruptured. The kinematic finite-fault joint inversion results reveal a complex rupture process of the mainshock on two fault segments with different geometries. The rupture initiated on the southeastern fault segment with a gentle dip angle and then jumped to the vertical northwestern fault segment. The main rupture length and duration are approximately 11 km and 8 s, respectively. The released total scalar seismic moment during the rupture process is 4:6x1017 N center dot m, corresponding to a moment magnitude of Mw 5.7. Our results suggest that the nucleation and the rupture initiation and propagation of the 2019 Ms 6.0 Changning earthquake sequence were likely controlled by the intrinsic structure and stress heterogeneities of the involved seismogenic faults, as well as the variation in pore-fluid pressure caused by the long-term water injection in the Changning salt mining area and adjacent areas.

On 25 June 2020, a Mw 6.3 earthquake struck Yutian county. The earthquake nucleated on a normal fault in the northwestern Tibetan Plateau. Through jointly inverting local seismic and teleseismic waveform data as well as Interfemmetric Synthetic Aperture Radar (InSAR) Line-of-Sight (LOS) displacement data, the spatiotemporal rupture properties of this earthquake were resolved to investigate and better understand regional crustal extension mechanisms. Inversion results show that this event ruptured along one main asperity concentrated within a depth range of 4.1-13.3 km with a maximum slip of similar to 0.9 m. The rupture front propagated initially around the hypocenter and then unilaterally toward the north. The whole process lasted approximately 8 s and released a total scalar seismic moment of 3.2 x 10(18) Nm (Mw 6.3). The energy-based average stress drop was estimated in the range of 1.9-3.0 MPa. The radiation efficiency was estimated to be 10.0-15.8%, which is relatively low, demonstrating that most of the accumulated strain energy dissipated during rupturing. We suggest that the earthquake was possibly related to the nearby Cenozoic volcano in consideration of the dissipated rupture of the source and the west-dipping causative fault located on the western edge of the volcano. Both the westward motion of the western Kunlun block and the eastward motion of the Kunlun-Qaidam block were responsible for the EW-trending crustal extension in the Yutian district. We supposed that the 2020 normal faulting earthquake was primarily attributed to the different velocities of the Kunlun-Qaidam block moving eastward. Additionally, according to an analysis of coseismic Coulomb stress interactions, the previous 2008 Mw 7.1, 2012 Mw 6.2, and 2014 Mw 6.9 Yutian earthquakes contribute small Coulomb stress disturbance on the hypocenter of the 2020 Mw 6.3 Yutian earthquake which suggest that previous Yutian earthquakes have a limited effect on triggering the 2020 event.

On 8 August 2017, an M-s 6.6 earthquake occurred in the northeastern Tien Shan orogenic belt. To reveal the source characteristics of this earthquake completely, the teleseismic and near-field seismic waveform data were collected as well as the coseismic Interferometric Synthetic Aperture Radar displacement data, and the methods of the backprojection and the finite-fault joint inversion were adopted. The backprojection of the teleseismic recordings indicates a unilateral rupture propagating 15 km westward. Two stages of the rupture were recognized from the backprojection results. in the first similar to 5 s, the rupture took place near the hypocenter, with an accelerating energy release but a small rupture velocity; then the rupture extended to the west, with a decelerating energy release but a relatively fast rupture velocity. The joint inversion of the multiple datasets shows a major slip asperity of about 24 km x 18 km. The asperity extended mainly to the west, with a duration of approximately 10 s. The average rupture velocity over the asperity was estimated to be approximately 2.0 km/s, which is close to that 1. 9 km/s estimated by the backprojection. It is interesting that the high-frequency sources were aligned almost on the margin of the slip asperity. Moreover, the occurrence of the earthquake sequence is found to relate with the low-V-P/V-S zone, implying a tectonic property, which controls the nucleation and rupture of earthquakes.

In order to analyze the relationship between the impoundment of the Zipingpu Reservoir and occurrence of the 2008 Wenchuan Ms8.0 earthquake,the diffused pore pressure on the hypocentral zone of the Wenchuan earthquake is gotten by the quantitative calculation in the pore-elastic layer medium.Based on it,stress variances before the Wenchuan earthquake on the hypocentral zone are also simulated.The results show that Coulomb stress on the hypocentral zone is about-0.001 MPa,indicating a very low relation between the reservoir impoundment and the earthquake occurrence.

水库地震是由于水库蓄水或者水位变化,使得库区附近地震的次数和震级明显增高的现象.1939年米德湖4.6级地震发生后,水库地震的问题被首次提出.与构造地震相比,水库地震通常具有震源浅、震中烈度高、易于发生滑坡崩塌等次生灾害的特点.事实上,一些发生在水库区的地震常常与库区的构造活动相关,属于构造型水库地震.如何区分这类构造型水库地震和构造地震是当前面临和亟待解决的问题.水库地震发生的机理是解决该问题的关键.本文围绕水库蓄水后引起的库区应力变化和地震发生机理开展了研究.实现了孔弹耦合介质中解析解形式的扩散孔隙压的求解;获取了作为典型水库的三峡水库、紫坪铺水库库区的主要断层面上的孔隙压变化量、正应力变化量、剪应力变化量和库仑应力变化量;分析了引起库区应力变化的主要原因及蓄水与库区地震活动的关系;通过比较不同水库的计算结果,初步尝试性地提炼和总结了水库地震活动的发生机理.

Combination of velocity, Poisson's ratio, and attenuation structure is a powerful tool to investigate the characteristics of media, and it is especially useful to infer the lithology, porosity, water content of rocks, fluid infiltration depth, and distribution in the reservoir area. Using data from 4967 earthquakes recorded by a high-density seismic network, we carried out high-resolution 3D V-P, V-P/P-S Q(P), and Q(S) tomography research in the Three Gorges Reservoir area of China, where the world's largest high-capacity hydroelectric power station is located. We found two regions with significantly low V-P, high V-P/V-S, and low Q(P) and Q(S); one is the region between the Shuitianba fault and the western edge of the Huangling dome, and the other is the Badong region on the upstream Yangtze River, where more than 2000 earthquakes occurred in four months after the reservoir was first impounded. We infer that there is significant fluid infiltration in the two regions. The interior area of the Zigui basin is shown as a low-V-P region at depths of 0-6 km, which is consistent with the depth to crystalline basement. The Three Gorges dam area is a region with moderately high V-P, low V-P/V-S, and prominently high Q(P) and Q(S), demonstrating no obvious water infiltration around the dam area. Combining present results with previous studies, we draw a conclusion that the distance away from main rivers, lithologies, and the presence of faults are the main factors contributing to the fluid infiltration distribution beneath the reservoir region.

Crustal earthquake focal mechanisms are investigated in the southeastern margin of the Tibetan Plateau, where the Tibetan Plateau and stable South China Block merge. An updated database of focal mechanisms has been compiled by selecting 132 Global Centroid Moment Tensor solutions and by adding the 173 new solutions (3.5 ≤ Ms ≤ 7.4) estimated by waveform inversion in this study. A total of 305 mechanisms are included in this database. These solutions show regionally specific distributions with dominant strike-slip faulting and some normal and reverse faulting. Focal mechanism solutions have also been inverted for the stress tensor orientation to obtain the principal stress axes over the study region. Results show that the horizontal maximum principal σ 1 axes rotate clockwise with a wider range than the geodetically measured surface motion in the east, which is not limited to the Xianshuihe–Xiaojiang fault, but has some overlap with the Zhaotong–Lianfeng fault. Localized normal faulting stress regimes are observed in the Jinshajiang–Litang fault areas and the Baoshan sub-block. The minimum principal axes are oriented with a gradually changing trend from north–south to northwest–southeast, from north to south, indicating diverse compression stress patterns. Significant changes in the crustal stress field after the Wenchuan earthquake are preliminarily observed in the Baoshan sub-block where orientations of two principal axes have changed, and in the Jinggu–Ximeng sub-block areas where the strike-slip faulting stress pattern has transformed to normal faulting.

No M-S>5 earthquakes have been observed in the northwestern boundary of the Sichuan-Yunnan Block, the Jinshajiang fault and the Deqen-Zhongdian-Daju fault systems, since 1976. The M(S)5.1 earthquake and the M(S)5.9 earthquake, which sequentially took place in Shangri-La Deqen, Yunnan province-Derong Sichuan province on April 28th, 2013 and April 31th, 2013, supplement apparent gaps of the focal mechanisms spatial distributions image of Sichuan-Yunnan Block boundary fault systems and also provide important date for further researches in terms of both faulting activities and tectonic stress field characteristics. Using digitally broadband seismic records, we determined focal mechanisms of 10 moderate earthquakes (M-S>4) in this M(S)5.9 and M(S)5. 1 earthquake sequence. The focal mechanism solutions then supply data for the analyses of seismogenic fault activities and investigations of focal mechanisms and stress field characteristics in the Sichuan-Yunnan region. Regional broadband seismic records of China Seismograph Network were used to determine the focal mechanisms and focal depths of the M(S)5. 9 earthquake, M(S)5. 1 earthquake and other M-S>4 events by the whole wave inversion method called the Cut-and-Paste method. The Focal Mechanism Stress Inversion method was introduced to estimate the local crust stress field of the source region. Combining our researches of focal mechanisms of moderate and large earthquakes in recent years with the geologic structures, aftershock distributions, intensity features and geodynamic backgrounds, we analyzed the characteristics of focal mechanisms and stress field of this earthquake sequence.The following results are obtained from focal mechanisms and stress field solutions of the M(S)5. 9, M(S)5. 1 earthquake sequence: (1) The M(S)5. 9 earthquake and M(S)5. 1 earthquake are normal faulting events, so are other eight Ms>4 aftershocks. Results show seismogenic fault is a northwest-west striking active fault. The fault solutions of the M(S)5. 9 and M(S)5. 1 earthquakes are strike 299, dip 53, rake 73 and strike 290, dip 55, rake 72 respectively. (2) Spatial distributions of T-axes and P-axes of the M(S)5. 9 and M(S)5. 1 events are exhibited similar, whereas, the orientations of these two axes of the M(S)5. 9 aftershocks deviate from the ones of the main event in the following two days and relocate to the ones that are similar with the main-event later, indicating an obvious stress readjustment process in the seismic source region. (3) The maximum principal axes (maximum compression) are perpendicularly oriented in northwest-southeast and the minimum principal axes (minimum compression) are horizontally in north-northeast-south-southwest, leading to an obvious normal-faulting active region. These distributions are consistent with the trending of the surface maximum principal tensile stress field derived from the geodetic observations. (4) Combined our previous focal mechanisms determination, it can be concluded that the Shangri-La sub-block and the Baoshan sub-block are two interior regions in the Sichuan-Yunnan Block which are obviously under the normal-faulting regimes. However, the fault nodal planes and P-axes and T-axes strike differently in each boundary fault system. The orientations of nodal planes exhibit a clockwise rotation, with a gradual change from trending east-west in the northern section of the Jinshajiang fault and the middle section of the Litang fault to trending northwest-west in the Deqen-Zhongdian-Daju fault then to trending nearly north-south in the southwestern section of the Lijiang-Xiaojinhe fault and the northern margin of the Baoshan sub-block. The T-axes are correspondingly oriented in north-south or north-northwest-south-southeast further to northeast-southwest then to east-west or northeast-east-southwest-west, suggesting that stress states in these boundary faults are not consistent under the normal faulting regimes. In sum, this M(S)5.9, M(S)5.1 earthquake sequence is an apparent normal-faulting sequence. The stress field of seismic source experienced distinct stress readjustments during the development process of the whole sequence. Solutions of focal mechanisms of the Sichuan-Yunnan region in recent years show that the Shangri-La sub-block and the Baoshan sub-block are primarily under normal faulting stress patterns which are dominated by the horizontally tensile stress field. While the changes of the nodal planes and T-axes orientations in each boundary fault system indicate the existed influences of the sub-blocks interactions, terrain diversities under the background tectonic stress field.

利用CAP(Cut and paste)方法获取了川滇块体及周边区域2007年8月至2013年4月75次3.5级以上中等地震的震源机制解,结合哈佛大学历史地震震源机制解,分析了震源机制解和震源深度的空间分布特征,并探讨了其构造动力学背景.结果表明:1)川滇块体各不同断裂带、块体内部各次级块体之间、块体内外表现出不同的震源机制解空间分布特征,揭示出位于青藏高原东南缘的川滇块体及周边地区应力场的非均匀性;2)研究区各主要断裂带所反映的与构造背景作用一致的震源机制分布特征表明,川滇块体及周边近期断层破裂方式主要受到各个断裂带的构造活动以及次级块体之间相互作用的控制;3)丽江-小金河断裂带上特殊的震源机制特征和发震应力轴的分布特征,进一步证实了丽江-小金河断裂带对高原逃逸物质的抵挡和屏蔽的作用;4)震源深度分布特征表明,川滇块体及周边地震震源深度主要分布于15km的上地壳,优势分布在5～ 15km的范围,揭示出研究区的地壳脆性孕震层位于5～ 15km的上地壳.

<正>新丰江水库1959年截流蓄水,蓄水后第一个月即记录到地震活动。1962年3月19日,在水库蓄水两年半之后,库区发生6.1级强震。随后,库区大坝附近地震活动不断。60年来,共发生有记录的地震十万余次,其中4.0级以上地震58次。新丰江水库地震的发生,开始了中国大陆水库地震的监测和研究。多

<正>引言2012年6月15、24、30日,中国大陆西部新疆维吾尔自治区巴音郭楞蒙古自治州轮台县、云南省丽江市宁蒗彝族自治县和四川省凉山彝族自治州盐源县交界、新疆维吾尔自治区伊犁哈萨克自治州新源县和巴音郭楞蒙古自治州和静县交界,分别发生了MS5.4、MS5.7、MS6.6中强地震。地震发生后,我们采用CAP方法[1～5],使用中国地震局地球物