Three-dimensional characteristics of focal fault of 1995 M_S=6.5 Wuding earthquake occurring in Yunnan Province

By means of the hypocenter distribution and focal mechanism of Wuding Ms=6.5 earthquake sequence occurred in1995. the space orientation and activity characteristics of focal fault of Wuding earthquake have been studied fromthe three-dimensional space-time process. The results indicate that the focal fault of Wuding earthquake is a subsurface, NWW-trending, upright and right-lateral strike slip fault which is consistent with the intensity distributionin foe meizoseisfnal region. Although the large-scale NS-trending Tanglang-Yimen active fault passes through theearthquake region. it is irrelevant to the Ms=6.5 Wuding main earthquake. Since the relationship between thestrong earthquake and the shallow geological active fault can not be determined, the crustal deep structure shouldbe studied. The method proposed in the paper can be used to distinguish the focal fault in the deep crust.

Focal Fault of the 1999 Datong M_S5.6 Earthquake in Shanxi Province

Several earthquakes with M S≥5.0 occurred in the Datong seismic region in 1989, 1991 and 1999. The precise focus location of the earthquake sequence was made by the records of the remote sensing seismic station network in Datong. Using that data together with macro-intensity distribution and focal mechanism solutions, we analyze the difference among three subsequences. The results show that the focal fault of the 1999 M S5.6 earthquake was a NWW-trending left-lateral strike-slip fault. It is 16 km long and 12 km wide. It developed at the depth of 5 km and is nearly vertical in dip. The two previous earthquake subsequences, however, were generated by activity along NNE-trending right-lateral strike-slip fault. It can be found that the rupture direction of the 1999 earthquake has changed. It is generally found that a rupture zone has more than two directions and has different strength along these two directions. The complicate degree of focal circumstance is related to the type of earthquake sequences. There is the NE-trending Dawangcun fault and the NW-trending Tuanbu fault in the seismic region, but no proof indicates a connection between focal faults and these two tectonic faults. The feature that focal faults of three subsequences are strike-slip is different from that of the two tectonic faults. It is suggested that the 1999 earthquake subsequence was possibly generated by a new rupture.

Focal Fault and Stress Field of the Yao'an M 6.5 Earthquake on January 15, 2001

Using the 78 focal mechanism solutions of the fore shocks, main shock and after shocks of the earthquake sequence for the Yao’an earthquake, the characteristics of the focal faults and stress field for the earthquake sequence are analyzed. The results show that the main rupture plane of the Yao’an earthquake sequence is a tectonic fault with N50°W strike and steep dip and all the main shock, the fore shocks and the vast majority of after shocks occurred on the main rupture plane. A tectonic fracture with NNE-NE strike also participated in development process of the sequence. The focal stress field of the sequence dominated by principal compressional stress with nearly horizontal orientation SSE is consistent with the regional tectonic stress field. In the sequence development, the stress field in the focal region was complex with multi-azimuths and multi-action models and the focal rupture showed complex features with multi-directions and multi-patterns.

EARTHQUAKE FOCAL MECHANISM AND ITS TECTONIC SIGNIFICANCE ALONG THE TWO SIDES OF THE RED RIVER FAULT ZONE

The Red River Fault Zone is a gigantic slide-slip fault zone extending up to 1000km from Tibet to SouthChina Sea. It has been divided into the north, central and south segments according to the difference of thegeometry, kinetics, and seismicity on the land, but according to the contacted relationship between the old pre-Cenozoic block in Indochina Peninsula and the South China block, the Red River Fault Zone was divided into two parts extending from land to ocean, the north and south segments. Since the Tertiary, the Red River Fault Zone suffered first the sinistral movement and then the dextral movement. The activities of the north and the south segments were different. Based on the analysis of earthquakes and focal mechanism solutions,earthquakes with the focus depths of 0-33km are distributed over the whole region and more deep earthquakes are distributed on the northeastern sides of the Red River fault. Types of faulting activities are the thrust in the northwest, the normal in the north and the strike-slip in the south, with the odd type, viz. the transition type, in the other region. These show the Red River Fault Zone and its adjacent region suffered the extruding force in NNW direction and the normal stress in NEE direction and it makes the fault in the region extrude-thrust,horizontal strike-slip and extensional normal movement.

Study on the 3-D Velocity Structure around the Focal Region of the Anqing Earthquake in January 2011 and Discussion on the Causative Fault

The Anqing MS4.8 earthquake occurred on January 19, 2011, with the epicenter lying in the foreland deformation belt along the Yangtze River of the lower Yangtze block. After the earthquake, the field work team surveyed and collected building damage data, calculating and obtaining more accurate intensity distributions. The focal mechanism of the main shock was calculated tentatively using digital seismic wave data from provincial digital seismic networks using the FOCMEC program and the first motions of P, SV and SH waves with their amplitude ratios. Using the location results of the main shock and aftershocks by the Anhui seismic network, combining the three-dimensional crust velocity structure imaging results of the focal region by seismic tomography, and referring on intensity distribution of the elliptic major axis' predominant direction, we conclude by comprehensive analysis that the NE-trending Susong-Zongyang fault is possibly the causative fault of the Anqing earthquake.

2022年9月5日四川泸定MS6.8地震序列特征及发震构造

2022年9月5日发生的泸定MS6.8地震是鲜水河断裂周边近六年来规模最大的破坏性地震,其对于分析川滇块体东边界潜在的强震活动具有重要的指示意义.本研究采用双差定位技术对2009年1月1日至2022年9月27日在泸定震源区发生的11103次地震进行了精确定位,利用波形拟合技术反演了主震(MS6.8)、12次震级大于或等于3.0的余震以及贡嘎山附近两次震级大于或等于5.0的强震的震源机制解.结合序列b值结果,对泸定地震序列的特征和发震构造进行了分析研究.研究结果表明,泸定MS6.8地震的发震断层为磨西断裂,主震及其余震发生在黑沟—燕子沟镇—田湾段.这次地震填补了磨西断裂段稀疏地震活动的空白区域,发震断层段大致长度约为40 km,走向大致为160°,呈基本直立状态.序列地震以左旋走滑型为主要错动类型.自2009年以来,泸定MS6.8地震主震西侧贡嘎山附近持续发生地震活动,尤其是2015年该区域出现了显著的地震活动集中现象.主震后,贡嘎山附近的浅源地震活动非常活跃,以拉张破裂为主要特征.需要注意的是,这些浅源地震活动属于触发活动,与磨西断裂带的地震活动不同.b值分析结果显示,磨西断裂带南东段湾东村至田湾的b值较低,约为0.5左右;而北西段湾东村至黑沟的b值较高,处于0.8~1.0之间.这表明磨西断裂带上的应力具有明显的分段特征,能量主要在燕子沟至田湾高应力集中段释放.

基于震源机制和地震定位研究2022年山西古交M_(L)4.1地震的发震构造

2022年2月20日山西太原古交市发生了M_(L)4.1地震,古交及周边县市震感强烈。此次地震的震中位于历史地震相对稀少、煤矿资源丰富的吕梁隆起区,其发震断裂尚不明确。为了更好地认识这次地震的发震构造,文中利用山西省测震台网的近震波形资料反演了地震的震源机制解,并获得最佳矩心深度。反演结果显示,古交地震是一次左旋走滑型地震事件,矩震级为M_(W)3.96,最佳双力偶解为:节面Ⅰ,走向90°、倾角86°、滑动角-19°;节面Ⅱ,走向182°、倾角70°、滑动角-175°;最佳矩心深度为3km,属于极浅源地震。地震序列重定位结果显示震中的优势展布方向近EW,与区域内断层基本为NE走向的分布格局差异显著。利用聚类分析方法得到古交地震的发震断层面中心解,结果显示断层的走向为91°,倾角为70°。另外,古交地震序列的应力降显著偏小,低于同区域背景地震至少1个数量级。结合实地调研的矿区生产情况和区域地质构造,认为古交地震序列的发震机制可能与震源区附近的煤田开采活动有关,开采活动致使局部应力改变,从而导致先存隐伏断层活化,进而触发此次地震。

盖州青石岭震群断层面参数的确定及构造应力场特征研究

根据之前学者的双差定位结果,使用Fit_Fault软件包计算盖州青石岭震群发震断层面走向为114.30°,倾角74.67°。收集2001年至2011年辽宁地区的震源机制解资料,采用全局网格搜索算法,反演该地区的构造应力场,其主压应力轴为NEE向,主张应力轴为NNW向,整体呈现拉张的应力状态。并据此确定出青石岭震群发震断层的滑动角是35.49°,为左旋走滑性质。模拟在辽宁地区构造应力体系下各种产状的断层面上可产生的相对剪应力和相对正应力分布情况,发现该震群发震断层面上的相对剪应力和相对正应力分别为0.513和0.576。分析认为盖州青石岭震群是一处地质薄弱带,背景构造应力场在其最近一期的活动中未起主导作用。

夏县断层气氢高值异常探讨

运用2010—2023年夏县断层气氢浓度观测数据,分析其异常变化与地震活动的关联性,结合异常时段的震源机制解、GPS基线构建的山西南部构造应力场、地壳垂直形变特征,解释断层气氢浓度变化与区域构造活动的关系,分析认为夏县断层气氢测值自2022年6月出现的高值异常可能与地下应力场调整相关。

采用2021年青海玛多地震序列震源机制节面聚类确定发震断裂几何形态

地震断裂带形状是活动构造和地球动力学的重要资料。2021年发生的玛多地震序列提供了丰富的震源机制资料,为统计震源机制节面并估计玛多地震发震断层面形状提供了很好机会。文章对地震的震源机制资料进行基于密度的聚类来确定断层的几何形态。首先对收集到的玛多地震序列的震源机制解进行中心解求解,从而获得更为精准的数据,然后对其进行DBSCAN方法的聚类分析,得到断层的走向为113.5°,倾角为88.2°,通过震源机制反演应力场,并将应力场投影到断层上,得到断裂带的相对剪应力和相对正应力分别为0.84和-0.79,剪应力强度较大,滑动角为-0.72°。结果表明玛多地震是发震断裂受NE-SW的挤压和NW-SE的拉张,形成了较大的剪切力,从而促使近东西的江错断裂发生左旋走滑错动所致。

2024年新疆乌什M_(S)7.1地震发震断层产状及其动力学探讨

大地震发生后,断层面产状快速确定对震后趋势判定具有重要参考价值。本文收集2024年新疆乌什M_(S)7.1地震序列的震源机制数据,对多家机构计算的该地震震源机制解进行中心解求解后,对震源机制的节面进行聚类分析,去掉噪声点的震源机制得到两簇结果,发现标准差较小的第二簇中心节面可能为发震断层面。然后,为进一步通过断层面与应力场关系验证可能的发震断层面,收集了该地区历史震源机制资料,反演得到乌什地震震源区的构造应力场,发现该地区受到NW—SE向的挤压应力和近垂直向的拉张应力;将应力场分别投影到两簇节面上,发现标准差较小的第二簇中心节面处于剪应力较大区域,并且正应力也有利于该中心节面破裂。综合其他资料分析,本研究推测2024年新疆乌什M_(S)7.1地震发震断层为NW向、低倾角断层,走向为244.62°,倾角为46.19°。

2023年甘肃积石山MS6.2地震:一次逆冲为主的浅源强震

基于区域数字地震台网波形数据,在三维空间内利用波形拟合反演方法获得2023年12月18日甘肃积石山MS6.2地震最佳震源机制解,结果表明:节面Ⅰ走向305°,倾角56°,滑动角61°;节面Ⅱ为走向169°,倾角43°,滑动角125°;矩震级MW6.02。最佳拟合的空间精细位置为102.377°E, 35.968°N,深度9 km。结果显示,地震破裂沿NW方向扩展。初步推断此次地震的发震断层面为节面Ⅱ,发震断层是拉脊山北缘断裂(积石山东缘断裂),是一次逆冲为主,兼少量走滑分量的浅源强震事件。

近场地震动竖向与水平分量统计特性对比

为研究近场地震动竖向与水平分量特性,本文对PEER数据库中1202组近场强震的地震动进行统计分析,研究了竖向水平加速度峰值比PGAv/PGAh、反应谱之比Sav(T)/Sah(T)、竖向水平峰值到达时间的特性。讨论了PGAv/PGAh与断层距、场地条件、震源机制的关系,并采用对数正态分布拟合不同场地条件、不同震源机制的PGAv/PGAh分布;对比了不同断层距、不同场地条件Sav(T)/Sah(T)的变化规律;研究了近场强震水平竖向分量峰值加速度的大小和到达时间上的相关性,并对结构分析过程中竖向地震动的选择提出建议。结果表明:断层距、场地条件、震源机制都会影响近场地震动的竖向分量,断层距小、走滑断层、场地条件差都易产生较大的竖向地震动;断层距小、场地条件差时,地震动竖向分量高频成分越强,断层距大、场地条件好,竖向地震动低频成分越强;竖向分量峰值加速度的大小与到达时间和两正交水平分量峰值加速度的大小与到达时间的几何平均数有明显的线性关系。

Fault plane parameters of Tancheng M81/2 earthquake on the basis of present-day seismological data

The great Tancheng earthquake of M81/2 occurred in 1668 was the largest seismic event ever recorded in history in eastern China. This study determines the fault geometry of this earthquake by inverting seismological data of present-day moderate-small earthquakes in the focal area. We relocated those earthquakes with the double-difference method and found focal mechanism solutions using gird test method. The inversion results are as follows： the strike is 21.6°, the dip angle is 89.5°, the slip angle is 170°, the fault length is about 160 km, the lower-boundary depth is about 32 km and the buried depth of upper boundary is about 4 km. This shows that the seismic fault is a NNE-trending upright right-lateral strike-slip fault and has cut through the crust. Moreover, the surface seismic fault, intensity distribution of the earthquake, earthquake-depth distribution and seismic-wave velocity profile in the focal area all verified our study result.

考虑数据不平衡的轨道交通装备液压系统内泵泄漏智能诊断方法研究

针对液压系统内泵泄漏诊断的数据集不平衡问题,提出了一种两阶段处理方法,使用变分编码器对少数类样本进行合成,将少数类故障样本补全到和正常样本一致。再使用焦点损失对故障分类模型进行训练,增强分类器对难分类样本的诊断能力。所提出方法经过消融实验验证,能够有效处理不平衡数据集。

Focal Mechanism Solutions of the 2008 Wenchuan earthquake sequence from P-wave polarities and SH/P amplitude ratios:new results and implications

Abstract The 2008 Wenchuan earthquake, a major intraplate earthquake with Mw 7.9, occurred on the slowly deforming Longmenshan fault. To better understand the causes of this devastating earthquake, we need knowledge of the regional stress field and the underlying geodynamic processes. Here, we determine focal mechanism solutions （FMSs） of the 2008 Wenchuan earthquake sequence （WES） using both P-wave first-motion polarity data and SH/P amplitude ratio （AR） data. As P-wave polarities are more reliable information, they are given priority over SH/ PAR, the latter of which are used only when the former has loose constraint on the FMSs. We collect data from three categories： （1） permanent stations deployed by the China Earthquake Administration （CEA）; （2） the Western Sichuan Passive Seismic Array （WSPSA） deployed by Institute of Geology, CEA; （3） global stations from Incorporated Research Institutions for Seismology. Finally, 129 events with magnitude over Ms 4.0 in the 2008 WES are identified to have well-constrained FMSs. Among them, 83 are well constrained by P-wave polarities only as shown by Cai et al. （Earthq Sci 24（1）：115-125,2011）, and the rest of which are newly constrained by incorporating SH/P AR. Based on the spatial distribution and FMSs of the WES, we draw following conclusions： （1） the principle compressional directions of most FMSs of the WES are subhorizontal, generally in agreement with the conclusion given by Cai et al. （2011） but with a few modifications that the compressional directions are WNW-ESE around Wenchuan and ENE-WSW around Qingchuan, respectively. The subhorizontal compressional direction along the Longmenshan fault from SW to NE seems to have a leftlateral rotation, which agrees well with regional stress field inverted by former researchers （e.g., Xu et al., Acta Seismol Sin 30（5）, 1987; Acta Geophys Sin 32（6）, 1989; Cui et al., Seismol Geol 27（2）：234-242, 2005）; （2） the FMSs of the events not only reflected the regional stress state of the Longmenshan region, but also were obviously controlled by the faults to some extent, which was pointed out by Cai et al. （2011） and Yi et al. （Chin J Geophys 55（4）：1213-1227, 2012）; （3） while the 2008 Wenchuan earthquake and some of its strong aftershocks released most of the elastic energy accumulated on the Longmen- shan fault, some other aftershocks seem to occur just for releasing the elastic energy promptly created by the 2008 Wenchuan earthquake and some of its strong aftershocks. （4） Our results further suggest that the Longmenshan fault from Wenchuan to Beichuan was nearly fully destroyed by the 2008 Wenchuan earthquake and accordingly propose that there is less probability for great earthquakes in the middle part of the Longmenshan fault in the near future, although there might be a barrier to the southwest of Wenchuan and it is needed to pay some attention on it in the near future.

Normal Faulting Type Earthquake Activities in the Tibetan Plateau and Its Tectonic Implication

This paper analyzes various earthquake fault types, mechanism solutions, stress field as well as other geophysical data to study the crust movement in the Tibetan plateau and its tectonic implications. The results show that a lot of normal faulting type earthquakes concentrate in the central Tibetan plateau. Many of them are nearly perfect normal fault events. The strikes of the fault planes of the normal faulting earthquakes are almost in the N-S direction based on the analyses of the equal area projection diagrams of fault plane solutions. It implies that the dislocation slip vectors of the normal faulting type events have quite great components in the E-W direction. The extension is probably an eastward extensional motion, mainly a tectonic active regime in the altitudes of the plateau. The tensional stress in the E-W or WNW-ESE direction predominates the earthquake occurrence in the normal event region of the central plateau. A number of thrust fault and strike-slip fault type earthquakes with strong compressive stress nearly in the NNE-SSW direction occurred on the edges of the plateau. The eastward extensional motion in the Tibetan plateau is attributable to the eastward movement of materials in the upper mantle based on_seismo-tomographic results. The eastward extensional motion in the Tibetan plateau may be related to the eastward extrusion of hotter mantle materials beneath the east boundary of the plateau. The northward motion of the Tibetan plateau shortened in the N-S direction probably encounters strong obstructions at the western and northern margins. Extensional motions from the relaxation of the topography and/or gravitational collapse in the altitudes of the plateau occur hardly in the N-S direction. The obstruction for the plateau to move eastward is rather weak.

The seismicity and tectonic stress field characteristics of the Longmenshan fault zone before the Wenchuan M_S8.0 earthquake

The seismicity of Longrnenshan fault zone and its vicinities before the 12 May 2008 Wenchuan Ms8.0 earthquake is studied. Based on the digital seismic waveform data observed from regional seismic networks and mobile stations, the focal mechanism solutions are determined. Our analysis results show that the seismicities of Longmenshan fault zone before the 12 May 2008 Wenchuan earthquake were in stable state. No obvious phenomena of seismic activity intensifying appeared. According to focal mechanism solutions of some small earthquakes before the 12 May 2008 Wenchuan earthquake, the direction of principal compressive stress P-axis is WNW-ESE. The two hypocenter fault planes are NE-striking and NW-striking. The plane of NE direction is among N50°-70°E, the dip angles of fault planes are 60°-70° and it is very steep. The faultings of most earthquakes are dominantly characterized by dip-slip reverse and small part of faultings present strike-slip. The azimuths of principal compressive stress, the strikes of source fault planes and the dislocation types calculated from some small earthquakes before the 12 May 2008 Wenchuan earthquake are in accordance with that of the main shock. The average stress field of micro-rupture along the Longmenshan fault zone before the great earthquake is also consistent with that calculated from main shock. Zipingpu dam is located in the east side 20 km from the initial rupture area of the 12 May 2008 Wenchuan earthquake. The activity increment of small earthquakes in the Zipingpu dam is in the period of water discharging. The source parameter results of the small earthquakes which occurred near the initial rupture area of the 12 May 2008 Wenchuan earthquake indicate that the focal depths are 5 to 14 km and the source parameters are identical with that of earthquake.

Inversion of Stress Fields in the Middle Section of the Xiaojiang Fault and Its Adjacent Area

With waveform data of 613 earthquakes with ML ≥ 2. 5 in the middle section of the Xiaojiang fault and its adjacent area which occurred during January,1998 to September 2007,focal mechanisms were calculated by the direct wave amplitude ratio of S /P in the vertical component and their characteristics were analyzed. According to regional tectonic features of the middle section of the Xiaojiang fault and its adjacent area,the study region was partitioned into two zones with the Xiaojiang fault as the boundary,e. g. zone A and zone B (including the Xiaojiang fault). In order to research the faults stress in detail,the Xiaojiang fault zone was picked out for independent analysis. The study region was also partitioned into 1°× 1° cells with a 0. 5° step. The stress fields of zone A,B and the fault zone were inverted with the FMSI method (Gephart,1990). The results show that first, the faults are mainly of strike-slip in the middle section and its adjacent area,amounting to 81. 28%,69. 23% and 72. 97% in the A,B and fault zones,respectively. Secondly,the stress inversion also indicates that the directions of maximum principal stress σ1 in the A, B,and fault zones are approximately NNW,NWW and NWW,the stress action is mainly horizontal,and strike-slip faulting is dominant in the study area. On the other hand,the direction of the principal stress field in the central Yunnan block changed from NNW to NWW,however,in the region between the Yuanmou and Pudu River faults,the azimuth of the main compressive stress shows that the north-south slip is obvious. While the direction of the main compressive stress of the Xiaojiang fault zone is nearly NW; in the east of the Xiaojiang fault,the direction of principal compressive stress is NW to NNW in the eastern Yunnan block.

Complex time－space spreading and focal mechanism of the 1989 Batang earthquake swarm （M_s＝6．7）

On the basis of field observations, the complex time-space spreading pattern and focal mechanism of the 1989Batang earthquake swarm are studied in this paper. The fault motions of the epicenter area are analysed bythe leveling survey before and after strong events. From the given simplified focal mechanical model of theswarm,the process of fracture indicates that swarm strong ruptures are associated with the spreading and thevirgation of the barrier of irregularen en echelon source fault system.