Study on the earthquake fault of the Lulong ML=6.2 earthquake by precise relocation of aftershock

Introduction The Lulong county is about 75 km northeast to Tangshan in Hebei Province. An earthquake of ML=6.2 took place on October 19, 1982 about 4 km northeast to the county town. The epicenter of the mainshock is located at 39°57′N, 119°04′E by the Beijing seismic network. The 1976 Tang- shan 7.8 earthquake and the successive Luanxian 7.1, Ninghe 6.9 and the Lulong earthquake spread in NE-SW direction in space. SONG and WANG et al of Institute of Geophysics, China Seismolog…     

Study on the earthquake fault of the Lulong M L=6.2 earthquake by precise relocation of aftershock

We have selected 171 near-field records from 391 aftershock records of the Lulong, Hebei Province, earthquake in October 1982 and relocated the hypocenter of 45 aftershocks using the program Hypoinverse. The distribution of aftershocks reveals a set of earthquake faults: a WNW stretching fault truncates two NNE stretching faults. The two branches of faults show the conjugate structure which is often seen in brittle fracture. The NNE stretching faults are connected together. The Luanhe river valley near Lulong developed to a rudiment rift basin surrounded by a series of faults. The fault of Lulong earthquake is a strike-slip fault with tension component. This fault type matches with the activity of Zhangjiakou-Bohai seismic belt (Zhang-Bo belt) and also shows the action of Zhang-Bo belt as a boundary of two secondary active blocks that truncates the NNE fault. Foundation item: National Natural Science Foundation of China (40234038). Contribution No. 05FE3016, Institute of Geophysics, China Earthquake Administration.     

近场精确定位在卢龙MemLsub6.2地震发震构造研究中的应用

从1982年10月19日卢龙6.2级地震余震近场数字地震资料的391条记录中， 挑选出可用于精确定位的171条地震事件记录，采用Hypoinverse定位方法对45个事件进行了重定位. 精确定位的震中分布显示出一卧倒的反ldquo;Frdquo;形活动地震构造的形态， 两条NNE向断裂被一条WNW向断裂所截断，两组断裂呈脆性断裂常见的共轭状态产出， NNE向的断层正在相互贯通，卢龙附近的滦河河谷发育成了四面断裂包围的断陷盆地雏形. 卢龙地震的发震构造是一个走滑兼张性的断裂组合， 这样的构造与张家口——渤海地震带的整体活动习性相符， 也反映了张渤带作为一个二级地块的分界截断NNE向的一系列断层所起的作用.      

Large aftershocks triggering by Coulomb failure stress following the 2001<Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>=8.1 great Kunlun earthquake

The great Kunlun earthquake occurred on Nov. 14, 2001 in Qinghai Province, China. Five large aftershocks with magnitude larger than 5.0 occurred near the Kunlun fault after main shock. Calculations of the change in Coulomb failure stress reveal that 4 of 5 large aftershocks occurred in areas with Δσ_f>0 (10^?2–10^?1 MPa) and one aftershock occurred in an area with Δσ_f=?0.56 MPa. It is concluded that the permanent fault displacement due to the main shock is the main cause of activity of large aftershocks, but not the whole cause.     

Stress changes on major faults caused by <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript>7.9 Wenchuan earthquake,May 12, 2008

On May 12, 2008, a magnitude 7.9 earthquake ruptured the Longmenshan fault system in Sichuan Province, China, collapsing buildings and killing tens of thousands people. As predicted, aftershocks may last for at least one year, and moreover, large aftershocks are likely to occur. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settling people as to avoid future disasters. It is demonstrated that the redistribution of stress induced by an earthquake should trigger successive seismic activity. Based on static stress triggering theory, we calculated the coseismic stress changes on major faults induced by the Wenchuan earthquake, with elastic dislocation theory and the multilayered crustal model. We also discuss the stress distribution and its significance for future seismic activity under the impact of the Wenchuan earthquake. It is shown that coulomb failure stress (CFS) increases obviously on the Daofu-Kangding segment of the Xianshuihe Fault, the Maqu and Nanping segment of the Eastern Kunlun Fault, the Qingchuan Fault, southern segment of the Minjiang Fault, Pengxian-Guanxian Fault, Jiangyou-Guangyuan Fault, and Jiangyou-Guanxian Fault. The increased stress raises the probability of earthquake occurrence on these faults. Since these areas are highly populated, earthquake monitoring and early disaster alarm system are needed. CFS increases with a magnitude of 0.03–0.06 MPa on the Qingchuan Fault, which is close to the northern end of the rapture of Wenchuan earthquake. The occurrence of some strong aftershocks, including three events with magnitude higher than 5.0, indicates that the seismic activities have been triggered by the main shock. Aftershocks seem to migrate northwards. Since the CFS change on the Lueyang-Mianxian Fault located on the NEE of the Qingchuan Fault is rather small (±0.01 MPa), the migration of aftershocks might be terminated in the area near Hanzhong City. The CFS change on the western Qinling Fault is around 10 Pa, and the impact of static triggering can be neglected. The increment of CFS on the Pengxian-Guanxian Fault and Beichuan-Yingxiu Fault southwest to the main rupture is 0.005–0.015 MPa, which would facilitate earthquake triggering in these areas. Very few aftershocks in these areas indicate that the accumulated stress has not been released sufficiently. High seismic risk is predicated in these areas due to co-seismic CFS loading. The Wenchuan earthquake released the accumulated CFS on the Fubianhe Fault, the Huya Fault, the Ha’nan-Qingshanwan Fault, and the Diebu-Bailongjiang Fault. The decrement of CFS changes on the Longquanshan Fault east to Chengdu City is about 0.002 MPa. The seismic activity will be depressed by decrement of CFS on these faults. Supported by Knowledge Innovation Program of Chinese Academy of Sciences (Grant No. KZCX-SW-153), National Natural Science Foundation of China (Grant Nos. 40574011 and 40474028)     

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.     

Variation of stress during the rupture process of the 1995 <Emphasis Type="Italic">M</Emphasis><Subscript>L</Subscript>=4.1 Shacheng,Hebei, China,earthquake sequence

According to the rupture dynamics of earthquakes, variations of the apparent stress and the difference between the static stress drop and the dynamic stress drop during the rupture of earthquakes are analyzed for the July 20, 1995 M _L=4.1 Shacheng, Hebei, China, earthquake sequence. Results obtained show that the apparent stress for main-shock is about 5 MPa, and the average apparent stress for aftershocks 0.047 MPa. During the rupture of the main-shock, the dynamic stress drop is approximately 1.6 times greater than the static stress drop with the difference of nearly 2.7 MPa. The dynamic stress drop is less than the static stress drop for all aftershocks with the average difference of ?0.75 MPa. Therefore, when the mainshock occurs the final stress on the focal fault is higher than the dynamic frictional stress, corresponding to that the fault is abruptly locked. When the aftershocks occur the final stress on the focal fault is lower than the dynamic frictional stress, corresponding to that the fault overshoots. It can be seen from the above results that there could be some differences in the physic processes between the mainshock and the aftershocks.     

利用余震震中分析芦山MS7.0 地震发震构造

对2013年4月20日芦山M_S7.0地震主震震中周边29个地震台记录到的震后一年多的微、 小余震,利用Hypo71绝对定位方法进行定位,获得了约1960次地震的震源位置. 结果显示,芦山地震余震在平面上主要沿双石—大川分支断裂及其周边分布,在垂向上主要集中在大约5—20 km深度之间的两条余震交叉带上. 其中一条余震带倾向NW,倾角在12 km左右深度发生变化,浅部倾角较陡,该余震带延伸至地表与双石—大川分支断裂和新开店断裂之间的推测隐伏断裂位置相重合; 另一条余震带倾向SE,其延伸至地表的位置与双石—大川断裂非常接近,但与该断裂倾向相反. 主震震源位置与两条余震带相交的位置接近,且芦山地震主震的两个节面产状与这两条余震带的深部几何形态正好对应, 表明芦山地震主震可能是两条余震带所对应的两条断裂同时活动的结果.      

结合波形互相关的双差定位方法在2008年汶川地震余震序列中的应用

选取了汶川地震主震后的2008年5月12日——2009年8月31日, 震级为3.0le;MSle;5.0的余震4240次．利用波形互相关方法得到其P波到时,用双差定位方法对其进行定位,最终得到了2441次重新定位的结果．统计定位误差（两倍标准偏差）在E-W方向为0.4 km,N-S方向为0.4 km,垂直方向为0.7 km．定位结果表明,汶川地震的余震深度集中在10——20 km,震中分布与龙门山中央断裂带的走向关系密切．沿龙门山断裂的地震分布具有明显的分段性,西南段呈水平带状分布,东北段接近垂直分布,且在北川附近存在深度突变．这与龙门山断裂的地震在西南段多表现为逆冲,东北段多表现为走滑的现象相吻合．在深度剖面上地震的空间分布存在分立的特征,通过对比前人在此地区浅层的地震剖面资料, 发现地震空间分布与已探知的浅部断层有较好的对应关系．      

Focal depths for moderate-sized aftershocks of the Wenchuan <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>8.0 earthquake and their implications

Sliding-window cross-correlation method is firstly adopted to identify sPn phase, and to constrain focal depth from regional seismograms, by measuring the time separation between sPn and Pn phases. We present the focal depths of the 17 moderate-sized aftershocks (M _S⩾5.0) of the Wenchuan M _S8.0 earthquake, using the data recorded by the regional seismic broadband networks of Shaanxi, Qinghai, Gansu, Yunnan and Sichuan. Our results show focal depths of aftershocks range from 8 to 20 km, and tend to cluster at two average depths, separate at 32.5°N, i.e., 11 km to the south and 17 km to the north, indicating that these aftershocks are origin of upper-to-middle crust. Combined with other results, we suggest that the Longmenshan fault is not a through-going crustal fault and the Pingwu-Qingchuan fault may be not the northward extension of the Longmenshan thrust fault. Supported by the National Natural Science Foundation of China (Grant Nos. 40604009 and 40574040) and Special Project for the Fundamental R & D of Institute of Geophysics, China Earthquake Administration (Grant No.DQJB08B20)     

伽师地震区地壳细结构及发震断层的初步研究

199年至1998年伽师地区共出现9次震级为6.1-6.8级的强震. 在一个非常短的时间间隔内和非常小的地区范围内接连出现这么多次震级非常接近的地震,确实非常罕见. 为研究伽师强震区的深部构造背景和孕震机制,本文对伽师地震区的余震观测资料进行了分析处理. 利用联合反演技术同时得到了地震震源位置和地震区地壳三维速度结构. 余震震中沿一北北东向条带分布,与强震分布的两个条带中的北东向条带位置基本重合. 三维反演得到的速度结构结果表明,在地下12 km以下存在一条北北东向和一条北北西向的低速条带. 上述两低速条带与强震分布的两个条带位置很接近. 初步推测,低速条带对应了地壳深部的两条断裂. 在我们观测期间,北北东向断裂有微震活动,北北西向断裂相对平静.     

Holocene activities of the Taigu fault zone,Shanxi Province,and their relations with the 1303 Hongdong <Emphasis Type="Italic">M</Emphasis>=8 earthquake

The Taigu fault zone is one of the major 12 active boundary faults of the Shanxi fault-depression system, located on the eastern boundary of the Jinzhong basin. As the latest investigation indicated, the fault zone had dislocated gully terrace of the first order, forming fault-scarp in front of the loess mesa. It has been discovered in many places in ground surface and trenches that Holocene deposits were dislocated. The latest activity was the 1303 Hongdong earthquake M=8, the fault appeared as right-lateral strike-slip with normal faulting. During that earthquake, the Taigu fault together with the Mianshan western-side fault on the Lingshi upheaval and the Huoshan pediment fault on the eastern boundary of the Linfen basin was being active, forming a surface rupture belt of 160 km in length. Moreover, the Taigu fault were active in the mid-stage of Holocene and near 7 700 aB.P. From these we learnt that, in Shanxi fault-depression system, the run-through activity of two boundary faults of depression-basins might generate great earthquake with M=8.     

The seismotectonic and macroseismic features of the Wenchuan (Sichuan) earthquake (<Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">S</Emphasis></Subscript> = 8.0) of May 12, 2008

A disastrous earthquake with a magnitude M _S = 8.0 (M _W = 7.9), in China called “the 5.12 Wenchuan earthquake,” occurred on May 12, 2008, in Sichuan province on the border between the Sino-Tibetan Mountains and the Sichuan depression. The instrumental epicenter was registered in the southeastern part of Wenchuan county, and the hypocenter depth was 14 km. As the strongest and most destructive earthquake within mainland China, it caused numerous human losses and destruction of buildings and infrastructure. The seismic effect from the main shock and aftershocks was felt in many counties, towns, and villages, though Sichuan province suffered the most. The maximum intensity of the shocks was estimated at 11 degrees, according to the Chinese macroseismic scale. In the process of source opening, from the southern part of Wenchuan county to the vicinities of Quingchuan, a seismic fault system with a total length up to 240 km out-cropped on the earth’s surface, confined to the Longmenshan fault belt. The seismic fault system disturbed the original ground, resulting in the collapse or damage to various constructions, such as buildings, homes, bridges, roads, etc. Fault offsets had a dextral strike-slip and thrust kinematic combination. The earthquake generated several tens of thousands of landslides, rockfalls, and debris flows. Many dammed ponds appeared in the epicentral zone due to the activation of landslides. Thus, the geological effects turned out to be the most destructive factor in this case. At the same time, the seismic intensity of surface shaking was abnormally low even in direct proximity to the seismic fault system. Usually it was no more than 7–8 degrees. This macroseismic phenomenon may turn out to be rather typical for many major earthquakes.     

The aftershock sequence of the 5 August 2014 Orkney earthquake (M<Subscript>L</Subscript> 5.5), South Africa

More than 1000 aftershocks were recorded within a month after the occurrence of the M_L 5.5, 5 August 2014 Orkney earthquake. The events were relocated using the double difference method as part of an effort to identify the fault which might be the source of the events. A north–south trend of seismicity was revealed by the relocated events, with a diffuse cluster to the north of the main event. A depth profile shows these two clusters: one at a depth of about 2 km to the north of the main event and the other at depth between 3 and 6 km south of the main event. Focal mechanism solutions of 18 aftershocks were determined using first motion polarities from seismic stations of the Council for Geoscience cluster networks. Stress inversion analysis results from the focal mechanism solutions show a dominant extensional stress field in the region; the main event had a strike-slip fault plane solution. This is consistent with the regional stress field which is predominantly related to the East African rift system. It is possible that the occurrence of the main event triggered seismicity on shallower faults within the mining horizons oriented in a different direction to the fault on which the main event occurred. The area has a complex heterogeneous faulting structure as indicated by the observed low p values and complex focal mechanism solutions.     

Coseismic displacement estimate of the 2013 <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>7.0 Lushan,China earthquake based on the simulation of near-fault displacement field

Usually, GPS observation provides direct evidence to estimate coseismic displacement. However, GPS stations are scattered, sparse and cannot provide a detailed distribution of coseismic displacement. Strong ground motion records share the same disadvantages as GPS in estimating coseismic displacement. Estimations from InSAR data can provide displacement distributions; however, the resolution of such methods is limited by the analysis techniques. The paper focuses on estimating the coseismic displacement of the M _S7.0 Lushan earthquake on April 20, 2013 using a simulation of the wave field based on the elastic wave equation instead of a quasi-static equation. First, the media and source models were constructed by comparing the simulated velocity and the record velocity of the ground motion. Then simulated static displacements were compared with GPS records. Their agreement validates our results. Careful analysis of the distribution of simulated coseismic displacements near the fault reveals more details of the ground motion. For example, an uplift appears on the hanging wall of the fault, rotation is associated with the horizontal displacement, the fault strike and earthquake epicenter provide the main control on motion near the faults, and the motion on the hanging wall is stronger than that on the footwall. These results reveal additional characteristics of the ground motion of the Lushan earthquake.     

2016年运城4.4级地震序列精定位及发震构造分析

根据2016年运城4.4级地震序列资料,进行余震精定位、主震震源机制和发震构造等研究。地震震中分布结果显示,本次地震的发生构造与以往该地区震群型地震发震构造不同,构造单元相对简单,发生在盐湖北岸断裂附近。余震双差精定位结果显示,余震优势分布呈NNE向,NW向也有零星活动。精定位后震源深度集中分布在15-24 km,平均深度20.2 km,断层剖面深度集中分布在18-23 km,倾向NW,与盆地地形构造吻合。采用Snoke与CAP方法得到的震源机制解基本一致,此次序列的主震错断方式为走滑兼逆冲,节面B参数与中条山山前断裂东段走向和倾向接近。综合认为,本次运城地震序列的余震呈NNE向优势分布,精定位结合地震震源机制结果,推断此次地震序列发震断裂为中条山山前断裂的NNE向隐伏断裂。     

利用sPn震相测定芦山MS7.0级地震余震的震源深度

利用南北地震带南段密集流动地震台阵的观测数据,采用波形互相关方法拾取Pn波走时,应用滑动时窗相关法识别sPn震相,通过sPn与Pn震相之间的走时差测定了芦山地震序列中28个M_L4.0级以上余震的震源深度.结果表明,震源深度集中在10~20 km范围内,垂直余震带的北西-南东向震源深度剖面揭示,余震分布表现出西深东浅的特点,倾角大约为39°.这些余震在空间上具有较好的线性分布特征,推测可能发生在与主震有关的破裂面上或邻近位置,由此推测主震的破裂面倾角大约为39°.根据余震的空间分布特征,认为芦山地震的发震断层并非双石-大川断裂,可能是其东侧的一条隐伏断层.     

Preliminary analysis on the source properties and seismogenic structure of the 2017 <Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript>7.0 Jiuzhaigou earthquake

At GMT time 13:19, August 8, 2017, an M_s7.0 earthquake struck the Jiuzhaigou region in Sichuan Province, China, causing severe damages and casualties. To investigate the source properties, seismogenic structures, and seismic hazards, we systematically analyzed the tectonic environment, crustal velocity structure in the source region, source parameters and rupture process, Coulomb failure stress changes, and 3-D features of the rupture plane of the Jiuzhaigou earthquake. Our results indicate the following: (1) The Jiuzhaigou earthquake occurred on an unmarked fault belonging to the transition zone of the east Kunlun fault system and is located northwest of the Huya fault. (2) Both the mainshock and aftershock rupture zones are located in a region where crustal seismic velocity changes dramatically. Southeast to the source region, shear wave velocity at the middle to lower crust is significantly low, but it rapidly increases northeastward and lies close to the background velocity across the rupture fault. (3) The aftershock zone is narrow and distributes along the northwest-southeast trend, and most aftershocks occur within a depth range of 5–20 km. (4) The focal mechanism of the Jiuzhaigou earthquake indicates a left-lateral strike-slip fault, with strike, dip, and rake angles of 152°, 74° and 8°, respectively. The hypocenter depth measures 20 km, whereas the centroid depth is about 6 km. The co-seismic rupture mainly concentrates at depths of 3–13 km, with a moment magnitude (M_w) of 6.5. (5) The co-seismic rupture also strengthens the Coulomb failure stress at the two ends of the rupture fault and the east segment of the Tazang fault. Aftershocks relocation results together with geological surveys indicate that the causative fault is a near vertical fault with notable spatial variations: dip angle varies within 66°–89° from northwest to southeast and the average dip angle measures ~84°. The results of this work are of fundamental importance for further studies on the source characteristics, tectonic environment, and seismic hazard evaluation of the Jiuzhaigou earthquake.     

Phodong (Sikkim) earthquake of 14 February 2006 and its aftershocks—Coulomb stress analysis

The 14 February 2006 Phodong (Sikkim) earthquake of moderate magnitude (Mw 5.3) triggered several aftershocks that were recorded by a local seismic network. The thrust earthquake is part of the continuing earthquake activity in the Himalayan seismic belt region that occurs on the detachment or ramp under the Higher Himalaya. The aftershocks of the earthquake occurred in increased stress regions caused by the earthquake rupture. Triggering of aftershocks by such a moderate magnitude earthquake implies that the faults in the Himalaya are critically stressed and even a small change of stress, about 0.001–0.002 MPa, can trigger earthquakes on such faults.     

A discussion on corioli force effect and aftershock activity tendency of the <Emphasis Type="Italic">M</Emphasis>=8.1 Kunlun Mountain Pass earthquake on Nov. 14, 2001

Following the theory and definition of the Corioli force in physics, the Corioli force at the site of the M=8.1 Kunlun Mountain Pass earthquake on November 14, 2001, is examined in this paper on the basis of a statistical research on relationship between the Corioli force effect and the maximum aftershock magnitude of 20 earthquakes with M≥7.5 in Chinese mainland, and then the variation tendency of aftershock activity of the M=8.1 earthquake is discussed. The result shows: a) Analyzing the Corioli force effect is an effective method to predict maximum aftershock magnitude of large earthquakes in Chinese mainland. For the sinistral slip fault and the reverse fault with its hanging wall moving toward the right side of the cross-focus meridian plane, their Corioli force pulls the two fault walls apart, decreasing frictional resistance on fault plane during the fault movement and releasing elastic energy of the mainshock fully, so the maximum magnitude of aftershocks would be low. For the dextral slip fault, its Corioli force presses the two walls against each other and increases the frictional resistance on fault plane, prohibiting energy release of the mainshock, so the maximum magnitude of aftershocks would be high. b) The fault of the M=8.1 Kunlun Mountain earthquake on Nov. 14, 2001 is essentially a sinistral strike-slip fault, and the Corioli force pulled the two fault walls apart. Magnitude of the induced stress is about 0.06 MPa. After a comparison analysis, we suggest that the aftershock activity level will not be high in the late period of this earthquake sequence, and the maximum magnitude of the whole aftershocks sequence is estimated to be about 6.0.