Moment tensor inversion and source rupture process of the September 27, 2003 M_S=7.9 earthquake occurred in the border area of China, Russia and Mongolia

IntroductionOn September 27, 2003, an earthquake of MS=7.9 struck the border area of China, Russia and Mongolia. According to the field investigation from the Earthquake Administration of XinjiangAutonomous Region, the whole northern Tianshan region felt the hit. Buildings and structures within six counties and one city in Altay region, which is total about 0.11×106 km2 area, were damaged to different extent and caused certain economic losses. The epicenter determined by China National …     

2003年9月27日中、俄、蒙边界Ms7.9地震震源机制及破裂过程研究

从IRIS全球数字地震台网长周期记录中, 选取震中距位于30deg;~90deg;的垂直向远震P波资料, 反演了2003年9月27日中、 俄、 蒙边界MS7.9地震及10月1日MS7.3强余震的地震矩张量解, 研究了MS7.9地震的时空破裂过程. 参考余震的空间分布及周围断层走向, 确定MS7.9地震发震断层走向127deg;、倾角为79deg;、滑动角为171deg;. MS7.9地震震源破裂过程反演结果表明,整个破裂过程持续了37 s,释放标量地震矩0.97times;1020 Nmiddot;m. 破裂主要发生在长110 km, 宽30 km的中地壳以上,最大位错3.6 m. 起始破裂处不是滑动量最大的地方. 断层面上显示出两个显著的、滑动量超过2.0 m的破裂区. 破裂传播至MS7.3震源区附近时, 滑动量迅速减小,显示出破裂传播过程的受阻停止, 反映了障碍体引起的破裂过程的不均匀性.      

Tempo-spatial rupture process of the 1997 Mani, Xizang (Tibet), China earthquake of M S=7.9

An earthquake of M _S=7.4 occurred in Mani, Xizang (Tibet), China on November 8, 1997. The moment tensor of this earthquake was inverted using the long period body waveform data from China Digital Seismograph Network (CDSN). The apparent source time functions (ASTFs) were retrieved from P and S waves, respectively, using the deconvolution technique in frequency domain, and the tempo-spatial rupture process on the fault plane was imaged by inverting the azimuth dependent ASTFs from different stations. The result of the moment tensor inversion indicates that the P and T axes of earthquake-generating stress field were nearly horizontal, with the P axis in the NNE direction (29°), the T axis in the SEE direction (122°) and that the NEE-SWW striking nodal plane and NNW-SSE striking nodal plane are mainly left-lateral and right-lateral strike-slip, respectively; that this earthquake had a scalar seismic moment of 3.4×10²⁰ N·m, and a moment magnitude of M _W=7.6. Taking the aftershock distribution into account, we proposed that the earthquake rupture occurred in the fault plane with the strike of 250°, the dip of 88° and the rake of 19°. On the basis of the result of the moment tensor inversion, the theoretical seismograms were synthesized, and then the ASTFs were retrieved by deconvoving the synthetic seismograms from the observed seismograms. The ASTFs retrieved from the P and S waves of different stations identically suggested that this earthquake was of a simple time history, whose ASTF can be approximated with a sine function with the half period of about 10 s. Inverting the azimuth dependent ASTFs from P and S waveforms led to the image showing the tempo-spatial distribution of the rupture on the fault plane. From the "remembering" snap-shots, the rupture initiated at the western end of the fault, and then propagated eastward and downward, indicating an overall unilateral rupture. However, the slip distribution is non-uniform, being made up of three sub-areas, one in the western end, about 10 km deep ("western area"); another about 55 km away from the western end and about 35 km deep ("eastern area"); the third about 30 km away from the western end and around 40 km deep ("central area"). The total rupture area was around 70 km long and 60 km wide. From the "forgetting" snap-shots, the rupturing appeared quite complex, with the slip occurring in different position at different time, and the earthquake being of the characteristics of "healing pulse". Another point we have to stress is that the locations in which the rupture initiated and terminated were not where the main rupture took place. Eventually, the static slip distribution was calculated, and the largest slip values of the three sub-areas were 956 cm, 743 cm and 1 060 cm, for the western, eastern and central areas, respectively. From the slip distribution, the rupture mainly distributed in the fault about 70 km eastern to the epicenter; from the aftershock distribution, however, the aftershocks were very sparse in the west to the epicenter while densely clustered in the east to the epicenter. It indicated that the Mani M _S=7.9 earthquake was resulted from the nearly eastward extension of the NEE-SWW to nearly E-W striking fault in the northwestern Tibetan plateau. Contribution No. 99FE2016, Institute of Geophysics, China Seismological Bureau. This work is supported by SSTCC Climb Project 95-S-05 and NSFDYS 49725410.     

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.     

Moment tensor inversion of the November 6, 1988 M S=7.6, Lancang-Gengma, China, earthquake using long-period body-waves data

Moment tensor inversion was carried out to invert the source mechanism and source time function of the MS=7.6 November 6, 1988, Lancang-Gengma, Yunnan Province, China, earthquake. Waveforms of long-period body-waves recorded by China Digital Seismograph Network (CDSN) were used in the inversion. The inverted result shows one nodal plane of right-lateralstrike-slip faulting and another of left-lateral strike-slip faulting and a simple source time function of a duration of about 15 s and scalar seismic moment of 6.4(1020 N(m. From the geological dataand tectonic settings and also from field observations and epicentral distribution of aftershocks, the nodal plane striking in the azimuth of 313( is preferred as the fault plane. The pressure axis lies almosthorizontally in north-south direction.     

Moment tensor inversion of focal mechanism for the aftershock sequence of 1982 Lulong Ms=6.1 earthquake

Introduction Lulong is located about 70 km to the northeast of Tangshan city, Hebei Province. On October 19, 1982, a MS=6.1 earthquake took place about 4 km to the northeast of Lulong county with the epicenter of 39°57′N, 119°04′E and the original time of 20h45min (Beijing time). Lulong is lo-cated at the connection part of Yanshan block and North China plain block according to the divi-sion of active blocks in Chinese mainland (ZHANG et al, 2003). There is a WNW-trending tec-toni…     

Spatial and temporal rupture process of the January 26, 2001, Gujarat, India, M S=7.8 earthquake

The source parameters, such as moment tensor, focal mechanism, source time function (STF) and temporal-spatial rupture process, were obtained for the January 26, 2001, India, M _S=7.8 earthquake by inverting waveform data of 27 GDSN stations with epicentral distances less than 90°. Firstly, combining the moment tensor inversion, the spatial distribution of intensity, disaster and aftershocks and the orientation of the fault where the earthquake lies, the strike, dip and rake of the seismogenic fault were determined to be 92°, 58° and 62°, respectively. That is, this earthquake was a mainly thrust faulting with the strike of near west-east and the dipping direction to south. The seismic moment released was 3.5×10²⁰ Nm, accordingly, the moment magnitude M _W was calculated to be 7.6. And then, 27 P-STFs, 22 S-STFs and the averaged STFs of them were determined respectively using the technique of spectra division in frequency domain and the synthetic seismogram as Green’s functions. The analysis of the STFs suggested that the earthquake was a continuous event with the duration time of 19 s, starting rapidly and ending slowly. Finally, the temporal-spatial distribution of the slip on the fault plane was imaged from the obtained P-STFs and S-STFs using an time domain inversion technique. The maximum slip amplitude on the fault plane was about 7 m. The maximum stress drop was 30 MPa, and the average one over the whole rupture area was 7 MPa. The rupture area was about 85 km long in the strike direction and about 60 km wide in the down-dip direction, which, equally, was 51 km deep in the depth direction. The rupture propagated 50 km eastwards and 35 km westwards. The main portion of the rupture area, which has the slip amplitude greater than 0.5 m, was of the shape of an ellipse, its major axis oriented in the slip direction of the fault, which indicated that the rupture propagation direction was in accordance with the fault slip direction. This phenomenon is popular for strike-slip faulting, but rather rare for thrust faulting. The eastern portion of the rupture area above the initiation point was larger than the western portion below the initiation point, which was indicative of the asymmetrical rupture. In other words, the rupturing was kind of unilateral from west to east and from down to up. From the snapshots of the slip-rate variation with time and space, the slip rate reached the largest at the 4th second, that was 0.2 m/s, and the rupture in this period occurred only around the initiation point. At the 6th second, the rupture around the initiation point nearly stopped, and started moving outwards. The velocity of the westward rupture was smaller than that of the eastward rupture. Such rupture behavior like a circle mostly stopped near the 15th second. After the 16th second, only some patches of rupture distributed in the outer region. From the snapshots of the slip variation with time and space, the rupture started at the initiation point and propagated outwards. The main rupture on the area with the slip amplitude greater than 5 m extended unilaterally from west to east and from down to up between the 6th and the 10th seconds, and the western segment extended a bit westwards and downwards between the 11th and the 13th seconds. The whole process lasted about 19 s. The rupture velocity over the whole rupture process was estimated to be 3.3 km/s. Foundation item: 973 Project (G1998040705) from Ministry of Science and Technology, P. R. China, and the National Science Foundation of China under grant No.49904004. Contribution No. 02FE2026, Institute of Geophysics, China Seismological Bureau.     

Spatial damage distribution of August 16, 2003, Inner Mongolia, China, M S=5.9 earthquake and analysis

The spatial damage distribution of August 16, 2003, Inner Mongolia, China, M _S=5.9 earthquake is summarized through field investigation. The moment tensor solution and focal mechanism are inverted using the digital long-period waveform records of China Digital Seismograph Network (CDSN). The relation between the spatial damage distribution and focal mechanism is analyzed according to the focal mechanism, the aftershock distribution and the spatial damage distribution. The possible relation between the characteristics of ground motion and the tectonic background of the source region is discussed in terms of the global ground motion records, historical earthquake documents and the damage distribution. Investigation reveals that the meizoseismal region is in east-west direction, which is consistent with the nodal plane of focal mechanism inversion. The meizoseismal area is relatively large and the damage of single-story adobe houses or masonry houses is more severe. This may have relations with local seismotectonic environment. Foundation item: The Special Funds for Major State Basic Research Project (2002CB412706). Contribution No. 05FE3009, Institute of Geophysics, China Earthquake Administration.     

The source rupture feature of the southern Taiwan Straits earthquake of September 16, 1994 ( M S7.3) & the analysis of earthquake circumstance in southeastern coast of China

According to the source mechanism of the main shock and the distribution feature of the aftershocks occurring in the southern Taiwan Straits on Sept. 16, 1994, in this paper the authors analysed the source rupture feature of the major earthquake, demonstrated that this seismic sequence possessed the characteristics of a large intraplate earthquake. And according to the seismotectonic background and the historical seismicity in the area, the authors clarified the active characteristics of the seismically active belts along northwestern direction and analysed preliminarily the earthquake circumstance in the southeastern coast of China.     

Source process of the 14 November 2001 western Kunlun Mountain M S=8.1 earthquake

Based on digital teleseismic P-wave seismograms recorded by 28 long-period seismograph stations of the global seismic network, source process of the November 14, 2001 western Kunlun Mountain M _S=8.1 (M _W=7.8) earthquake is estimated by a new inversion method. The result shows that the earthquake is a very complex rupture event. The source rupture initiated at the hypocenter (35.95°N, 90.54°E, focal depth 10 km, by USGS NEIC), and propagated to the west at first. Then, in several minutes to a hundred minutes and over a large spatial range, several rupture growth points emerged in succession at the eastern end and in the central part of the finite fault. And then the source rupture propagated from these rupture growth points successively and, finally, stopped in the area within 50 km to the east of the centroid position (35.80°N, 92.91°E, focal depth 15 km, by Harvard CMT). The entire rupture lasted for 142 s, and the source process could be roughly separated into three stages: The first stage started at the 0 s and ended at the 52 s, lasting for 52 s and releasing approximately 24.4% of the total moment; The second stage started at the 55 s and ended at the 113 s, lasting for 58 s and releasing approximately 56.5% of the total moment; The third stage started at the 122 s and ended at the 142 s, lasting for 20 s and releasing approximately 19.1% of the total moment. The length of the ruptured fault plane is about 490 km. The maximum width of the ruptured fault plane is about 45 km. The rupture mainly occurred within 30 km in depth under the surface of the Earth. The average static slip in the underground rocky crust is about 1.2 m with the maximum static slip 3.6 m. The average static stress drop is about 5 MPa with the maximum static stress drop 18 MPa. The maximum static slip and the maximum stress drop occurred in an area within 50 km to the east of the centroid position. Foundation item: Joint Seismological Science Foundation of China (103066) and Foundation of the Seismic Pattern and Digital Seismic Data Application Research Office of Institute of Earthquake Science of the China Earthquake Administration.     

Moment tensor inversion and source rupture process of the September 27, 2003 <Emphasis Type="Italic">M</Emphasis><Subscript>S</Subscript>=7.9 earthquake occurred in the border area of China,Russia and Mongolia

We conducted moment tensor inversion and studied source rupture process for M _S=7.9 earthquake occurred in the border area of China, Russia and Mongolia on September 27 2003, by using digital teleseismic P-wave seismograms recorded by long-period seismograph stations of the global seismic network. Considering the aftershock distribution and the tectonic settings around the epicentral area, we propose that the M _S=7.9 earthquake occurred on a fault plane with the strike of 127°, the dip of 79° and the rake of 171°. The rupture process inversion result of M _S=7.9 earthquake shows that the total rupture duration is about 37 s, the scalar moment tensor is M ₀=0.97×10²⁰ N·m. Rupture mainly occurred on the shallow area with 110 km long and 30 km wide, the location in which the rupture initiated is not where the main rupture took place, and the area with slip greater than 0.5 m basically lies within 35 km deep middle-crust under the earth surface. The maximum static slip is 3.6 m. There are two distinct areas with slip larger than 2.0 m. We noticed that when the rupture propagated towards northwest and closed to the area around the M _S=7.3 hypocenter, the slip decreased rapidly, which may indicate that the rupture process was stopped by barriers. The consistence of spatial distribution of slip on the fault plane with the distribution of aftershocks also supports that the rupture is a heterogeneous process owing to the presence of barriers.     

2003年9月27日中蒙俄交界处7.9级大震的预测回顾

回顾了我们对2003年9月27日中蒙俄交界处7．9级地震(矩震级为8级)的预测。从Corioli力观点讨论了该震的强余震(矩震级7．3级)。     

Investigation of rupture process of the 1999 M =5.4 Xiuyan, Liaoning, earthquake sequence

Conclusions The sequence of the November 29, 1999 Xiuyan, Liaoning, earthquake withM _S=5.4 is relocated, and its rupture process is analyzed. Results are as follows: The rupture extended mainly before the January 12, 2000,M _S=5.1 earthquake. There are two phases of rupture extending: The first phase was before the November 29, 1999,M _S=5.4 earthquake, epicenters were situated within a small region with a dimension of about 5 km, and the focal depth increased. It shows that the rupture mainly extended from shallow part to deep in the vertical direction. The second phase was between theM _S=5.4 earthquake and theM _S=5.1 earthquake, earthquakes migrated along southeast, the focal depth decreased. It indicates that the rupture extended along southeast and from deep to shallow part. Foundation item: The Project of “Mechanism and Prediction of the Strong Continental Earthquake” (95-13-05-04). Contribution No. 01FE2017, Institute of Geophysics, China Seismological Bureau.     

Changes in source parameters of foreshocks and aftershocks of the 2001 Ms=6.0 Yajiang,Sichuan, earthquake

In this paper changes in focal mechanisms) parameters of wave spectra, and stress drops for the Ms=5.0 forcshock and Ms=6.0 mainshock in February 2001 in Yajiang County, Sichuan, and seismicity in cpiccntral region are studied. Comparison of focal mechanisms for the Yajiang earthquakes with distribution patterns of aftcrshocks, the nodal plane Ⅰ, striking in the direction of NEN, of the Yajiang M=5.0 event is chosen as the faulting plane, the nodal plane Ⅱ, striking in the direction of WNW, of the M=6.0 event as the faulting plane. The strikes of the two faulting planes are nearly perpendicular to each other. The level of stress drops in the cpicentral region before the occurrence of the M=6.0 earthquake increases, which is consistent with increase of seismicity in the epicentral region. The rate decay of the Yajiang earthquake sequence, changes in wave spectra for foreshocks and aftershocks,and focal mechanisms are complex.     

A source extent analysis of the Lancang earthquake ( M s=7.6) of 1988

Source extent parameters of the 1988 Lancang earthquake (M _s=7.6) were estimated by computing the second-central moment of displacement pulses of far field long period SH waves. We inverted the source duration T, the fault length L and the directivity parameter D by the least squares, and obtained that T=11.77 s, D=15.05 km·s, and L=70.94 km. We also find that this event is a symmetrically bilateral rupture and fault segments in two opposite rupture directions have the same value of 35 km in length. Combining analysis of aftershock distribution, the results would imply that there is an area in the joint part of the rupture area of two main shocks, which is not broken yet during the main shocks. Due to the dislocation accompanied during the main shocks, the strain would be rearranged. The joint part has enough strength to accumulate enough strain energy to excite a larger aftershock.     

Inversion of long-period body-wave data for the source process of the Gonghe, Qinghai, China earthquake

Long period body waves data recorded by the China Digital Seismograph Network (CDSN) are inverted for the seismic moment tensors of the April 26, 1990, Gonghe, QinghaiM _S=6.9 earthquake and itsM _S=5.0 after-shock occurred on May 7, 1990. In the inversion, the generalized reflection-transmission coefficient matrix method is used to generate Green’s function. From the inversion it is obtained that the rupture process of theM _S=5.0 aftershock is relatively simple, and that of the main shock is rather complex. There are at least two events during main shock rupture process with an interval about 35 seconds. The focal mechanisms of two events are roughly the same as that of the aftershock, all of them were mainly reverse dip-slipping faulting with minor left-lateral strike-slip motion. These results indicate that the Gonghe earthquake was the result of the farther extension of one NWW-SEE striking buried fault on the southern margin of Gonghe basin from shallower depth to deeper depth and from NW to SE under the action of a nearly horizontal NE direction compressive stress. Contribution No. 95A0111, Institute of Geophysics, SSB, China.     

An analysis of anomalous weather and climate around the M S=6.2 earthquake of Zhangbei region

Through analysis we found that some mesoscale anomalous regions (10¹–10² km) of meteorological parameters such as the special drought areas, unusual warm areas, the largest snowfall center, low pressure area together with the epicenter area of M _S=6.2 Zhangbei earthquake on January 10, 1998 are located at the same area, i.e. there appears the “Five areas corresponding” phenomenon. Meanwhile, three times of low pressure evolution are generated and develop in the earthquake area in five days after the occurrence of the earthquake. The abnormal variation of the lower limit of frozen soil layer shows indirectly that unusual warm in earthquake areas are related to the upward thermal conduction from the deeper layer of earth surface.     

智利M_W8.3地震与M_W8.8地震的震源过程及其引起的库仑应力特征

2015年9月17日6时54分32秒(北京时间)智利中部伊拉佩尔附近(震中31.57°S,71.67°W)发生了一次M_w8.3大地震,在此次地震震中以南约500 km处的马乌莱地区曾于2010年2月27日14时34分11秒发生过一次M_w8.8强震(震中36.12°S,72.90°W),两次地震余震分布区之间有约75 km的地震空区.本文利用远场体波与面波波形,基于有限断层模型,反演了这两次地震的震源破裂过程.结果显示这两次地震均为逆冲型大地震,2015年伊拉佩尔M_w8.3地震的平均滑动角度为107°,平均滑动量为2.43 m,平均破裂速度为1.82 km·s~(-1),标量地震矩为3.28×10~(21)Nm,95%的标量地震矩在104 s内得到了释放.最大滑动量约8 m,位于沿走向75 km,深度8 km处.2010年马乌莱M_w8.8地震的平均滑动角度为109°,平均滑动量为4.95 m,平均破裂速度1.90 km·s~(-1),标量地震矩为1.86×10~(22)Nm,95%的标量地震矩在121 s内得到了释放.最大滑动量约12.5 m,位于沿走向100 km,深度21 km处.2015年伊拉佩尔M_w8.3地震浅部更大的滑动量应该是其引起了较大海啸的一个原因.基于破裂滑动分布,我们计算了这两次地震引起的周边俯冲带上静态库仑应力变化,结果显示两次地震均显著增加了周边俯冲带上的库仑应力,2010年马乌莱地震使得2015.年伊拉佩尔地震震源区附近的库仑应力增加了(0.01~0.15)×10~5Pa,从应力积累的角度看,2010年马乌莱地震有利于2015年伊拉佩尔地震的发生,对后者的发生起到了促进作用.