Inversion of near-field waveform data for earthquake source rupture process ( II ): Inversion of Chi Chi earthquake source, 20 September, 1999, Taiwan

The new inversion algorithm developed based on the recent progress in the nonlinear programming study by us is used to invert the earthquake source process of Chi Chi earthquake M _w7.6, 20 Semptember, 1999, Taiwan. A curve fault model is constructed in our inversion to make the fault model close to the real rupturing fault to reduce the influence from the discrepancy between the constructed fault model and the real rupturing fault. The results show that (1) the rupture process of the Chi Chi earthquake source lasted about 32 seconds and the main faulting occurred between 6th to 21st second after the start of the ruptures and the high slip area were mainly located at the northern segment of the fault. (2) The slip was dominated by thrust faulting. The average rake angle was 64.5°, which was very consistent with those inverted by USGS, Harvard and CWB (Central Weather Bureau of Taiwan). The amount of the moment inverted in this paper was 7.76×10²⁰ NM, which was a slightly bigger than those inverted by USGS and Harvard. (3) A clear nucleation step existed in the source faulting process and it lasted about 6 seconds. The moment release rate accelerated obviously at the end of the nucleation step. The faulting started from the southern segment and mainly occurred at the northern segment after 10 seconds. At the end of this paper, we analyzed the reliability of the inversion result via comparing with the GPS observations and discussed its scientific signification.     

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

Introduction An earthquake of MS=7.8 occurred near the Gujarat of India on January 26, 2001, which was one of the most deadly earthquakes since there was the record in the Indian history (Bendick, et al, 2001; Gupta, et al, 2001). The USGS of USA determined the origin time of the earthquake to be 3h16min41s (UTC), and the epicenter location to be 70.32篍, 23.40篘. Shortly after the earthquake, the moment tensor solutions or focal mechanisms and other related parameters were offered by s…     

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.     

四川芦山 MS7．0地震震源机制解

使用中国数字地震台网记录的区域和远震宽频带波形,通过频率域和时间域多步反演,研究四川芦山 MS 7．0地震基于点源模型的震源机制解和有限断层模型。考虑到使用不同波形资料类型、盆地效应及简化的一维速度模型等给反演震源参数带来的影响,经测试比较,结果表明：使用区域波形和本区域简化一维速度模型,波形拟合误差最小。此次地震表现为高倾角纯逆冲型事件矩心在水平方向上位于震中偏南西向约4．5 km,最佳波形拟合矩心深度约17 km,矩震级 MW约6．6。推断此地震大多数能量主要在震源偏南西约4．5 km 处释放。     

A combined study of macroseismic data and focal mechanisms applied to the West-Bohemian earthquake,Czechoslovakia

The main shock of the West-Bohemian earthquake swarm, Czechoslovakia, (magnitudem=4.5, depthh=10 km) exhibits an irregular areal distribution of macroseismic intensities 6° to 7° MSK-64. Four lobes of the 6° isoseismal are found and the maximum observed intensity is located at a distance of 8 km from the instrumentally determined epicentre. This distribution can be explained by the energy flux of the directS wave generated by a circular source, the hypocentral location and focal mechanism of which are taken from independent instrumental studies. The theoretical intensity, which is assumed to be logarithmically proportional to the integrated squared ground-motion velocity (i.e.,I=const+log v ² (t)dt), fits the observed intensity with an overall root-mean-square error less than 0.5°. It is important that the present intensity data can also be equally well explained by the isotropic source. The fit was attained by means of a horizontally layered model though large fault zones and an extended sedimentary basin suggest a significant lateral heterogeneity of the epicentral region. The results encourage a broader application of the simple modelling technique used.     

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.     

The 14 August 2003 Lefkada Island (Greece) earthquake: Focal mechanisms of the mainshock and of the aftershock sequence

The central area of the Ionian Sea is dominated by the Cephalonia Transform Fault Zone (CTFZ) with a pronounced dextral strike-slip component of motion. The CTFZ has two main segments: the Lefkada Segment (LS) in the north and the Cephalonia Segment (CS) in the south. On 14 August 2003 an M_w 6.2 earthquake ruptured the Lefkada Segment and produced extensive damage, especially to the western coast of the island. Teleseismic waveform modelling revealed the multiple source character of the mainshock, which occurred as three sub-events along a ∼N12^∘E line. The first sub-event occurred at a depth of about 15 km, followed 2.5 s later by the second and largest sub-event at a depth of 11 km and the third sub-event 14 s after the second at a depth of 15 km. The total moment from the body waves of this sequence is about 22.3×10¹⁷ Nt m (M_w 6.2) with a source duration of ∼15 s. The rupture started at the northern part of the Lefkada fault Segment and propagated southwards. The second and third sub-events are located at 7 and 40 km to the south-east in respect to the first sub-event. The focal mechanisms of the two strongest sources indicate strike-slip faulting along the NE–SW trending Lefkada segment (sub-event 2: Strike = 12^∘, Dip = 81^∘, Rake = 174^∘; sub-event 3: Strike = 20^∘, Dip = 63^∘, Rake = −179^∘). Moment tensor inversion applied to regional broad band waveforms obtained from the Greek National Seismographic Network provided focal mechanisms for 23 aftershocks with magnitudes ranging from M_w 3.6 to 5.4. The aftershock sequence presented spatial and temporal variation. The aftershocks were concentrated in two clusters one at the northern part of the activated area and another at the southern part. Most of them were of strike-slip character, following the major tectonic lines of the area, although low-angle thrust and reverse faulting mechanisms were also observed. Thrust and reverse type mechanisms are mainly concentrated in the northern and mainland part of the Lefkada Island which probably indicates the segmented character of the fault and probable activation of adjacent structures.     

2014年云南鲁甸M_W6.1地震:一次共轭破裂地震

烈度与余震分布显示2014年云南鲁甸MW6.1(MS6.5)地震的发震构造较复杂.为深入了解鲁甸地震的发震断层与破裂特征,本文考虑了单一断层破裂和共轭断层破裂的情况,对震中距250km范围内的近震资料(宽频带资料和强震资料)和远震体波资料进行了反演,得到了鲁甸地震的破裂过程,探讨了滑动分布与余震分布之间的关系.根据反演得到的滑动分布、震源时间函数和波形拟合,认为鲁甸地震是一次在北西向主压应力与北东向主张应力的统一应力场下发生的两条共轭断层先后破裂的一次复杂地震事件.在破裂开始后0~2s,破裂主要发生在ENE—WSW向(近东西向)的断层上,随后NNW—SSE向(近南北向)断层开始破裂,释放了大部分的地震矩.由于近南北向断层南段(即震中以南)的破裂规模较大,且以左旋走滑为主,对近东西向断层的西段起到了一定程度的解锁作用,可能是震中以西无明显主震破裂但存在密集余震分布的主要原因.     

Mechanical features of Shanxi Datong earthquake swarm sequence and their numerical simulation by using finite element method

The seismicity characteristics of the earthquake swarm occurred in Datong area in Shanxi Province during 1989–1993 period are studied in detail. The focal mechanisms and rupture processes of the principal earthquakes in it are also estimated in this paper. A mechanical model of fracture system of asymmetrical conjugate array is suggested from the view point of fracture propagation in order to simulate the mechanical features in the seismic source respect of the Datong swarm sequence. The computation results of displacement field, stress field and strain energy distribution from this model by using finite element method are presented. In addition, some recorded data of geothermal precursors before these three climaxes of the Datong earthquake swarm sequence are discussed mainly in this paper. Contribution No. 94A0093, Institute of Geophysics, SSB, China.     

Inversion of near-field waveform data for earthquake source rupture process (Ⅱ): Inversion of Chi Chiearthquake source, 20 September, 1999, Taiwan

The new inversion algorithm developed based on the recent progress in the nonlinear programming study by us is used to invert the earthquake source process of Chi Chi earthquake Mw7.6, 20 Semptember,1999, Taiwan. A curve fault model is constructed in our inversion to make the fault model close to the real rupturing fault to reduce the influence from the discrepancy between the constructed fault model and the real rupturing fault. The results show that (1) the rupture process of the Chi Chi earthquake source lasted about 32 seconds and the main faulting occurred between 6th to 21st second after the start of the ruptures and the high slip area were mainly located at the northern segment of the fault. (2) The slip was dominated by thrust faulting. The average rake angle was 64.5°, which was very consistent with those inverted by USGS, Harvard and CWB (Central Weather Bureau of Taiwan). The amount of the moment inverted in this paper was 7.76×1020 NM, which was a slightly bigger than those inverted by USGS and Harvard. (3) A clear nucleation step existed in the source faulting process and it lasted about 6 seconds. The moment release rate accelerated obviously at the end of the nucleation step. The faulting started from the southern segment and mainly occurred at the northern segment after 10 seconds. At the end of this paper, we analyzed the reliability of the inversion result via comparing with the GPS observations and discussed its scientific signification.     

应用国家数字地震台网的宽频带资料测定国内中强地震的震源机制解

利用剪切位错点源作为震源模型,采用横向均匀的水平层状介质模型作为地壳近似模型,使用F-K波数积分法计算格林函数并拟合理论地震图.在已有理论方法的基础上,利用国家数字地震台网所记录的宽频带数字资料,选取了2002～2003年间发生在国内Ms≥5.4的5个地震事件进行震源机制解测定.并且,将测定结果与HRV的矩心矩张量解、USGS的地震矩张量解、中国地震信息网的速报震源机制解分别进行了对比,通过实际计算表明测定结果是可靠的.从而初步验证了在中国国家数字地震台网中使用F-K波数积分法测定震源机制解的可行性和准确性.     

The 1992 Cairo earthquake: A case study of a small destructive event

On 12 October 1992, an earthquake, magnitude mb = 5.9 and M s = 5.2, hit the City of Cairo, Egypt. It was this century's largest earthquake in northern Egypt with related destruction in the City of Cairo, the Nile Valley and the Nile Delta areas. Our source parameter determinations show that the 1992 earthquake had a normal faulting mechanism, seismic moment M o = 5.2 × 1017 Nm, centroid depth of 23 km and a source time function duration of 3 seconds. The mechanism is compared with those corresponding to two other events that occurred in the northern Red Sea. The similarity between the mechanisms as well as the spatial distribution of the geological faults around Cairo suggest seismic activity along the extension of the stress field of the Red Sea rift system to the area around the City of Cairo. This situation affects the level of seismic hazard in the Cairo area. The 1992 earthquake belongs to an unusual class of relatively small, M w > 6.0, yet destructive earthquakes. The damage caused by these events is usually attributed to their shallow focal depth, 5 km, and to amplification of seismic waves in the local soil beneath the damaged structures. However, the Cairo earthquake deviates from other events of this class since the focal depth was determined to be 23 km. We calculated synthetic accelerograms for the 1992 earthquake with the loose sediments observed in the Nile Valley, and show that this enhanced the amplitude of the acceleration by a factor of two. However, the determined accelerations, about 0.5 m s-2, cannot alone explain the relatively large amount of damaged structures. Hence, a major cause to the destruction is likely the poor state of construction of the Cairo buildings.     

2021年11月17日江苏省盐城市大丰区海域MS 5.0地震快速测定与应急产品产出

2021年11月17日13时54分（北京时间）江苏盐城市大丰区海域发生M_S 5.0地震。江苏盐城、南通等地区震感强烈,上海普遍有感,山东和浙江等部分地区亦有震感。上海地震台在震后7 min发布人工速报结果,并启动地震应急产品产出工作,产品包括地震基本参数、历史地震、地震构造、震源机制、仪器烈度和余震精定位等数据。本次地震发生在黄海南部坳陷地区,震源机制解表明该地震为一次走滑型事件,断层呈NE走向,余震主要沿主震NE方向拓展约20 km,深度集中在15 km以内范围。     

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.     

2011年1月19日安徽安庆M4.8地震的震源机制

我们用5个区域数字地震台网84个台站记录到的清楚的149个P波(P<,n>、R<,g>)的初动符号,求得了2011年1月19日安庆M4.8地震的震源机制解.节面Ⅰ:走向223°,倾角90°,滑动角-90°;节面Ⅱ:走向45°,倾角0°,滑动角-90°.根据野外考察得出的这次地震极震区长轴走向结果,可以推断,223°走向...     

Source time functions of the Gonghe，China earthquake retrieved from long－period digital waveform data using empirical Green＇s function technique

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A priliminary study on the sequence characteristics and focal mechanism solution of the Jiashi strong earthquake swarm

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Seismotectonics and seismic quietness of the Oranie region (Western Algeria): The Mascara earthquake of August 18th 1994, Mw = 5.7, Ms = 6.0

The plate dynamics in the central western Mediterranean region is characterised by a collision between the Eurasian and African plates. In response to this dynamics, many systems of faults and folds having a NE-SW and E-W trending have been generated along the Tellian Atlas of Algeria. The Oranie region (north western Algeria) has experienced some significant earthquakes in the last centuries, the most important one is that of Oran city on February 9^th 1790, I_o = XI which destroyed the town completely and caused the loss of many lives. Since 1790 no other event was so disastrous except that of August 18^th 1994, M_w = 5.7, which struck Mascara province (Algeria) at 01 h 13 mn GMT. Since the beginning of this century the region has been dominated by a seismic quietness. Thus, no event with magnitude larger than 5.5 have occurred in this area. In relation with this recent event, a seismotectonic framework summarising the tectonic, seismicity and focal solution results is presented. The Maximum Observed Intensities Map (MOI) made for Algeria (Bezzeghoud et al., 1996) is also used to show that the Mascara region is located in an VIII-X intensity zone, which explain partially the casualties caused by the 18/08/1994 (M_w = 5.7) earthquake. This earthquake is not anomalous compared to historical records but is unusual compared to recorded seismicity of this century. The seismotectonic map made in this study and also the review of the focal solutions given by the EMSC, Harvard, and other authors shows that our event is probably associated with a source belonging to a system of faults located in the vicinity of the village of Hacine where the maximum damage was observed.     

The November 14, 2001 west of Kunlun Mountain Pass earthquake: An earthquake with unsaturated surface wave magnitude

Introduction According to the Rapid Earthquake Information Release of CNDSN (Department of Earth- quake Monitoring and Prediction, China Earthquake Administration, 2002), an earthquake with surface wave magnitude MS=8.1 shook west of Kunlun Mountain Pass (KMP) at the juncture of Xinjiang, Qinghai and Xizang on November 14, 2001. This is the largest and the only MS>8.0 earthquake in Chinese mainland over 50 years since the August 15, 1950 MS=8.6 (MW=8.6) Chayuearthquake in Tibeta…     

2007年8月15日秘鲁M_W8.0地震的破裂震相记录特征

对2007年8月15日秘鲁近海MW8.0地震在全球数字地震仪台网(GSN)83个台的宽频带数字记录进行分析,在短周期(SP)、中长周期(SK)和长周期(LP)记录上均识别出在震源破裂过程中由主要破裂激发出的破裂震相。由破裂震相分析结果可以看出:该地震是一个由4次主要破裂构成的地震事件,第2次破裂、第3次破裂和第4次破裂与第1次破裂之间的时间间隔平均约9.1 s、48.5 s和63.3 s,与震源破裂时间函数的结果一致。4次主要破裂激发的破裂震相在GSN台站可识别的震中距范围为1.4°—153.8°。本文给出的破裂震相记录特征,为分析识别其他地震的破裂震相提供了参考。从地震记录图上直接识别破裂震相有助于快速判断震源破裂过程,有助于快速判断震灾损失。