A Preliminary Analysis of the Seismogenic Structure of the Akto MS6.7 Earthquake Sequence on November 25, 2016

In this study,data from the Xinjiang regional network and IRIS shared global stations are used to relocate the Akto M_S6. 7 earthquake sequence on November 25,2016 by using double difference location method. Three earthquakes of M_S4. 8,M_S6. 7 and M_S5. 0 are inverted by using the g CAP method,and the focal mechanism solutions are obtained.According to the results of relocating,the location of the main shock is 39. 22°N,73. 98°E,the distribution of the earthquake sequence is about 70 km in length,and the focal depth is mainly within the range of 5-20 km. The plane and depth profiles of the earthquake sequence show that aftershocks extended in SEE direction after the main shock and the dip angle of fault plane is steep. Focal mechanism results show that the three earthquakes are characterized by strike-slip movement. Based on the results of field geological investigation,it is inferred that the seismogenic fault of the Akto earthquake is Muji fault,which is located at the northernmost end of the Kongur extensional system.The possible cause of this earthquake is that the Indian Plate continues to push northward,and during this compression process,the Indian Plate is affected by the clockwise rotation of the Tarim basin,which causes the accumulation of right-lateral action of the Muji fault,resulting in this earthquake.     

Relocation of the MS5.7 Earthquake Sequence in Songyuan, Jilin Province, 2018 and the Analysis of Its Seismogenic Structure

The 2018,Songyuan,Jilin M_S5. 7 earthquake occurred at the intersection of the FuyuZhaodong fault and the Second Songhua River fault. The moment magnitude of this earthquake is M_W5. 3,the centroid depth by the waveform fitting is 12 km,and it is a strike-slip type event. In this paper,with the seismic phase data provided by the China Earthquake Network, the double-difference location method is used to relocate the earthquake sequence,finally the relocation results of 60 earthquakes are obtained. The results show that the aftershock zone is about 4. 3km long and 3. 1km wide,which is distributed in the NE direction. The depth distribution of the seismic sequence is 9km-10 km. 1-2 days after the main shock,the aftershocks were scattered throughout the aftershock zone,and the largest aftershock occurred in the northeastern part of the aftershock zone. After 3-8 days,the aftershocks mainly occurred in the southwestern part of the aftershock zone. The profile distribution of the earthquake sequence shows that the fault plane dips to the southeast with the dip angle of about 75°. Combined with the regional tectonic setting,focal mechanism solution and intensity distribution,we conclude that the concealed fault of the Fuyu-Zhaodong fault is the seismogenic fault of the Songyuan M_S5. 7 earthquake. This paper also relocates the earthquake sequence of the previous magnitude 5. 0 earthquake in 2017. Combined with the results of the focal mechanism solution,we believe that the two earthquakes have the same seismogenic structure,and the earthquake sequence generally develops to the southwest. The historical seismic activity since 2009 shows that after the magnitude 5. 0 earthquake in 2017,the frequency and intensity of earthquakes in the earthquake zone are obviously enhanced,and attention should be paid to the development of seismic activity in the southwest direction of the earthquake zone.     

Mainshocks and aftershocks of the 2002 molise seismic sequence, southern Italy

In October and November 2002, the Molise region (southern Italy) was struck by two moderate magnitude earthquakes within 24 hours followed by an one month long aftershocks sequence. Soon after the first mainshock (October 31st, 10.32 UTC, M_w 5.7), we deployed a temporary network of 35 three-component seismic stations. At the time of occurrence of the second main event (November 1st, 15.08 UTC, M_w 5.7) the eight local stations already installed allowed us to well constrain the hypocentral parameters. We present the location of the two mainshocks and 1929 aftershocks with 2 < M_L < 4.2. Earthquake distribution reveals a E-trending 15 km long fault system composed by two main segments ruptured by the two mainshocks. Aftershocks define two sub-vertical dextral strike-slip fault segments in agreement with the mainshock fault plane solutions. P- and T-axes retrieved from 170 aftershocks focal mechanisms show a coherent kinematics: with a sub-horizontal NW and NE-trending P and T-axes, respectively. For a small percentage of focal mechanisms (∼ 10%) a rotation of T axes is observed, resulting in thrust solutions. The Apenninic active normal fault belt is located about 80 km westward of the 2002 epicentral area and significant seismicity occurs only 20-50 km to the east, in the Gargano promontory. Seismic hazard was thought to be small for this region because neither historical earthquake are reported in the Italian seismic catalogue or active faults were previously identified. In this context, the 2002 seismic sequence highlights the existence of trans-pressional active tectonics in between the extensional Apenninic belt and the Apulian foreland.     

Relocation and seismogenic structure of the 1998 Zhangbei-Shangyi earthquake sequence

Introduction The January 10, 1998 Zhangbei-Shangyi, Hebei Province, earthquake has been the third large event of magnitude 6.0 and greater since the 1976 great Tangshan earthquake of magnitude 7.8 in the northern China (33皛42癗, 110皛124癊). Before this event, there were only two events of magnitude 6.0 and greater occurred in or around the Tangshan area since 1976: the M=6.9 Ninghe, Tianjin, earthquake of November 15, 1976 and the M=6.2 Hangu, Tianjin, earthquake of May 12, 1977. The …     

2020年云南巧家MS5.0地震微震检测及发震构造初步探讨

利用匹配定位方法对2020年5月18日云南巧家Ms5.0地震震后24h震源附近台站.记录的连续波形进行遗漏地震扫描和定位,共识别出327个地震事件,约为台网目录的2.4倍,最小完整震级由最初的ML1.9降至ML1.1.随后,依据最新目录计算了震后震源区的b值,并结合余震展布形态,初步分析此次地震发震构造.研究结果显示,...     

The 1990 Zaisang earthquake of magnitude 7.3 in Kazakstan

The Zaisang earthquake (M _s=7.3) on June 14, 1990 occurred in the boundary between China and Kazakstan. During the great shock, 3 persons lost their lives, 30 people had been hurt and 340 houses collapsed. The intensity of the epicentre is VIII Degree. The economic losses equals to about 320 000 000 Yuan. This large earthquake was occurred on a new born fault, it belongs to main shock-aftershock type earthquake. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 360–365, 1993.     

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.     

Determination of focal depth by two waveformbased methods: A case study for the 2008Panzhihua earthquake

With the 2008 M_S6.1 Panzhihua earthquake as a case study, we demonstrate that the focal depth of the main shock can be well constrained with two approaches: (1) using the depth phase sPL and (2) using full waveform inversion of local and teleseismic data. We also show that focal depths can be well constrained using the depth phase sPL with single broadband seismic station. Our study indicates that the main shock is located at a depth of 11 km, much shallower than those from other studies, confirming that the earthquake occurs in upper crust. Aftershocks are located in the depth range of 11-16 km, which is consistent with a ruptured near vertical fault whose width is about 10 km, as expected for an M_S6.1 earthquake.     

The milos island earthquake of March 20, 1992 and its tectonic significance

An earthquake sequence took place on March 20, 1992, in Milos island (Greece) and lasted for about ten days. The main shock registered a magnitudeM _s =5.3 and a depth of 9.6 km. The majority of the events were shallower than 5 km. Theb value of the sequence (b=0.96) is characteristic for tectonic rather than volcanic activity. Geological, tectonic and seismological observations show that in the island of Milos the seismic energy is mainly released along fault zones. Minor swarm activity was also detected.Recent seismic activity is due to the reactivation of the tectonic graben which traverses the central part of the island in NW-SE trend.     

Relocation of the 1998 Zhangbei-Shangyi earthquake sequence using the double difference earthquake location algorithm

Introduction On January 10, 1998, at 11h50min Beijing Time (03h50min UTC), an earthquake of ML=6.2 occurred in the border region between the Zhangbei County and Shangyi County of Hebei Province. In total 87 events with ML3.0 were recorded by Beijing Telemetry Seismic Network (BTSN) before March of 1999. Before relocation the preliminary hypocenters determined by BTSN showed an epicentral distribution of 25 km long and 25 km wide without any predominate orientation. The epicentral a…     

Crustal Heterogeneity in the Source Region of the 2004 Mid Niigata Prefecture Earthquake: Inversion Analysis of Coda Envelopes

The 2004 Mid Niigata Prefecture earthquake (M_JMA 6.8) and its aftershock sequences generated complicated, i.e., several conjugate fault planes in their source region. In order to understand the generating process of these earthquakes, we estimated a 3-D distribution of relative scattering coefficients in the source region. The large slip area during the main shock rupture seems to be bounded by strong heterogeneous zones with larger scattering coefficients. Hypocenters of the main shock and major large aftershocks with M 5-6 classes tend to be located close to stronger scattering areas. We found that one of these strong heterogeneities already existed before the occurrence of the M 5.9 aftershock on November 8. We suppose that heterogeneous structures in the source region of this earthquake sequence affected the initiation and growth of ruptures of the main shock and major large aftershocks.     

The reservoir-associated earthquakes of April 1983 in western Thailand: Source modeling and implications for induced seismicity

On 22 April 1983, a very large area of Thailand and part of Burma were strongly shaken by a rare earthquake (m _b=5.8,M _s=5.9). The epicenter was located at the Srinagarind reservoir about 190 km northwest of Bangkok, a relatively stable continental region that experienced little previous seismicity. The main shock was preceded by some foreshocks and followed by numerous aftershocks. The largest foreshock ofm _b=5.2 occurred 1 week before the main shock, and the largest aftershock ofm _b=5.3 took place about 3 hours after the main shock. Focal mechanisms of the three largest events in this earthquake sequence have been studied by other seismologists using firts-motion data. However, the solutions for the main shock and the largest aftershock showed significant inconsistency with known surface geology and regional tectonics. We reexamined the mechanisms of these three events by using teleseismicP-andS-waveforms and through careful readings ofP-wave first motions. The directions of theP axes in our study range from NNW-SSE to NNE-SSW, and nodal planes strike in the NW-SE to about E-W in agreement with regional tectonics and surface geology. The main shock mechanism strikes 255°, dips 48°, and slips 63.5°. The fault motions during the main shock and the foreshock are mainly thrust, while the largest aftershock has a large strike-slip component. The seismic moment and the stress drop of the mainshock are determined to be 3.86×10²⁴ dyne-cm and 180 bars, respectively. The occurrence of these thrust events appears to correlate with the unloading of the Srinagarind reservoir. The focal depths of the largest foreshock, the main shock, and the largest aftershock are determined to be 5.4 km, 8 km, and 22.7 km, respectively, from waveform modeling and relative location showing a downward migration of hypocenters of the three largest events during the earthquake sequence. Other characteristics of this reservoir-induced earthquake sequence are also discussed.     

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.     

Surface fault traces,fault plane solution and spatial distribution of the aftershocks of the September 13, 1986 earthquake of Kalamata (Southern Greece)

A shallow earthquake ofM _S=6.2 occurred in the southern part of the Peloponnesus, 12 km north of the port of the city of Kalamata, which caused considerable damage. The fault plane solution of the main shock, geological data and field observations, as well as the distribution of foci of aftershocks, indicate that the seismic fault is a listric normal one trending NNE-SSW and dipping to WNW. The surface ruptures caused by the earthquake coincide with the trace of a neotectonic fault, which is located 2–3 km east of the city of Kalamata and which is related to the formation of Messiniakos gulf during the Pliocene-Quaternary tectonics. Field observations indicate that the earthquake is due to the reactivation of the same fault.A three-days aftershock study in the area, with portable seismographs, recorded many aftershocks of which 39 withM _S1.7 were very well located. The distribution of aftershocks forms two clusters, one near the epicenter of the main shock in the northern part of the seismogenic volume, and the other near the epicenter of the largest aftershock (M _S=5.4) in the southern part of this volume. The central part of the area lacks aftershocks, which probably indicates that this is the part of the fault which slipped smoothly during the earthquake.     

Rupture process of the M L=4.1 earthquake in Huailai Basin on July 20, 1995

On July 20, 1995, an earthquake of M _L=4.1 occurred in Huailai basin, northwest of Beijing, with epicenter coordinates 40.326°N, 115.448°E and focal depth 5.5 km. Following the main shock, seismicity sharply increased in the basin. This earthquake sequence was recorded by Sino-European Cooperative Huailai Digital Seismograph Network (HDSN) and the hypocentres were precisely located. About 2 hours after the occurrence of the main shock, a smaller event of M _L=2.0 took place at 40.323°N, 115.447°E with a focal depth of 5.0 km, which is very close to the main shock. Using the M _L=2.0 earthquake as an empirical Green’s function, a regularization method was applied to retrieve the far-field source-time function (STF) of the main shock. Considering the records of HDSN are the type of velocity, to depress high frequency noise, we removed instrument response from the records of the two events, then integrated them to get displacement seismogram before applying the regularization method. From the 5 field stations, P phases in vertical direction which mostly are about 0.5 s in length were used. The STFs obtained from each seismic phases are in good agreement, showing that the M _L=4.1 earthquake consisted of two events. STFs from each station demonstrate an obvious “seismic Doppler effect”. Assuming the nodal plane striking 37° and dipping 40°, determined by using P wave first motion data and aftershock distribution, is the fault plane, through a trial and error method, the following results were drawn: Both of the events lasted about 0.1 s, the rupture length of the first one is 0.5 km, longer than the second one which is 0.3 km, and the rupture velocity of the first event is 5.0 km/s, larger than that of the second one which is about 3.0 km/s; the second event took place 0.06 s later than the first one; on the fault plane, the first event ruptured in the direction γ=140° measured clockwise from the strike of the fault, while the second event ruptured at γ=80°, the initial point of the second one locates at γ=−100° and 0.52 km from the beginning point of the first one. Using far-field ground displacement spectrum measurement method, the following source parameters about the M _L=4.1 earthquake were also reached: the scalar earthquake moment is 3.3×10¹³ N·m, stress drop 4.6 MPa, rupture radius 0.16 km. Contribution No. 99FE2022, Institute of Geophysics, China Seismological Bureau. This study is supported by the Chinese Joint Seismological Science Foundation (95-07-411).     

滇西南地区汉母坝-澜沧断裂晚第四纪构造活动的地质地貌证据

通过对汗母坝-澜沧断裂晚第四纪地质、地貌实地调查与测量,并结合前人研究成果,讨论了该断裂晚第四纪最新构造活动特征。综合分析认为,汗母坝-澜沧断裂为一条以右旋走滑为主的全新世活动断裂,长约120 km,整体走向NNW。该断裂活动习性具有明显的分段特征,北段称为汗母坝断裂,是1988年耿马7.2级地震的发震断裂;南段称为澜沧断裂,是1988年澜沧7.6级地震的发震断裂之一。晚第四纪以来其新活动形成了丰富的断错地貌现象,如冲沟和山脊右旋位错、断层沟槽、断层垭口、断层陡坎、断陷凹坑等。根据断裂断错地貌特征的相应资料估计,该断裂晚第四纪右旋走滑速率约为(4.7±0.5) mm/a。     

2022门源MS6.9地震地表破裂带震害特征调查

2022年1月8日,青海省海北藏族自治州门源县发生M_S6.9地震,震中位于青藏高原东北缘地区祁连—海原断裂带的冷龙岭断裂和托勒山断裂构造转换区域(37.77°N,101.26°E)。震后野外现场考察结果表明,此次地震形成的同震地表破裂带总长度约为26 km,整体走向NWW向,破裂性质以左旋走滑局部逆冲为主。断层错动造成的破坏形式以雁列式组合的张裂隙、张剪裂隙、挤压鼓包、断层陡坎等为主。其中,道河至硫磺沟段地表破裂最为强烈,规模大且连续性好,造成的震害最为显著,地表破裂规模向东、西两端逐渐衰减。破裂带穿过区域内多条河流,造成显著的冰面破裂变形,并沿河岸形成一系列的边坡崩塌、滚石等地质灾害。综合破裂带及震害规模分析,宏观震中位于道河至硫磺沟地区。     

Earthquake activity in the Aswan region,Egypt

The November 14, 1981 Aswan earthquake (M _L= 5.7), which was related to the impoundment of Lake Aswan, was followed by an extended sequence of earthquakes, and is investigated in this study. Earthquake data from June 1982 to late 1991, collected from the Aswan network, are classified into two sets on the basis of focal depth (i.e., shallow, or deeper than 10 km). It is determined that (a) shallow seismicity is characterized by swarm activity, whereas deep seismicity is characterized by a foreshock-main shock-aftershock sequence; (b) the b value is equal to 0.77 and 0.99 for the shallow and deep sequences, respectively; and (c) observations clearly indicate that the temporal variations of shallow seismic activity were associated with a high rate of water-level fluctuation in Lake Aswan; a correlation with the deeper earthquake sequence, however, is not evident. These features, as well as the tomographic characteristics of the Aswan region (Awad andMizoue, this issue), imply that the Aswan seismic activity must be regarded as consisting of two distinct earthquake groups.We also relocated the largest 500 earthquakes to determine their seismotectonic characteristics. The results reveal that the epicenters are well distributed along four fault segments, which constitute a conjugate pattern in the region. Moreover, fault-plane solutions are determined for several earthquakes selected from each segment, which, along with the 14 November 1981 main shock, demonstrate a prominent E-W compressional stress.     

Critical evaluation of strong motion in Kocaeli and Düzce (Turkey) earthquakes

Turkey was struck by two major events on August 17th and November 12th, 1999. Named Kocaeli (M_w=7.4) and Düzce (M_w=7.2) earthquakes, respectively, the two earthquakes provided the most extensive strong ground motion data set ever recorded in Turkey. The strong motion stations operated by the General Directorate of Disaster Affairs, the Kandilli Observatory and Earthquake Research Institute of Bogazici University and Istanbul Technical University have produced at least 27 strong motion records for the Kocaeli earthquake within 200 km of the fault. Kocaeli earthquake has generated six motions within 20 km of the fault adding significantly to the near-field database of ground motions for M_w>=7.0 strike–slip earthquakes. The paper discusses available strong motion data, studies their attenuation characteristics, analyses time domain, as well as spectral properties such as spectral accelerations with special emphasis on fault normal and fault parallel components and the elastic attenuation parameter, kappa. A simulation of the Kocaeli earthquake using code FINSIM is also presented.     

Fault deformation anomaly and intermediate and short-term prediction of the Jingtai M s=5.9 earthquake

The time-space distribution characteristics of fault deformation anomaly in the near-source region and its outlying zone in the seismogenic process of the Jingtai M _s=5.9 earthquake occurred on June 6, 2000 in Gansu Province is studied preliminarily. The distribution scope of fault deformation anomaly before the earthquake is wide, the anomaly shape is complicated and the pattern anomalous zone of fault deformation (strain) information index is obvious. The shape and amplitude of fault deformation anomaly in different regions differ significantly, which is closely related with the tectonic location of anomaly. The fault deformation anomaly of α, β, and γ phases along the western segment of Haiyuan fault zone shows the process from the quasi-linearity to non-linearity of fault movement in the near-source region, matches the high-value anomalous area of fault deformation (strain) information index, and reflects the high strain accumulation in the seismogenic region. However, the anomaly of abrupt jump and cusp with a large amplitude occurred in the areas far from the earthquake, such as Liupanshan fault zone which is the tectonic convergent section does not reflect the strain accumulation of its location, maybe it is a sign that the regional tectonic stress field is strengthened in the seismogenic process. Based on the above-mentioned facts and combined with the preliminary summary of experiences and lessons in the intermediate and short-term prediction of the Jingtai M _s=5.9 earthquake, we study and explore the application of fault deformation anomaly to earthquake judgment. Foundation item: National Key Basic Research Development Program (G1998040703 and G1998040705), and State Scientific and Technological Project of the “Ninth Five-Year Plan” (96-913-09-01-02-03 and 96-913-09-02-02-03), China.