The tectonic settings and seismogenic tectonics of the M5.7 Jiujiang earthquake in 2005,Jiangxi Province,China

The M5.7 Jiujiang earthquake in 2005 was a mid-strong one, stronger than expected to occur in the region. This paper discusses the neo-tectonic settings of this earthquake, and it is thought that the earthquake region is located in the transitional belt, a potential area inducing weak to moderately strong earthquakes, between two large different tectonic units. The results of the reconnaissance work and on-the-spot investigation after earthquake indicate that the occurrence of the M5.7 Jiujiang earth- quake is closely related with the NE-trending fault on the western margin of Ruichang Basin. From its controlling to the landforms and Quaternary depositions, geological profiles, ESR dating, etc., the ac- tivity of the Dingjiashan-Langjunshan fault bounding the basin is discussed. It suggests that this fault displays an active one in Middle Pleistocene by the outcrop. Based on the activity of the fault, and the direction and location of the ground fissures, the isoseismal lines and the nodal plane of the focal mechanism solution, it is inferred that the Dingjiashan-Langjunshan fault is the seismogenic tectonics of the M5.7 Jiujiang earthquake, and the intersection point between this fault and the active NW ones is the possible origin of location of this earthquake. Our study shows that this earthquake is not an event exceeding expectation, and that the active and invisible characteristics of the causative fault are typical in the eastern area of China.     

Rupture behavior and deformation localization of the Kunlunshan earthquake (<Emphasis Type="Italic">M</Emphasis><Subscript><Emphasis Type="Italic">w</Emphasis></Subscript>7.8) and their tectonic implications

Earthquake surface rupture is the result of transformation from crustal elastic strain accumulation to permanent tectonic deformation. The surface rupture zone produced by the 2001 Kunlunshan earthquake (M _w 7.8) on the Kusaihu segment of the Kunlun fault extends over 426 km. It consists of three relatively independent surface rupture sections: the western strike-slip section, the middle transtensional section and the eastern strike-slip section. Hence this implies that the Kunlunshan earthquake is composed of three earthquake rupturing events, i.e. the M _w =6.8, M _w =6.2 and M _w ⩽=7.8 events, respectively. The M _w =7.8 earthquake, along the eastern section, is the main shock of the Kunlunshan earthquake, further decomposed into four rupturing subevents. Field measurements indicate that the width of a single surface break on different sections ranges from several meters to 15 m, with a maximum value of less than 30 m. The width of the surface rupture zone that consists of en echelon breaks depends on its geometric structures, especially the stepover width of the secondary surface rupture zones in en echelon, displaying a basic feature of deformation localization. Consistency between the Quaternary geologic slip rate, the GPS-monitored strain rate and the localization of the surface ruptures of the 2001 Kunlunshan earthquake may indicate that the tectonic deformation between the Bayan Har block and Qilian-Qaidam block in the northern Tibetan Plateau is characterized by strike-slip faulting along the limited width of the Kunlun fault, while the blocks themselves on both sides of the Kunlun fault are characterized by block motion. The localization of earthquake surface rupture zone is of great significance to determine the width of the fault-surface-rupture hazard zone, along which direct destruction will be caused by co-seismic surface rupturing along a strike-slip fault, that should be considered before the major engineering project, residental buildings and life line construction. Supported by the National Natural Science Foundation of China (Grant No. 40474037) and the National Basic Research Program of China (Grant No. 2004CB418401)     

Stress interactions between normal faults and adjacent strike-slip faults of 1997 Jiashi earthquake swarm

During a 4-month period starting from 21 January, 1997, an earthquake swarm of seven major events (Ms≥6.0) struck the Jiashi region at the northwestern corner of the Tarim Basin in Xinjiang,, China. Previous relocation studies suggested that these strong earthquakes had occurred along at least two parallel rupture zones. According to the relocated hypocenters and focal mechanisms of the events, we have constructed fault models for these seven earthquakes to calculate the Coulomb stress changes produced by each of these events. Furthermore, we extended our model calculations to include an ad- jacent 1996 Ms=6.9 Artushi earthquake, which occurred one year before the Jiashi earthquake swarm. Our calculations show that the Coulomb stress change caused by the preceding events was around 0.05 MPa at the hypocenter of the 4th event, and higher than 0.08 MPa at the hypocenters of the 2nd, 3rd, 5th and 6th events. Our results reveal a Coulomb stress interactive cycle of earthquake triggering between two adjacent normal and strike-slip faults.     

A seismic gap on the Anninghe fault in western Sichuan,China

Through integrated analyses of time-varying patterns of regional seismicity, occurrence background of strong and large historical earthquakes along active faults, and temporal-spatial distribution of accu- rately relocated hypocenters of modern small earthquakes, this paper analyzes and discusses the im- plication of a 30-year-lasting seismic quiescence in the region along and surrounding the Anninghe and Zemuhe faults in western Sichuan, China. It suggests that the seismic quiescence for ML≥4.0 events has been lasting in the studied region since January, 1977, along with the formation and evaluation of a seismic gap of the second kind, the Anninghe seismic gap. The Anninghe seismic gap has the background of a seismic gap of the first kind along the Anninghe fault, and has resulted from evident fault-locking and strain-accumulating along the fault during the last 30 years. Now, two fault sections either without or with less small earthquakes exist along the Anninghe fault within the An- ninghe seismic gap. They indicate two linked and locked fault-sections, the northern Mianning section and the Mianning-Xichang section with lengths of 65 km and 75 km and elapsed time from the latest large earthquakes of 527 and 471 years, respectively. Along the Anninghe fault, characteristics of both the background of the first kind seismic gap and the seismicity patterns of the second seismic gap, as well as the hypocenter depth distribution of modern small earthquakes are comparable, respectively, to those appearing before the M=8.1 Hoh Xil earthquake of 2001 and to those emerging in the 20 years before the M=7.1 Loma Prieta, California, earthquake of 1989, suggesting that the Anninghe seismic gap is tending to become mature, and hence its mid- to long-term potential of large earthquakes should be noticeable. The probable maximum magnitudes of the potential earthquakes are estimated to be as large as 7.4 for both the two locked sections of the Anninghe fault.     

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)     

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.     

Surface rupture zone of the 1303 Hongtong M=8 earthquake, Shanxi Province

Based on the latest displacement of Huoshan piedmont fault, Mianshan west-side fault and Taigu fault obtained from the beginning of 1990‘s up to the present, the characteristics of distribution and displacement of surface rupture zone of the 1303 Hongtong M = 8 earthquake, Shanxi Province are synthesized and discussed in the paper. If Taigu fault, Mianshan west-side fault and Huoshan piedmont fault were contemporarily active during the 1303 Hongtong M = 8 earthquake, the surface rupture zone would be 160 km long and could be divided into 3 segments, that is, the 50-km-long Huoshan piedmont fault segment, 35-km-long Mianshan west-side fault segment and 70-km-long Taigu fault segment, respectively. Among them, there exist 4 km and 8 km step regions. The surface rupture zone exhibits right-lateral features. The displacements of northern and central segments are respectively 6～7 m and the southern segment has the maximum displacement of 10 m. The single basin-boundary fault of Shanxi fault-depression system usually corresponds to M ≈ 7 earthquake, while this great earthquake (M = 8) broke through the obstacle between two basins. It shows that the surface rupture scale of great earthquake is changeable.     

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.     

Studying the viscosity of lower crust of Qinghai-Tibet Plateau according to post-seismic deformation

The viscosity of lower crust of Qinghai-Tibet Plateau on earth should be determined. It has become a predominant problem in quantitative research on geodynamics. Its order of magnitude will have a great influence on the results of quantitative modeling. To obtain the viscosity of lower crust of Qinghai-Tibet Plateau, this parameter was calculated by three methods. The first is based on the estimation on the temperature state of Qinghai-Tibet Plateau in the deep part, and the viscosity of lower crust of northern Plateau was recomputed with strain rate derived from rheology law and GPS observation. Effective viscosity of middle crust in Kunlun region is between 10²⁰ and 10²² Pa·s, and that of lower crust is between 10¹⁹ and 10²¹ Pa·s; the second is based on three kinds of rheological models used to fit the post-seismic deformation recorded by cross-over fault GPS sites set after M _s8.1 Kunlun earthquake in 2001. The viscosity of lower crust obtained by this method is of 10¹⁷ Pa·s order of magnitude. However, higher viscosity is required to fit the data of south fault better, and the lower one is required to fit the data of north fault better. The viscosity of lower crust, which was obtained by fitting the cross-over fault post-seismic deformation after M _s7.6 Luhuo earthquake in 1973, is of 10¹⁹ Pa·s order of magnitude. Non-linear relationship between effective viscosity and strain rate is ignored in the former research of effective viscosity. This research shows the difference of effective viscosity obtained from laboratory experiment, and shorter and longer time post-seismic deformation after large earthquakes can be explained in phase. Supported by National Basic Research Program of China (Grant No. 2004CB418405), National Natural Science Foundation of China (Grant No. 40774048), Important Direction Item of Knowledge Innovation Project of Chinese Academy of Sciences (Grant No. KZCX2-YW-123) and Basic Scientific Research Project of Earthquake (Grant No. 02076902-05)     

1994年9月16日台湾海峡南部7.3级地震的构造环境和震前地震活动图象

从构造单元，活动断裂，地球物理场，海底地形等方面分析了１９９４年９月１６日台湾海峡７．３级地震发生的构造环境。研究认为，地震发生在陆壳向过渡壳变化的位置上。历史上地震活动沿深部构造变异带分布，该区地质构造，震源深度，地震类型，地震波特性等都有别于海峡中，北部，具有南海系的特征。     

Holocene activities of the Taigu fault zone, Shanxi Province, and their relations with the 1303 Hongdong M =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. Foundation item: Chinese Joint Seismological Science Foundation (201017). Contribution No. 2003A004, Institute of Crust Dynamics, China Earthquake Administration.     

2020年7月12日唐山5.1级地震分析

对2020年7月12日唐山5.1级地震的发震特点、地震的性质、发震构造以及破裂机制进行初步分析,推测唐山断裂可能为其控震断裂。地震前唐山地区和震中所处的华北构造区的地震活动性异常以缺震和显著平静为主,表明该区域地壳应力积累到了一定程度。分析认为：此次唐山5.1级地震属于1976年7月28日唐山7.8级大震震区内的地震起伏活动;此次地震的序列本身并不丰富,震区烈度偏低、有感范围大。     

2022年青海门源MS6.9地震余震的空间迁移特征

精确识别和定位中强地震序列中的微震事件对于准确判定发震构造具有重要的意义。文章对2022年1月8日发生在青藏高原东北缘的青海门源M_S6.9地震序列进行精定位及微震检测研究。首先运用双差定位法对2022年1月8—16日由中国地震台网中心记录的1 010个门源地震序列原始目录进行重定位,得到404个精定位地震目录。分别采用原始地震目录(CENC)和双差重定位目录(HypoDD),对震源区150 km范围内9个台站的连续波形数据进行微震检测。结果表明,基于CENC目录识别的余震个数是原始目录地震数量的3.0倍,基于HypoDD目录识别的地震个数是原始目录的2.1倍,是HypoDD目录的5.8倍;两种地震目录的微震检测均使得M_L震级完备性从1.7级降低至1.1级。新的地震目录空间位置显示,主震发生后余震主要沿托莱山断裂向西侧扩展,8分钟以后,在托莱山断裂和冷龙岭断裂均发生破裂。根据本研究获取的更高空间分辨率的地震序列,同时结合震源机制解,认为2022年门源M_S6.9地震初始破裂位于近E-W向的托莱山断裂,并触发了NW-SE向的冷...     

Numerical simulation of strong ground motion for the <Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript>8.0 Wenchuan earthquake of 12 May 2008

The Wenchuan earthquake of 12 May 2008 is the most destructive earthquake in China in the past 30 years in terms of property damage and human losses. In order to understand the earthquake process and the geo-morphological factors affecting the seismic hazard, we simulated the strong ground motion caused by the earthquake, incorporating three-dimensional (3D) earth structure, finite-fault rupture, and realistic surface topography. The simulated ground motions reveal that the fault rupture and basin structure control the overall pattern of the peak ground shaking. Large peak ground velocity (PGV) is distributed in two narrow areas: one with the largest PGV values is above the hanging wall of the fault and attributed to the locations of fault asperities and rupture directivity; the other is along the northwestern margin of the Sichuan Basin and caused by both the directivity of fault rupture and the amplification in the thick sediment basin. Rough topography above the rupture fault causes wave scattering, resulting in significantly larger peak ground motion on the apex of topographic relief than in the valley. Topography and scattering also reduce the wave energy in the forward direction of fault rupture but increase the PGV in other parts of the basin. These results suggest the need for a localized hazard assessment in places of rough topography that takes the topographic effects into account. Finally, had the earthquake started at the northeast end of the fault zone and ruptured to the southwest, Chengdu would have suffered a much stronger shaking than it experienced on 12 May, 2008. Supported by the U.S. National Science Foundation (Grant Nos. EAR 0738779 and OCE 0727919), the National Basic Research Program of China (Grant No. 2004CB418404), and partially by the National Nature Science Foundation of China (Grant No. 40521002)     

Earth's rotation-triggered earthquakes before the 2018 MS5.7 Xingwen earthquake

For earthquakes (M_L≥2.0) that occurred from January 2006 to October 2018 around the M_S5.7 Xingwen earthquake occurred on 16 December 2018 in Xingwen, Sichuan province, China, we statistically investigated the correlation between the phase of Earth's rotation and the occurrence of earthquakes via Schuster's test to determine the signals that triggered earthquakes before the M_S5.7 Xingwen event. The results were evaluated based on the P-value where a smaller P-value corresponded to a higher correlation between the occurrence of an earthquake and Earth's rotation. We investigated the spatial distribution of P-values in the region around the epicenter of the M_S5.7 Xingwen event, and obtained a result exhibiting a extremely low-P-value region. The M_S5.7 event occurred inside near the northern boundary of this region. Furthermore, we analyzed the temporal evolution of P-values for earthquakes that occurred within the extremely low-P-value region and found that some extremely low P-values (less that 0.1%), i.e., significant correlation, were calculated for earthquakes that occurred before the M_S5.7 Xingwen earthquake. Among sixty-one earthquakes with the lowest P-value, occurred from May 2014 to April 2018, a vast majority of them occurred during the acceleration of Earth's rotation. The lower P-value obtained in this study reveals that the Xingwen source body probably was extremely unstable prior to the occurrence of the M_S5.7 Xingwen earthquake.     

Spatio-temporal rupture process of the 2008 great Wenchuan earthquake

Focal mechanism and dynamic rupture process of the Wenchaun M _s8.0 earthquake in Sichuan province on 12 May 2008 were obtained by inverting long period seismic data from the Global Seismic Network (GSN), and characteristics of the co-seismic displacement field near the fault were quantitatively analyzed based on the inverted results to investigate the mechanism causing disaster. A finite fault model with given focal mechanism and vertical components of the long period P-waves from 21 stations with evenly azimuthal coverage were adopted in the inversion. From the inverted results as well as aftershock distribution, the causative fault of the great Wenchuan earthquake was confirmed to be a fault of strike 225°/dip 39°/rake 120°, indicating that the earthquake was mainly a thrust event with right-lateral strike-slip component. The released scalar seismic moment was estimated to be about 9.4×10²⁰-2.0×10²¹ Nm, yielding moment magnitude of M _w7.9–8.1. The great Wenchuan earthquake occurred on a fault more than 300 km long, and had a complicated rupture process of about 90 s duration time. The slip distribution was highly inhomogeneous with the average slip of about 2.4 m. Four slip-patches broke the ground surface. Two of them were underneath the regions of Wenchuan-Yingxiu and Beichuan, respectively, with the first being around the hypocenter (rupture initiation point), where the largest slip was about 7.3 m, and the second being underneath Beichuan and extending to Pingwu, where the largest slip was about 5.6 m. The other two slip-patches had smaller sizes, one having the maximum slip of 1.8 m and lying underneath the north of Kangding, and the other having the maximum slip of 0.7 m and lying underneath the northeast of Qingchuan. Average and maximum stress drops over the whole fault plane were estimated to be 18 MPa and 53 MPa, respectively. In addition, the co-seismic displacement field near the fault was analyzed. The results indicate that the features of the co-seismic displacement field were coincident with those of the intensity distribution in the meizoseismal area, implying that the large-scale, large-amplitude and surface-broken thrust dislocation should be responsible for the serious disaster in the near fault area. Supported by the National Basic Research Program of China (Grant No. 2004CB418404-4) and the National Natural Science Foundation of China (Grant Nos. 40574025 and 40874026)     

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.     

Source mechanism of strong aftershocks (<Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript>⩾5.6) of the 2008/05/12 Wenchuan earthquake and the implication for seismotectonics

Dozens of >M5, hundreds of >M4, and much more >M3 aftershocks occurred after the 2008/05/12 Wenchuan earthquake, which were well recorded by permanent and portable seismic stations. After relocated with P arrival, the >M3 aftershocks show two trends of distribution, with most of the aftershocks located along the north-east strike consistent with Longmenshan fault system, yet there is a north-west trend around the epicenter. It seems that substantially more aftershocks occur in regions with crystalline bedrocks. Then we collected waveform data from National Digital Seismograph Network and regional seismograph network of China, and employed “Cut and Paste” method to obtain focal mechanisms and depths of the big aftershocks (M⩾5.6). While most of those aftershocks show thrust mechanism, there are some strike slip earthquakes in the northern-most end of the rupture. Focal mechanisms show that the events located on the southern part of central Beichuan-Yingxiu Fault (BY) are mainly thrust earthquakes, which is consistent with initial mechanism of the main shock rupture. In the north part the aftershocks along the BY are also dominated by thrust slip, which is quite different from the right slip rupture of the main shock. Around Qingchuan-Pingwu Fault, the focal mechanisms are dominated by right-slip rupture with large depths (∼18 km). So we suspected that in the north part the main shock might rupture on two faults: Beichuan Fault and Qingchuan-Pingwu Fault. The complex pattern of aftershock mechanisms argues for presence of a complicated fault system in the Longmenshan area. Supported by Knowledge Innovation Project of Chinese Academy of Sciences (Grant Nos. KZCX3-SW-153, KZCX2-YW-116-1), National Natural Science Foundation of China (Grant No. 40604004), and National Basic Technology R & D Program (Grant No. 2006BAC01B02-01-02).     

Is clustered seismicity an indicator of regional stress? Insights from earthquake sequences in Yongning-Luguhu faulted basin,Southwest China

Using hypocenter relocation, moment tensor inversion, stress field inversion, and fault slip tendency analysis, this study systematically investigated three M5.5–5.8 earthquake sequences that occurred after 2000 in the Yongning-Luguhu faulted basin in the middle of the Lijiang-Xiaojinhe fault zone within the Sichuan-Yunnan block, Southwest China. Our results show that since the 2008 Wenchuan Earthquake, the tectonic stress pattern in this area may have changed and that b-values estimated for the earthquake sequences show evidence of an increasing trend in stress in the study area. Seismicity in the small-scale faulted basin adjacent to the large-scale fault zone is a possible indicator of regional stress. We also note that the aftershocks of the M5.7 earthquake sequence in 2012 and the M5.5 earthquake sequence in 2022 show relatively clear fluid diffusion-triggering characteristics. Overpressure of deep fluids is still the main factor driving seismic activity in the region, and we propose that the background tectonic stresses have not yet reached critical levels.     

Spectral element analysis on the characteristics of seismic wave propagation triggered by Wenchuan <Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript>8.0 earthquake

The 2008 Wenchuan earthquake occurred in an active earthquake zone, i.e., Longmenshan tectonic zone. Seismic waves triggered by this earthquake can be used to explore the characteristics of the fault rupture process and the hierarchical structure of the Earth’s interior. We employ spectral element method incorporated with large-scale parallel computing technology, to investigate the characteristics of seismic wave propagation excited by Wenchuan earthquake. We calculate synthetic seismograms with one-point source model and three-point source model respectively. The AK135 model is employed as a prototype of our numerical global Earth model. The Earth’s ellipticity, Earth’s medium attenuation, and topography data are taken into consideration. These wave propagation processes are simulated by solving three-dimensional elastic wave governing equations. Three-dimensional visualization of our numerical results displays the profile of the seismic wave propagation. The three-point source, which is proposed from the latest investigations through field observation and reverse estimation, can better demonstrate the spatial and temporal characteristics of the source rupture process than the one-point source. We take comparison of synthetic seismograms with observational data recorded at 16 observatory stations. Primary results show that the synthetic seismograms calculated from three-point source agree well with the observations. This can further reveal that the source rupture process of Wenchuan earthquake is a multi-rupture process, which is composed by at least three or more stages of rupture processes. Supported by National Basic Research Program of China (Grant No. 2004CB418406), National Natural Science Foundation of China (Grant Nos. 40774049 and 40474038), and Computer Network Information Center, Chinese Academy of Sciences (Grant No. INF105-SCE-02-12)