Seismicity gap and seismic quiescence before 1999 Jiji(Chi-Chi) M_W7.6 earthquake

The September 21, 1999, Jiji （Chi-Chi） Mw7.6 earthquake is the strongest event occurred since 1900 in Taiwan of China. It is located in the middle segment of the western seismic zone of Taiwan. Based on several versions of China earthquake catalogue this study found that a seismic gap of M≥5 earthquakes appeared, in and around the epicenter region, 24 years before and lasted up to the mainshock occurrence. This study also noticed that there existed a lager seismically quiet region of M≥4 earthquakes, which lasted for about 2.5 years before the mainshock occurrence, The spatial variation pattern of regional seismicity before the mainshock seems to match with its coseismic source rupture process. The mentioned seismicity gap and seismic quiescence might be an indication of the preparation process of the Jiji strong earthquake.

Numerical Modeling of Neotectonic Movements and State of Stresses in the Central Seismic Gap Region,Garhwal Himalaya

This paper presents finite element modeling （FEM） to simulate the present-day stress field and crustal deformation using NE-SW structural section in the central Seismic Gap region of the Garhwal Himalaya. Our study deals with the effect of geometrical characteristics and rock layer parameters on the upper crust. Modeling results show that two types of tectonic regimes developed in the central Seismic Gap region： the geotectonics of the northern part has been controlled by regional compression, whereas southern part is characterized by regional extension. Correspondingly, thrust faults are induced in the northern part and normal faults are extensively developed in the southern front. Those evidences noticeably indicate that the compressive tectonic environment of the Himalaya becomes change into the extensional tectonic regime in its front. The computed shear stress accumulation along the northern fiat of Main Himalayan Thrust （MHT） implies that considerable amount of interseismic stress is building up along the MHT system in the Himalaya, which ultimately release through the possible future great Himalayan earthquake （M 〉 8）. The comparison between our modeled stress field, faulting pattern and horizontal shortening rate with the distribution of the microseismic events, focal mechanism solutions, active faulting and GPS data in the central Seismic Gap region shows good agreement.

Study of a 3-D seismic gap

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Seismogenic structure of the 2016 Ms6.4 Menyuan earthquake and its effect on the Tianzhu seismic gap

On January 21, 2016, a strong earthquake with a magnitude of Ms6.4 occurred at Menyuan, Qinghai Province of China. In almost the same region, there was another strong earthquake happened in 1986, with similar magnitude and focal mechanism. Based on comprehensive analysis of regional active faults, focal mechanism solutions, precise locations of aftershocks, as well as GPS crustal deformation, we inferred that the Lenglongiing active fault dips NE rather than SW as suggested by previous studies. Considering the facts that the 2016 and i986 Ms6.4 Menyuan earthquakes are closely located with similar focal mechanisms, both of the quakes are on the north side of the Lenglongling Fault and adjacent to the fault, and the fault is dipping NE direction, we suggest that the fault should be the seismogenic structure of the two events. The Lenglongling Fault, as the western segment of the well-known Tianzhu seismic gap in the Qilian-Haiyuan active fault system, is in a relatively active state with frequent earthquakes in recent years, implying a high level of strain accumulation and a high potential of major event. It is also possible that the Lengiongiing Fault and its adjacent fault, the Jinqianghe Fault in the Tianzhu seismic gap, are rupturing simultaneously in the future.

A Study on the Prediction Method of Strong Earthquakes Based on Dynamic Seismicity Patterns with Dynamic Implications

Seismic gap method is one of the effective earthquake prediction methods using seismicity patterns. However, this method has some limitations and uncertainty when using it singly in predicting earthquakes. This paper puts forward the prediction method using the dynamic seismicity pattern with dynamic implications. This method considers the formation and evolution of the seismic gap on the basis of plate movement and structural characteristics. Through analysis of 26 cases of earthquakes of MS≥5.0 occurring in East China and South China, this paper obtains the relationship between the main shock with seismic gap and active fault's location, as well as the relationship between the seismic gap and location and strike of active faults. Meanwhile, this paper provides a dynamic explanation of the differences in the formation and evolution patterns of the seismic gap between the two regions, thus providing the physical basis for and reducing the uncertainty of predicting earthquakes using the seismic gap method.

Rapid report of June 1,2022 M_(W) 5.9 Lushan earthquake,China with geodetic and teleseismic data

Timely response to earthquake characterization can facilitate earthquake emergency rescue and further scientific investigations.On June 1,2022,M_(W) 5.9 earthquake occurred in the southern area of the Longmenshan fault zone.This event also happened at the south end of the Dayi seismic gap and is the largest earthquake that has occurred in this seismic gap since the 1970 M 6.2 event.The slip-distribution model constrained by the seismic waveforms suggests a thrust-dominated faulting mechanism.The main slip occurs at a depth of~14 km,and the cumulative energy is released in the first 6 s.The variations of Coulomb stress caused by the mainshock show a positive change in the southwest area of the Dayi seismic gap,indicating possible activation of future earthquakes.In addition,we emphasize the importance of rapid estimation of deformation for near-field hazard delineation,especially when interferometric radar fails to image coseismic deformation in a high relief terrain.

Present status and prospective on the seismic hazard studies of major seismic gaps in the Sichuan-Yunnan region

Several seismic gaps have been identified along the Xianshuihe-Xiaojiang fault zone and the Longmenshan fault zone in the Sichuan-Yunnan region, including the Daofu-Shimian gap on the Xianshuihe Fault, the Anninghe-Zemuhe gap, parts of the Xiaojiang Fault, and the Dayi gap between the Wenchuan and Lushan earthquakes. The seismic hazards of the Daofu-Shimian and Dayi gaps have aroused great concern and caused considerable debate. This paper reviews the recent geophysical observations and multidisciplinary studies conducted by domestic and international researchers in these areas, focusing on seismogenic structures, seismic activity, fault locking, stress fields, as well as earthquake triggering, and further analyzes the current controversial viewpoints about the seismic hazards in these regions. Based on the comprehensive analysis, we argue that the kinematic status along the Daofu-Shimian segment may vary significantly along the fault strike. Due to limitations in data and models, there is no consensus on the seismogenic potential of this area. Conversely, the Dayi seismic gap presents a high seismic potential. Furthermore, we delve into the scientific problems associated with major seismic gaps in the Sichuan-Yunnan region and make prospects for the strategy of future work. We propose a research framework that combines dense array and distributed acoustic sensing, artificial intelligence and big data technologies, joint inversion of multiple geophysical fields, as well as numerical simulation techniques for comprehensive seismic hazard analysis in seismic gaps.

Characteristics of seismic activity before the M_S8.0 Wenchuan earthquake

The characteristics of spatio-temporal seismicity evolution before the Wenchuan earthquake are studied. The results mainly involve in the trend abnormal features and its relation to the Wenchuan earthquake. The western Chinese mainland and its adjacent area has been in the seismically active period since 2001, while the seismic activity shows the obvious quiescence of M≥7.0, M≥6.0 and M≥5.0 earthquakes in Chinese mainland. A quiescence area with M≥7.0 has been formed in the middle of the North-South seismic zone since 1988, and the Wenchuan earthquake occurred just within this area. There are a background seismicity gap of M≥5.0 earthquakes and a seismogenic gap of ML〉4.0 earthquakes in the area of Longrnenshan fault zone and its vicinity prior to the Wenchuan earthquake. The seismic activity obviously strengthened and a doughnut-shape pattern of M≥4.6 earthquakes is formed in the middle and southern part of the North-South seismic zone after the 2003 Dayao, Yunnan, earthquake. Sichnan and its vicinity in the middle of the doughnut-shape pattern show abnormal quiescence. At the same time, the seismicity of earthquake swarms is significant and shows heterogeneity in the temporal and spatial process. A swarm gap appears in the M4.6 seismically quiet area, and the Wenchuan earthquake occurred just on the margin of the gap. In addition, in the short term before the Wenchuan earthquake, the quiescence of earthquake with ML≥4.0 appears in Qinghai-Tibet block and a seismic belt of ML〉3.0 earthquakes, with NW striking and oblique with Longmenshan fault zone, is formed.

Characteristics of seismic activity before Chile M_W8.8 earthquake in 2010

The tempo-spatial variation of seismic activity before great Chile Mw8.8 earthquake on February 27, 2010 is studied. Some results are as follows： ①Two types of seismic gaps appeared before the Chile MwS.8 shock. One is background gap of Mw≥8.0 earthquakes with 360 km length since 1900, the other is seismogenic gap formed by M≥5.5 earth- quakes with 780km length five years before the Chile earthquake; ②There was only one Mw7.1 earthquake in the middle and southern part of Chile from 1986 to 2010. The obvious quiescence of Mw≥7.0 earthquake is the long-term background anomaly for the Chile earthquake; ③ The quiescence of M≥6.5 earthquakes appeared in South American block and its vicinity during the period from 2007 to 2009, and the quietude state has been disappeared three months before the Chile Mw8.8 earthquake; ④ The deep and intermediate-depth earthquake activity has been noticeablely strengthened in the subduction zone of South American block since 1993; ⑤The great Chile earthquake shows that global seismicity is still in the active period of Mw≥8.5 earthquakes since 2004. Based on the characteristics of the former two active periods, several great earthquakes with Mw≥8.5 would take place in a few years. In addition, the circum-Pacific seismic belt would be the main region for Mw≥8.0 earthquakes.

The Characteristics of Seismic Activity Before the Devastating Earthquake with M_W9.0 Off the West Coast of Northern Sumatra

The characteristics of seismic activity in different time-spatial domain before the M_W9.0 earthquake were studied. The results are as follows:The activity of the deep earthquakes in the north boundary zone of the Australian plate had been evidently strengthened since 1994, showing an increased frequency, magnitude and depth, especially in regards to the heterogeneous distribution of the earthquake depth (namely between 500km and 689km). Meanwhile the shallow earthquakes of M≥7.0 in the Sumatra island and its vicinity had been obviously strengthened too, and formed a strengthening area with a length of about 1000 km and width 300 km. ②The time distribution of global strong earthquakes with M≥7.0 shows that the character of anomalous seismic quiescence-activity one year before the M_W9.0 earthquake and during its active period, the strong earthquakes formed a seismic belt striking in NWW direction. At the same time, there is a seismic gap formed by earthquakes of M≥5.0 in the epicenter and its neighboring region. ③Two deep earthquakes of M≥7.0 occurred in the west and in the east of the north boundary zone of the Australian plate half year ago. It is notable that one of them occurred in the Sumatra island where no deep earthquake with M≥6.0 has occurred in the past thirty years. ④The space distribution of moderate shocks occurring three days ago exhibited a NWW-strike seismic belt along the north boundary zone of the Australian plate. ⑤The activity of volcanoes distributed in the north boundary zone of the Australian plate had been strengthened in the past 4 years, especially several months before the occurrence of the M_W9.0 earthquake.

Characteristics of seismic activity before several large Sumatra, Indonesia, earthquakes

Two great earthquakes of MS8.5 and MS8.3 determined by the China Earthquake Networks Center (CENC) occurred successively on September 12 and 13, 2007 in the sea area to the south of Sumatra, which is another group of large earthquakes after MS8.7 event on December 26, 2004 and MS8.5 event on March 29, 2005. The

Identification of expected seismic activity areas by forecasting complex seismic-mode parameters in Uzbekistan

In this paper, the author proposed a methodology to reveal expected seismic activation places for coming years by a complex of forecasting parameters of a seismic mode. Areas in Uzbekistan where currently observed anomalies in various parameters of a seismic mode has been revealed. By number of displayed abnormal signs the areas has been ranked based on probability of occurrence of strong earthquakes there. It has prepared schemes of the synoptic forecast of expected seismic activation places in case of occurrence of strong earthquakes in the Central-Asian region.

Application of digital image processing to the determination of spatial distribution of earthquakes

This paper has proposed a practical method for determining the spatial distribution pattern of earthquakes by means of digital image processing and has given some calculation results. This method can overcome the artificial arbitrariness which is usually inevitable in determining the spatial distribution of earthquakes. Meanwhile, the form of seismic gaps and the method for identifying seismic gaps have also been discussed. It should be pointed out that the method proposed in this paper is a new idea in this respect. However, this method is still unable to determine the seismic gap uniquely. In order to identify the real seismic gap, comprehensive analysis of the variation of other relevant parameters (e. g., the b-value, etc. ) should also be made. The results are as follows; Thephenomenon that an area of high seismic activity is surrounded by low seismicity areas, i. e., the seismic clustering pattern is the general form of spatial distribution of earthquakes; whereas a low seismicity area surrounded by high seismicity areas, i. e., the seismic gap pattern, is only an important form of spatial distribution of earthquakes. Earthquakes often occur in a certain seismic clustering area or on its margin.

Characteristics of Seismic Activity in a 3-Dimensional Space before the Wen'an Earthquake,Hebei Province by Relocation of Small Earthquakes in the China's Capital Area

Characteristics of seismic activity before the M5. 1 earthquake in Wen＇an, Hebei Province on July 4, 2006 are analyzed by relocation of small earthquakes in the China＇s capital area, and some results are obtained as follows：① The seismic activity of ML ≥ 3.0 in the middle part of the Hebei plain seismic belt displayed a feature of strengthening （ lasting 43 months） quiescence （17 months） five years before the Wen＇an earthquake. Simultaneously, the strain release curve showed a variation process of accelerating-flatting. ② A seismogenic gap in a three-dimensional space, located at the depth of 15km - 20km, with 70km long in latitude direction and 90km long in longitude orientation, was formed by M≥2.0 earthquakes four years prior to the Wen＇an earthquake. The initial rupture point of the Wen＇an earthquake is situated at the bottom of the gap. ③ The focal depths of earthquakes with ML ≥ 2. 0 in the middle part of the Hebei plain seismic belt gradually increased from 10km to 30kin during the period from April, 2003 to October, 2004. Meanwhile, the seismic activity obviously strengthened in the middle and lower crust （from 20km to 30km in depth）.

In situ stress state and seismic hazard in the Dayi seismic gap of the Longmenshan thrust belt

In the Longmenshan thrust belt,the Dayi seismic gap,an area with few earthquakes,is located between the ruptures of the 2008 Wenchuan Earthquake and the 2013 Lushan Earthquake,with a length of approximately 40–60 km.To date,however,the extent of the seismic hazard of the Dayi seismic gap and whether this gap is under high stress are still hotly debated.To further evaluate the seismic hazard of the Dayi seismic gap with regard to stress,two boreholes(1,000 and 500 m deep)were arranged to carry out hydraulic fracturing in situ stress measurement on either side of the Shuangshi-Dachuan fault zone.This zone has a high seismic hazard and the capacity to undergo surface rupture.Through the analogy of this new data with stability analysis using Byerlee’s Law and existing stress measurement data collected before strong earthquakes,the results show that the area surrounding the Shuangshi-Dachuan fault zone in the Dayi seismic gap(Dachuan Town)is in a state of high in situ stress,and has the conditions necessary for friction slip,with the potential hazard of moderate to strong earthquakes.Our results are the first to reveal the in situ stress profile at a depth of 1,000 m in the Dayi seismic gap,and provide new data for comprehensive evaluation of the seismic hazard in this seismic gap,which is of great significance to explore the mechanism of earthquake occurrence and to help mitigate future disaster.