| The active block hypothesis is a scientific hypothesis to explain the relationship between tectonic blocks and seismic activity on the basis of the evolution of fault block tectonics theory after the introduction of plate tectonics in China.For seismologists,it is crucial to understand how active blocks control regional strong earthquake activity.In recent decades,several devastating earthquakes with M≥7.0 have occurred in the central and eastern part of the Qinghai-Tibet Plateau.For example,the M_S7.5 Mani earthquake in Xizang in 1997,the M_S8.1 Kunlun Mountain earthquake in Qinghai in2001,the M_S7.3 Yutian earthquake in Xinjiang in 2008,the M_S8.0 Wenchuan earthquake in Sichuan in 2008,the M_S7.1 Yushu earthquake in Qinghai in 2010,and the M_S7.0 Jiuzhaigou earthquake in Sichuan in 2017.By observing the occurrence locations of the destructive earthquakes mentioned above,it is not difficult to find that these large earthquakes are located in the boundary active tectonic belt of Bayankhara block on the Qinghai-Tibet Plateau,indicating that the occurrence locations of large earthquakes are also regular.However,due to the wide range of Bayan Kera block,there are many and complex large active faults in the block,so this paper selected a secondary block within the Bayankera block,namely Minshan active block,as a sample to study the temporal and spatial regularity of earthquake occurrence on the active block.The main part of Minshan active block is Minshan uplift,which is bounded by Huya fault in the east and Minjiang Fault in the west.The main reason for choosing Minshan active block as the sample to study the temporal and spatial laws of earthquakes on the active block is that several large earthquakes with M≥7.0 have occurred in a short period of time on both sides of a relatively narrow active block in the plate,and such seismic activities with such intensity and frequency are rare in a secondary block.However,due to the unique geographical conditions and strong uplift of the eastern margin of the Qinghai-Tibet Plateau,it is relatively difficult to carry out field work in this area due to the high mountain and deep water.Moreover,the field geological environment is not well,and the surface active faults are poorly preserved and exposed due to the external dynamic effects such as strong erosion and weathering.This also restricts us from the pure geological perspective of the Minshan active block around the seismic structure and their relationship with more in-depth research.Because of this,this paper chooses to study Minjiang fault and Huya fault in Minshan active block from the perspective of seismic geology.The study of seismic sequence of a major earthquake is an important means to dissect seismogenic structure.In a certain period of time after the occurrence of a major earthquake,a large number of small earthquakes will still occur in the fault plane and its vicinity.Therefore,the spatial distribution of the source location of small earthquakes can accurately describe the characteristics of the source body of historical large earthquakes.Many scholars have studied or discussed the seismogenic structure characteristics of the Diexi earthquake(M7.5)in 1933,Songpan-Pingwu earthquake(M_S7.2)in 1976and Jiuzhaigou earthquake(M_S7.0)in 2017 by using seismic sequence analysis method.However,there is a lack of systematic research on the temporal and spatial development characteristics of the four M≥7.0 earthquakes around the Minshan active block.In terms of data processing,this study tries to use kernel density analysis to draw the density distribution map of seismic sequence,which is more intuitive and accurate than the traditional density map.Based on this,the seismogenic faults of several large earthquakes in Minshan active block are analyzed,the seismogenic laws of Minshan active block are summarized,the seismic gaps are delineated,and the maximum magnitude and seismogenic probability are calculated.(1)39,076 small earthquakes in Minshan active block and its adjacent areas during 2000-2019 were extracted from seismic phase observation reports provided by Sichuan Earthquake Monitoring Center using hypo DD localization program(double difference localization method),including 245,003 P wave traveltime and 242,130 S wave traveltime.In addition,The specific locations of 9,034 earthquakes in the study area were selected from the catalogue of earthquakes recorded by instruments in the same area from 1972 to 1999.The data of 48,110 seismic events in minshan active block and its adjacent areas were obtained by combining the two parts of data.(2)By comparing the distribution of seismic events before and after precise location,it can be seen that the aggregation of seismic events after precise location is significantly improved and has the characteristics of linear arrangement.Based on the location of small earthquakes in the Minshan active block and its adjacent areas from2000 to 2019 and the earthquake catalogue from 1972 to 1999,four large earthquakes with M>6.5(Diexi M7.5 in 1933,Songpan-Pingwu earthquake swarm in 1976)in the study area since 1933 are obtained.Jiuzhaigou M_S7.0,2017 earthquake sequences in different periods,and epicenter distribution maps were plotted.It was found that these earthquakes were all distributed along the boundary zone of Minshan active block in space,indicating that the active block had an obvious control effect on the gestate and occurrence of regional large earthquakes.Meanwhile,the results of regional seismic relocation and the spatial distribution of earthquake sequences in different periods define the seismogenic faults and their locations of the four major earthquakes.(3)According to the distribution characteristics of seismic sequence in time and space,it is considered that the strike of the seismogenic fault of the 1976 Songpan M_S7.2 earthquake is basically the same as that of the 2017 Jiuzhaigou M_S7.0 earthquake,but there are about 60°differences in strike between them and that of the 1976 Pingwu M_S7.2 earthquake.The seismogenic faults of these three major earthquakes may be divided into two faults.The seismogenic faults of Jiuzhaigou M_S7.0 earthquake in 2017and Songpan M_S7.2 earthquake in 1976 are Shuzheng faults in NW direction,while the seismogenic faults of Pingwu M_S7.2 earthquake in 1976 are the northern segment of Huya fault in nearly SN direction.From the perspective of seismicity,the seismogenic fault of Diexi M7.5 earthquake in 1933 should be the southern segment of Minjiang Fault.The 2017 Jiuzhaigou M_S7.0 earthquake occurred in the gap between the 1976Songpan M_S7.2 earthquake and the Minjiang Fault.(4)Before the Songpan M_S7.5 earthquake in 1976 and Jiuzhaigou M_S7.0earthquake in 2017,there were intensive small earthquakes along the main earthquake region,indicating that the occurrence of these large earthquakes has precursors in terms of seismicity.This paper argues that in the Minshan block peripheral probably there are two seismic zone,located in the southern section of Huya fault and the middle section of the Minjiang fault,and the Minshan block peripheral earthquake types are likely to belong to the foreshocks main shock-aftershock type,because the area is densely populated and high altitude,from the perspective of earthquake prediction and social security,It is suggested to strengthen the monitoring of these two seismic gaps;(5)With Huya fault has been basically,Minjiang fault in the middle but keep the‘silence’for many years,this paper calculates the maximum earthquake magnitude in the central part of the Minjiang fault and the probability of the earthquake,on the basis of fracture length of the middle 50±5 km,and using Monte Carlo method to generate fault zone length of random Numbers,software programming calculated in the MATLAB software,The numerical solution and uncertainty of the maximum earthquake magnitude are obtained by statistical method,and the range of the maximum earthquake magnitude in the middle section of minjiang Fault is～Ms7.3... |
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