| China is the country with the most frequent earthquakes and the most serious earthquake disasters on the global mainland.Earthquakes such as Wenchuan M_S8.0 in2008,Yushu M_S6.9 in 2010,Lushan M_S7.0 in 2013 and Jiuzhaigou M_S7.0 in 2017,all caused heavy casualties and national economic losses.So,seismic risk analysis is one of the key scientific problems that the government and society must face.Earthquake is in fact the result of the sudden instable rupture after the strain accumulates and reaches the limit state in the active fault zone under the action of regional tectonic stress.As the largest active fault zone in eastern China,the Tanlu fault zone is also the main seismic tectonic belt in North China.On its northern segment,a series of strong earthquakes occurred there,such as Bohai M_S7.4 in 1969,Haicheng M_S7.3 in 1975.On its middle-southern segment,there were also quite a few strong earthquakes occurred,such as Anqiu M_S7 in 70 BC and Tancheng M_S8?in 1668.The Yishu segment of Tanlu fault zone(which is also called Yishu fault zone or Shandong segment of Tanlu fault zone)is the segment with the best exposure,the largest scale and the strongest neotectonics activity,where 25 M_S?5 earthquakes occurred in history.Because it is located in the east of China where the economy is relatively developed and the population is dense,the seismic risk analysis of the Yishu fault zone and surroundings has a strong social demand.Affected by the subduction of the Pacific plate,the Tohoku M_W9.0 earthquake in Japan occurred on March 11,2011,and after this giant earthquake,the seismic activity in the Yishu fault zone and surroundings increased significantly.Since the Tanlu fault zone and the Japan trench belong to the same tectonic system and are both affected by the subduction of the Pacific plate,the giant earthquake would inevitably lead a direct impact on the dynamic environment and seismic potential on both sides of the Yishu fault zone,which makes the demand of seismic risk analysis more pressing.The energy of earthquake preparation mainly comes from the accumulation of strain energy caused by differential crustal motion,and it is necessary to extract the dynamic quantitative information of crustal deformation for seismic risk analysis.The high precision and large scale crustal deformation state provided by GPS under the global unified reference frame plays an important role in the study of regional tectonic setting and seismogenic environment and therefore is widely used in seismic risk analysis.Based on this,the previous researchers had tried to reveal the crustal deformation characteristics of the Yishu fault zone and surroundings with GPS observation in North China,but there are still some scientific problems as followings to be solved:1)The Yishu fault zone and surroundings are located at the intersection of the Eurasian plate,the Pacific plate and the North American plate,where the crustal dynamic environment is extremely complicated.The Tohoku M_W9.0 earthquake in Japan caused by the subduction of the Pacific plate undoubtedly had a direct impact on the dynamic environment of the region.The regions on both sides of the Yishu fault zone belong to the North China Plain block and the Ludong-Huanghai block,and the plate interaction that caused the Tohoku M_W9.0 earthquake,must be reflected in the blocks on both sides of the Yishu fault zone,and constitute the dynamic environment that affects the seismic activity in this area.Therefore,the evolution of the relative motion between the blocks on both sides of the Yishu fault zone before and after the Tohoku M_W9.0 earthquake and its influence on regional seismic activity deserve further discussion.2)The Tohoku M_W9.0 earthquake caused significant coseismic deformation in North China and directly affected the crustal deformation on both sides of the Yishu fault.Since this giant earthquake,the seismicity in this area has enhanced significantly,and quite a few significant seismic events occurred,such as Laizhou M_L5.0 earthquake and its sequence,Rushan earthquake swarm,and Changdao earthquake swarm.Therefore,the coseismic influence of the Tohoku M_W9.0 earthquake on crustal deformation in the Yishu fault zone and surroundings and its influence on regional tectonic stress,seismic activity and seismic potential need to be further analyzed.3)The Yishu fault zone and surroundings are directly affected by the subduction of the Pacific plate and we should pay attention to the current characteristics of tectonic activity and the geodynamic process reflected in it.Therefore,quantitative analysis of the latest crustal deformation characteristics and its impact on the earthquake potential is indispensable.To solve these problems above,this paper takes the Yishu fault zone and surroundings as the study area,and based on the high-density and high-precision GPS observation in this area,combining cross-fault leveling,fixed-point geophysical observation,regional geological structure and seismicity,the following four aspects of work are carried out:1)The deformation field with high spatial resolution in the study area was constructed based on high-density GPS observations.The changes of crustal deformation were studied in different periods of pre-seismic,co-seismic and post-seismic of the Tohoku M_W9.0 earthquake.The evolution process of the deformation field is given through window sliding,which improves the temporal resolution.Thus,we get the crustal deformation state in the study area with high spatial and temporal resolution.2)The crustal dynamic environment in the study area and its influence on seismicity are researched.Basing on the sliding block model,we studied the temporal correlation between the relative motion evolution of the two sides'blocks of Yishu fault zone and the regional seismicity parameter before and after the Tohoku M_W9.0earthquake,and hereby try to describe the seismic strain energy accumulation/release process by establishing the regression relationship between the relative motion and seismic energy release.The results confirm the controling effect of the relative motion between active blocks to the regional seismicity from evolution process,which provide the basis for regional seismic risk analysis.3)With 112 continuous GPS observations,we obtained the coseismic deformation field with high spatial resolution in the study area caused by the Tohoku M_W9.0earthquake.Being verified by the coseismic stress/strain response of fixed-point geophysical observations and the seismic b-value,and based on the seismic moment superposition analysis,the influence of the Tohoku M_W9.0 earthquake on the tectonic stress,seismicity and seismic potential in the study area are discussed.The results show that the coseismic deformation field led different coseismic effects on the south and north segments of the Yishu fault zone,with a tensile effect on south segment and a compressive effect on the north,and produced significant shear strain in the East Shandong uplift and the West Shandong fault block,which might change the stress characteristics of these regions,accumulate seismic moments,and contribute to the enhancement of seismicity in these regions after the earthquake.4)The deformation characteristics of the study area since the Tohoku M_W9.0earthquake and tectonic activity of the Yishu fault zone were analyzed,and hereby the seismic risk was studied.Since the Tohoku M_W9.0 earthquake,the Jiaodong peninsula area and the uplift of the West Shandong fault block had a high seismic moment accumulation rate,and accordingly,had an obvious seismic moment release rate during the same period.The current tectonic activity of the Yishu fault zone is weak,in a state of low slip rate.The coseismic deformation of the Tohoku M_W9.0 earthquake might have released the strain energy in strike-slip direction of the Yishu fault zone and the post-seismic tensioning might release that in dip-slip direction,and they both helped to delay its seismic potential.However,due to coseismic extrusion and fewer strain energy released by the Tohoku M_W9.0 earthquake,the northern segment is still locked now.The last strong earthquake in this segment has yet a long elapse time,its seismic risk is relatively high. |
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