Characteristics And Mechanism Of Strong Earthquake Activity Within Tectonic Belt Near Western Ordos Block

Strong earthquakes in continental China are controlled by the movement and deformation of active blocks.The main tectonic deformation and strong earthquakes in continental China occur on the active block boundary zone,and a few strong earthquakes with lower magnitude occur in the continuous deformation of the blocks.The movement and deformation of active blocks have led to widespread distribution of strong earthquakes in continental China,west strong and east weak,alternation of active and quiet,along block and belt.The spatiotemporal evolution patterns and mechanisms of paleoearthquakes and historical strong earthquakes in the active block boundary zone have great significance to reveal the spatiotemporal evolution pattern and mechanism of the strong earthquakes in continental China and to assess seismic risk.The tectonic belt near weatern Ordos block is located in the northern part of the North-South seismic belt,which is the intersection area of Ordos active block,Alxa block and Qaidam-Longxi block.In the south of the area,there are several left lateral strike slip faults and thrust faults at the end,which belong to the Qinghai Tibet Plateau tectonic domain;In the north,there are Yinchuan basin and Helan Mountain controlled by many normal faults,which form typical basin mountain structure and belong to North China tectonic domain.The right lateral shear zone of Sanguankou-Niushoushan-Luoshan faults is formed in the middle of the two tectonic domains,which forms the boundary between the arc structure in the northeastern margin of the Qinghai Tibet Plateau and the basin mountain structure in the western margin of North China.The study of historical strong earthquakes and paleoearthquakes in this area is more,which provides conditions for studying the characteristics and mechanism of strong earthquakes within the tectonic belt near western Ordos block.This paper summarizes the data of paleoearthquakes and historical earthquakes in the region,and studies the characteristics and mechanism of strong earthquake activity by means of statistical analysis and Coulomb stress changes model.The main conclusions are as follows:1.The recurrence mode of strong earthquakes on a single segment of fault in the western margin of Ordos active block is mainly quasi-periodic,and the quiet and active periods of strong earthquakes alternate in the region.In the last four active periods of historical strong earthquakes,strong earthquakes along the western margin of Ordos active block migrate from east to west in time and space,which is also reflected in the spatial and temporal distribution of the latest paleoearthquake event.Based on 95paleoearthquakes,the coefficient of variation of earthquake recurrence on a single segment of fault is 0.051?0.7,of which 83%is less than 0.5;By Monte Carlo simulation,the coefficient of variation is between 0.109±0.026?0.704±150 when the error obeys normal distribution,and the coefficient of variation is between 0.130±0.03?0.708±0.149 when the error obeys the uniform distribution;Based on the bootstrap sampling method,the probability of earthquake random recurrence is calculated,and the proportion P_d of faults obeying quasi-periodic recurrence mode is 67%.2.Through numerical simulation of viscoelastic Coulomb stress changes,it is found that the triggering effect is not only obvious between the earthquakes from 1219to 1920,cumulative Coulomb stress changes increased 0.01?0.1 MPa by the pre-sequence earthquakes but also the the Coulomb stress changes on the seismogenic fault increases rapidly before the occurrence of these strong earthquakes.It is also found that the cumulative Coulomb stress changes between the three strong earthquakes in 1219-1352 are negative,and it has no obvious effect on the stress of the strong earthquake fault after 1495.The difference of the characteristics of Coulomb stress between the active period(1495—1920)and the quiet period(1219—1495)indicates that the rapid increase of Coulomb stress on the fault plane is an important reason for triggering strong earthquakes,and the Coulomb stress triggering will lead to more active strong earthquake activity in the boundary zone of the block.3.Based on the paleoearthquake data,the Coulomb stress interaction between structures in typical strike slip fault system,strike-thrust fault system and normal fault system is studied,which shows that the structural style has a certain control effect on the triggering of strong earthquakes.The Lenglongling-Jinqianghe-Maomaomaoshan-Laohushan-Haiyuan faults and the Lenglongling-Tianqiaogou-Huangyangchuan-Xiangshan-Tianjingshan faults are characterized by nearly linear geometric distribution and similar fault mechanical mechanism(mainly left lateral strike slip).The high Coulomb stress changes at the end of the fault caused by strong earthquakes promotes the rupture of adjacent faults and promotes the active period of strong earthquakes.For example,between the west and middle sections of Haiyuan fault and the west and east sections of Xiangshan-Tianjingshan fault,due to the large length width ratio of fault rupture during the occurrence of strong earthquake,the negative Coulomb stress will be formed.However,due to the end effect,the Tianqiaogou-Huangyangchuan faults play a transitional role in them,contributing to the Tianqiaogou-Huangyangchuan faults strong earthquake active period between the western and eastern segments of Xiangshan-Tianjingshan fault and the western and middle segments of Haiyuan fault.If we consider the change of deep dip of fault,the interaction between the north section of the Yellow river fault on the eastern boundary of Yinchuan Basin and the east Helan fault on the western boundary shows that the rupture of the north section of the Yellow river fault will form a positive Coulomb stress in the shallow part of the east Helan fault and Luhuatai fault(about 13 km above),while the rupture of the east Helan fault will form a negative Coulomb stress dominated stress environment in the basin,a positive Coulomb stress changes occur in the shallow part of the fault(about 3 km)in the basin.This kind of fault interaction is closely related to the geometric distribution,mechanical properties and deep structure of faults.4.Based on the analysis of cumulative co-and post-seismic Coulomb stress changes,seismic gaps and strong earthquake recurrence probability model,it is concluded that the fault near the north-south distribution on the western margin of Ordos active block has relatively high seismic risk.The Coulomb stress changes on the fault surface indicate that the accumulation Coulomb stress on the south section of Huanghe fault,Yunwushan fault,Liupanshan fault,Qingshuihe fault,Yantongshan fault and other faults near north-south distribution is large.These areas also are the main parts of the historical seismic gaps.And the south section of the Yellow river fault and the Sanguankou fault calculated by the probability model will have relatively high recurrence probability of strong earthquakes in the next 20 years.Therefore,the seismic risk of the faults near the north-south distribution on the western border zone of the Ordos active block should be paid attention to.In addition,the recurrence probability of strong earthquakes calculated by the normal distribution probability model in the next 20 years is also higher in the east Luoshan fault,Laohushan fault,the west and middle section of Haiyuan fault and the north end of Liupanshan fault with strong earthquake records.