| Earthquake prediction is a difficult and attractive frontier subject in the field of contemporary natural sciences.The prospect of earthquake prediction and the deep research of earthquake prediction are closely related to the determination of seismic parameters and medium parameters with the development of seismology itself.With the development of informationization and large data,the advantages of modern digital seismic record are further amplified,the bandwidth,the dynamic range is large and the resolution is high.The new seismic parameters,the medium parameters and the tectonic environment and the seismic source The feature provides an infinite possibility,which brings new hope for the deepening of the earthquake prediction research and the improvement of the forecast level.North of the north-south seismic belt has been in recent years has been strong earthquake zone.The physical properties of the underground media will change including elasto-plasticity,phase transition,density,etc.somewhat in the middle.And the change of the attribute will lead to the change of the wave velocity in the source area.In this paper,we get the the imaging characteristics of the north-south seismic belt by ambient noise and the 6-60 s phase velocity imaging of the Rayleigh surface wave.Then the characteristics velocity of the change of wave velocity is analyzed before and after in the 2013 Minxian-Zhangxian and Meng Yuan Earthquake.The following preliminary conclusions are now available(1)The velocity distribution of the north-south seismic belt is mainly consistent with the distribution of the block.Most of the northern part of the north-south seismic belt is at low speed.The whole velocity distribution has great transverse inhomogeneity,and the velocity distribution is more obvious.And the 104°boundary can be divided into different structural characteristics of the East and West two parts.It is probably due to the thinning of the crustal thickness in the northern part of the north-south seismic belt,which is mainly from west to east,and the north-south direction is relatively uniform.The result is similirity related to the anomaly of the Bouguer gravity anomaly from west to east.(2)The dynamic wave velocity evolution process in the northern part of the north-south seismic belt in 2015 shows the stress and energy accumulation process in the northern section of the north-south seismic belt.The results show that the whole North-South seismic zone is more active and the wave velocity is more obvious.The variation of the wave velocity is0.007%,and the variation range is different in different regions.Long Xi block,south China block and Qaidam block have a wave velocity decreased;A-La-shan block is a significant wave velocity rise,just in the vicinity of the epicenter can find the volatility of the wave velocity.(3)During the earthquake in 2013 Minxian-Zhangx Ian Earthquake,there was a significant anomaly in the wave velocity before and after the earthquake,and there was a decrease in the Rayleigh surface wave group near the earthquake source area.It occurred in the transition zone of the low speed and high speed,which belonged to the transition zone.The general high and low speed conversion zone is think of the boundary of the active block,and most of the great earthquakes occur on the active tectonic belt of this boundary.The low velocity zones are often considered as the areas where the crust is prone to deformation,and high-speed and low-speed boundaries are considered to be prone to stress concentration.(4)The imaging results show that the apparent velocity of the source area is decreased whenthe period is T=10s in the 2016 Meng Yuan Earthquake.The tidal velocity of the Qilian blockin the south of the focal area is lower at T = 12 s,14s and 16 s,and the obvious velocity isincreased on the north side of the source area.Before the earthquake,it may be broken.Thestress in the belt and its surrounding area is gradually accumulated and the wave velocity isgradually increased.The north side of the source area may be the area of stress accumulationin the earthquake area.Because the north side and the east side is rigid Gobi-Alashan blockand Ordos block,subject to a certain block.The results of the post-earthquake imaging showthat the velocity of the west side of the source is significantly decreased at T = 10 s,12s and14s,and the velocity decreases from 10s-14 s in the three months after the earthquake.TheQilian block and Qaida block.The body is in the velocity reduction area;in the east of thesource area there is no obvious increase in speed,may be the stress and energy migrationeastward.The evolution of the entire wave velocity changes,from a certain extent above thereaction of the earthquake,energy accumulation and release process.In the process of long geological tectonic movement,natural earthquakes are the result of tectonic activity,and the late seismicity is the continuation of pre-seismic activity.By analyzing the current seismic activity,a series of inversion can reflect the scale of tectonic activity,the stress distribution in the depth of the crust and the possible stress concentration,which is of great significance to us to study the occurrence of earthquakes... |
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