Crustal And Upper Mantle Velocity Structure And Seismic Risk Of The Haiyuan Fault And Its Adjacent Area

The Haiyuan Fault tectonic zone and adjacent areas are located in the northern part of the NorthSouth seismic zone in China,at the intersection of several geological units of the Qilian block,the Alxa block,and the Ordos block.There are many active faults in the area,and strong earthquakes occur frequently,among which the 1920 Ms8.5 Haiyuan earthquake occurred on the Haiyuan fault.The Haiyuan earthquake is one of the destructive earthquakes that have occurred in modern times.However,with the accelerated urbanization,there is an increasing number of major projects and high-rise buildings.Earthquakes will cause serious losses of people’s lives and properties.Therefore,it is urgent to carry out earthquake prevention and mitigation work in the Haiyuan Fault tectonic zone and adjacent areas.In this paper,we use seismic tomography to identify the subsurface structure of the study area,and then use near-field strong ground motion to simulate the characteristics of surface ground shaking distribution,which can provide scientific support for earthquake prediction and mitigation in the Haiyuan Fault tectonic zone,and provide a theoretical basis for urban earthquake prevention and mitigation.The main work and conclusions of the paper are as follows:1.For the Haiyuan Fault tectonic zone and adjacent areas,1277 seismic event waveform data recorded by 227 broadband seismic stations in the study area were screened and used.29,593 high quality Pwave initial arrival times were finally obtained through intercepting,de-instrumentalizing,filtering,and manual picking steps,which can be used for subsurface structure inversion in the study area.2.Using the seismic travel-time laminar imaging technique based on the equation of the equation,the inversion of the obtained P-wave initial arrival time data was used to obtain the subsurface 3D velocity structure and azimuthal anisotropy structure model in the Haiyuan Fault tectonic zone and adjacent areas.The imaging results reveal that:(1)There are significant low-velocity anomalies and weak azimuthal anisotropy in NW-SE direction in the lower crust of the Qilian block,and the main deformation mode of the Qilian block is extrusion deformation in combination with existing research data.(2)A significant high-velocity anomaly is observed beneath the Alxa block and extends into the east Qilian orogenic belt,which may imply that the lithosphere of the Alxa block subducted beneath the east Qilian orogenic belt.(3)The two devastating earthquakes on the generalized Haiyuan fault in the study area had different gestation structures.The 1920 Ms 8.5 Haiyuan earthquake occurred at the boundary of the high-velocity anomaly and the orientation anisotropy of the source area changed dramatically.It is believed that the high-velocity anomaly near the Haiyuan fault is the concave and convex body of the Haiyuan earthquake mechanism,and the body is easy to accumulate stresses due to the change in orientation anisotropy direction.In additional,the 1927 Ms 7.9 Gulang earthquake occurred in the normal upper crust,and strong low-velocity anomalies are found below this earthquake.3.The ground motion characteristics of the 1920 Haiyuan earthquake were simulated.Using the subsurface structure model of the Haiyuan fault obtained in the previous step and the research results related to the Haiyuan earthquake,a kinematic source model was constructed,and the ground motion characteristics of the Haiyuan earthquake were obtained by numerically simulating the ground motion within the period of 2～80s using element-by-element parallel spectral-element method,and further calculating the theoretical seismic intensity.The simulation results show that the earthquake propagation is mainly along the fault rupture direction,with the directional effect of strong ground motion,and the most serious earthquake damage is mainly concentrated along the fault and distributed along the fault direction,and the maximum intensity of the extreme earthquake area can reach Ⅺ～Ⅻ degrees.The simulation results provide a reliable basis for seismic disaster prevention and mitigation planning and siting of major projects in the region.