Dynamic Mechanism Of The 1556 Huaxian Earthquake (M=8(1/4)) With Particularly Severe Earthquake Damage

Huaxian earthquake in 1556 recorded the largest number of deaths in history,about 830,000 deaths.With the exploration of active faults in Huaxian seismic fault zone and the study of ancient earthquakes in recent years,scholars have paid attention to the seismic dynamic mechanism in this area.Huaxian earthquake occurred in Weihe basin,Weihe basin is the best natural conditions in Shaanxi area,where the terrain is flat,fertile soil,farming and animal husbandry developed.Therefore,the study of Huaxian earthquake is helpful to explore the seismogenic mechanism of Huaxian earthquake and the earthquake prevention and disaster reduction of Weihe basin.The propagation velocity of fault rupture usually affects the spatial distribution of strong ground motions and the degree of earthquake disasters.However,there is no comprehensive study on how the rupture speeds affect seismic hazards.To this end,this study changed the rupture speeds of the fault propagation from small value to large magnitude(i.e.,from subshear wave velocity to supershear rupture speed),and at the same time,the finite element method was used to calculate the rupture scenarios and the corresponding ground motions.The spatial distribution of the peak ground acceleration(PGA)is calculated with the final dislocations on the fault kept the same in all cases,that is,the moment magnitude of the earthquakes generated under different rupture conditions remains unchanged,which is Mw=7.0.The calculation results show that,in general,the faster the rupture propagation speed,the larger the PGA is,and also wider the distribution of high-value areas.Accordingly,the more serious the earthquake disaster will be.Overall,the seismic hazard will be graver for the supershear rupture earthquake than that of subshear rupture event.However,when the rupture speed is exactly equal to the shear wave speed of the medium,the peak value of PGA near the fault is the largest,and the earthquake disaster is the most serious.In addition,this study also found that when the rupture propagation speed is equal to(?)times of the shear wave speed,which is called Eshelby velocity,no Mach wave appears.This is inconsistent with the general phenomenon of supershear rupture that generates Mach fronts.Therefore,this research is of important scientific significances and practical values for in-depth understanding of earthquake source process,ground motion effects and seismic hazard assessment.In this paper,the two-dimensional model is extended to three-dimensional.Based on the real terrain data,seismic profile data and previous geological survey fault data,a three-dimensional finite element model is constructed.The finite element method is used to simulate the full elastic wave field of Weihe basin during the Huaxian earthquake.The rationality of the calculation is verified by comparing the seismic damage and isoseismic line distribution obtained by historical records and field visits.The results show that the interaction between the soft sedimentary layer of the Weihe Basin and the basin edge effect of the Weihe Basin is the main reason for the wide range of influence of the ground motion and the persistence of the seismic wave.At the same time,the main reason for the large difference in the attenuation between the north and south of the isoseismic line is the uneven distribution of dislocations in the upper and lower plates and the absorption of seismic waves by the mountains in the northern Qinling Mountains.The results of this paper are of great significance to estimate the destructiveness of earthquakes and urban disaster prevention in cities with historical earthquakes but without seismic records.