The Fine Vs Structure Of The Seismic Gap Between The Wenchuan And Lushan Earthquakes Revealed By Joint Inversion Of Receiver Functions And Dispersion Curves

In this paper,the dense broadband seismic stations deployed on the Eastern margin of the Tibet Plateau are collected to extract the noise-based Rayleigh wave dispersion curves and the receiver functions.The two datasets were used to invert the three-dimensional high-resolution S-wave velocity structure around the un-ruptured segment(the seismic gap)between the Wenchuan and Lushan earthquakes in the southern Longmenshan region.Our results show that the upper crust thrust of the Songpan-Ganzi block is overridden onto the hard and high-velocity Sichuan Basin basement(Yangtze Craton),which indicates that the growth mechanism and geodynamic model of the Longmenshan are more in line with model with the detachment and overthrst of the upper crust.A very obvious feature of the low-velocity in the depth of 14-40 km and a thickened Moho was shown in the seismic gap between the Wenchuan and Lushan earthquakes.Our results show that there is a low-velocity in the seismic gap between the Wenchuan and Lushan earthquake,and the thickened crust and high Poisson's ratio indicate that a partial melting processing may occur beneath the seismic gap.Partial melting at the bottom of the crust may increase the temperature of the F1-F3 section of the Longmenshan fault in the seismic gap to 300-400 °C,and cause these faults to become aseismic(ie,less seismic activity than other sections of the Longmenshan fault system).And it may also result in many hot springs or thermal anomalies on the surface above the low velocity body.This can explain why seismic activity is low in seismic gap areas.The seismic activity of the three main faults,i.e,the Wenchuan-Maoxian fault(F1),Yingxiu-Beichuan fault(F2),and Pengxian-Gungxian fault(F3),above a high-temperature and low-velocity body may be very low,but the fault F6(Shuangshi-Dachuan fault,Dayi fault,or other blind faults in the basin)above a high velocity body might has strong earthquake risk.According to the Culomb Stress calculated by the previous study and ~60 km fault length,we deduce that a major earthquake on these faults is possible and that the magnitude could be larger than that of the Dayi earthquake(M 6.2)and be up to M 7.