Coseismic Radiation Of Large Earthquakes And Its Relationships To Fault Complexities

Understanding the rupture process of large earthquakes and their energy radiation patterns are important in seismology.With the help of back-projection method,the frequency-dependent behavior of seismic energy radiation,as well as its relationship with fault structures,has been widely identified in subduction zone megathrust earthquakes,continent-continent collision large earthquakes and continental medium-sized earthquakes.In this paper,we focus on two large continental earthquakes,and try to relate their radiation pattern with fault complexities.Our work contains:(1)improving the spatial resolution of the compressive-sensing back-projection method with window-time adjustment technique;(2)studying the supershear rupture process and high-frequency radiation of the Mw 8.1 Kunlunshan earthquake occurred in 2001;(3)studying the rupture process and frequency-dependent radiation pattern of the Mw 7.9 Wenchuan earthquake.Based on the sparsity constraint of earthquake subevents,the compressive-sensing back-projection method shows advantages over the classic back-projection methods mainly in several aspects:a higher spatial resolution,ability to resolve multiple sources,and better capability to resolve frequency-dependent seismic radiation sources.Large earthquakes usually rupture hundreds of kilometers long,so teleseismic waveforms are almost aligned perfectly at the first break while they become slant as subevents move far away from the epicenter,especially for an array with a large aperture.Slant waveforms of the later arrived P phase are truncated by the sliding time window with a tapered boxcar shape,which leads to inaccurate subevent locations after inversion.To address this problem,we propose to set the sliding window with window time adjustment utilizing reference rupture fronts calculated at every time step.In this way,the windowed waveforms are aligned again using those reference front.Synthetic tests demonstrate the effectiveness of this method(for unilateral rupture earthquakes).We apply the improved compressive-sensing back-projection method to data from European seismic stations and study the rupture process of the 2001 Kunlun earthquake.We found that the Kunlunshan earthquake has apparent increase of rupture speed near the Hongshui River,which suddenly accelerates to about 5.0 km/s,where a supershear rupture occurs.Moreover,the triple junction of faults at the Kunlun Pass seems not to hinder the supershear rupture,and high speed remains until the end of the rupture.The average rupture velocity of the earthquake is about 3.5 km/s.We believe that the fault structure disturbance and local tensile stress conditions near the Hongshui River jointly produce the super-shear rupture process.The long and straight main Kunlunshan fault is favorable to maintain the supershear velocity.We also found that the seismic subevents in the mid-and high-frequency band are similar,which are distributed on the complex fault structures in the Kunlunshan Fault Mountains,where faults are bent,bifurcated,and intersected.We believe that these fault complexities cause disturbances in earthquake rupture speed that induce high-frequency energy release.With the same method,we select data from regional stations in Alaska to invert for the rupture process of the 2008 Wenchuan earthquake.We find that the rupture speed of the Wenchuan earthquake also has a significant acceleration.The initial speed is slow,only about 1.5 km/s.Then it suddenly accelerates to 3.0 km/s near Xiaoyudong fault.The total average speed is about 2.8 km/s.We assume that the sudden release of an asperity near Xiaoyudong fault accelerates the rupture.We also find that the coseismic energy radiation is frequency dependent:the lower frequency(<0.3 Hz)radiation sources are concentrated on the smooth fault plane;the higher frequency(>0.3 Hz)radiation sources are distributed in the fault complexities.Their complementarity indicates different rupture behaviors of energies from different frequency components.The low-frequency result corresponds to a continuous rupture process with large slip;while a large amount of high frequency energies is released when the rupture is disturbed.The rupture speed regulates the close relationship between the high-frequency radiation and the fault complexities.