Aftershock Distribution Of 2010 M_w6.9 Yushu,China Earthquake

On April 14,2010 at 07:49:36 Beijing Time,a strong earthquake with a magnitude of Mw 6.9 occurred in Yushu County,Yushu Tibetan Autonomous Prefecture,Qinghai Province,China,triggering massive human casualties and property damage.This earthquake was one of the largest earthquakes to occur on the Qinghai-Tibet Plateau in this century.The earthquake caused a rupture of approximately 75 kilometers on the Yushu segment of the Yushu-Ganzi-Xianshui fault,which is one of the most active rupture zones in the eastern Qinghai-Tibet Plateau.The mainshock was preceded by an Mw 4.6 foreshock and was followed by numerous aftershocks.Identifying lost events and relocating aftershocks are very important to supplement the missing events in the original seismic catalog.It can improve the accuracy of seismic localization,and provide a deeper understanding of the spatial distribution,source evolution,and triggering mechanism of aftershocks beneath the seismic zone.In this paper,We collected the most complete observation data of aftershocks of the Yushu earthquake,detected and relocated the aftershocks of the 2010 Mw 6.9 Yushu earthquake using matched filter technique(MFT)and HypoDD double-difference relocation algorithm.First,we collected waveform data continuously monitored from February 28,2010 to August 30,2011,from a total of 50 permanent and temporary observation stations.After preprocessing the data and picked both P and S arrivals for all catalog events,we selected earthquakes with SNR larger than 5 as templates for scanning on continuous waveforms.A total of 4,819 events were detected within three months after the main shock,which is more than twice the number of earthquakes detected in the same time period before MFT.Then,we used the improved catalog for cross-correlation calculation to get the travel time differences of P-waves and S-waves,and selected more than 3.98 million event pairs with cross-correlation coefficients not less than 0.7.Finally,we successfully relocated 2,233 aftershock events using the HypoDD double-difference relocation algorithm.Before relocation,the root mean square(RMS)of the residual for the earthquake group was 2.34 s,which decreased to 0.24 s after the double-difference relocation.Finally,we obtained the high-resolution distribution characteristics of aftershocks.Combined with the b-value characteristics,Coulomb rupture stress,slip distribution,and interferometric deformation characteristics,we analyzed seismic activity and explored the mechanism of the Mw 6.9 Yushu earthquake comprehensively.The relocation results show that most of the aftershocks are distributed along the GanziYushu fault and are located in areas where Coulomb stress has increased.The earthquake events can be divided into three groups.Group 1 and Group 2 are separated by the Longbao Lake,and there is a stretching stress field with northeastward offset between the two groups.Among them,Group 1 is located about 40 km northwest of the main shock,which is the gathering area for most of the large aftershocks.Compared with the other two groups,the b-value is higher,and it is affected by pull-apart basin near the Longbao Lake.When the main shock occurred,the accumulated stress before the main shock was slowly released by the strike-slip fault,forming a flower-like structure.Group 2 is located around the epicenter of the main shock and the Mw 5.9 largest aftershock,with a total length of about 30 km.The Mw 4.6 largest foreshock caused negative static Coulomb stress in the main shock nucleation area.Group 3 is located near Yushu Town,mainly on the southeast side of the Ganzi-Yushu fault zone.The cross section shows that the depth of the aftershocks generally decreases compared with Group 1,and the events are concentrated around the main fault on the surface,with a near-vertical dip angle.The overall stress is high,and after the main shock occurred,it migrated to the surface with larger slip.Based on the activity patterns of each group of aftershocks,we speculate that the seismogenic processes of the Mw 6.9 Yushu earthquake are as follows:a foreshock sequence led by the largest Mw 4.6 earthquake occurred on the conjugate fault of the Ganzi-Yushu main fault 2 hours before the mainshock,hindering the immediate outbreak of the mainshock after the foreshock sequence.The stress was eventually released 2 hours after the largest foreshock.Within six months after the mainshock,more than 2,000 aftershock events occurred along the Ganzi-Yushu fault and migrated to the southeast and northwest sides of the mainshock zone.Aftershocks that migrated to the southeast side lasted for more than a month and stopped gradually.The overall magnitude of these events was small and occurred along the main fault,inducing large surface slip.Aftershocks that migrated to the northwest side lasted for three months and occurred continuously.The overall magnitude of these events was relatively large,and the stress accumulation process was slower.After analyzing the high-resolution spatial-temporal distribution and activity of aftershocks of the Mw 6.9 Yushu earthquake,which is one of the typical strong earthquakes occurred in the Qinghai-Tibet Plateau in this century,we found that it is beneficial to deepen our understanding of the movement of the surrounding blocks and the earthquake process in the Qinghai-Tibet Plateau.This will further contribute to the earthquake warning and disaster prevention in key monitoring areas such as the Qinghai-Tibet Plateau.