Spatial-temporal Evolution And Fault Zone Imaging Before And After The Wenchuan Earthquake Using The Waveform Matched Filtering Method

The 12 May 2008 Mw 7.9 earthquake occurred along the Longmenshan Fault(LMSF)in Sichuan province of China.Field observations revealed that the primary rupture was along the LMSF extending up to ~330km long.The Wenchuan earthquake is the largest intra-plate high-angle thrust earthquake recorded by seismograph.After the earthquake,many studies over the last decade focused on the subsurface structure,source process,and postseismic response of the 2008 earthquake by varies of geological and geophysical methods.Different tectonic models have been proposed to explain the occurrence of the Wenchuan earthquake,however,the aftership,postseismic activities and the geometry of seismogenic faults ruptured during the 2008 Wenchuan earthquake is still in debate.Furthermore,very few systematic study on the comprehensive seismic activity of Longmenshan fault zone before the Wenchuan earthquake had carried out,which is of key importance for the study of the occurrence mechanism of the Wenchuan earthquake.Seismic waveform data recorded by dense broadband seismic array has greatly improved the spatial resolution and acuracy of small earthquake locations.However,the massive data recorded by dense seismic array also poses a challenge to the manual processing of seismic data.The Waveform Matched Filtering method(WFMT),which adopts the cross-correlation technique in the means of searching for the "similar earthquakes",can quickly pick up the missing earthquake events not in the conventional earthquake catalogue.The relocated earthquakes can provide additional constraints for fault activity and deep geometry,and has been widely used in microseismic detection and imaging of the earth’s interior.Therefore,the combination of the above two methods can efficiently and quantitatively study the deep activity of the fault zone.During this study,the Nov.18,2017 Mainling Ms6.9 earthquake occurred in Linzhi city,Tibet,China.Since the earthquake occurred in the Namgabawa structural junction near the Eastern Himalayan Syntaxis,it quickly attracted the attention of scholars and many research institutes started the post-earthquake research work.During this time,a dense temporary broadband seismic array was deployed in this area by the Institute of Geology,China Earthquake Administration.Using the continuous seismic waveform data recorded by the array,we relocated the aftershocks in the standard catalog using the double-difference seismic location method,and then used the waveform matched filtering method to detect the early aftershocks of this earthquake.We detected aftershocks in the first 7 days after the main shock.The total number of aftershocks were about 10 times more than earthquake bulletin by China Seismic Network.The results systematically showed the temporal and spatial evolution and distribution of aftershocks.As a test run of the WFMT mathod,this study shows that the method along with the double-difference seismic location can be used to detect and locate microseismics quickly and systematically,which provides a method basis for further studying the spatial and temporal evolution and distribution of microseismics before and after the Wenchuan Mw 7.9 earthquake in 2008.From October 2006 to August 2009,the Western Sichuan Seismic Array(WSSA)was in operation around the central and southern part of the LMSF.The array includes 297 broadband stations with Gurap CMG-3ESPC broadband sensors and RefTek-130B01 data acquisition system,and the average station spacing is 10-30 km.The WSSA covered the southern 2/3 segment of the Longmen Shan fault activated during the Wenchuan earthquake,providing a unique opportunity to systematically study the seismicity before and after the Wenchuan earthquake.In this study,we apply the WFMT and the double-difference seismic location method to detect and locate the small earthquakes before and after the Wenchuan mainshock.Our goals are to explore detailed spatio-temporal evolutions related to the nucleation process,deep fault structure,deep deformation and the seismogenic mechanism of the Wenchuan earthquake.Firstly,we systematically detected and analyzed the evolution and distribution of the early aftershocks of Wenchuan earthquake.Based on a relocated catalogue of aftershocks,1273 aftershock events were selected as template earthquakes after signal-to-noise ratio detection.The WFMT method was used to scan the continuous waveform data within the first 7 days after the main shock.We obtain up to ~5.3 and ~10 times more aftershocks than the standard bulletin and the relocated template catalogue,respectively.We compared the results with the distribution of the deep ruptures,the distribution of long-term aftershocks to investigate the early afterslip rupture extension.We also compared the inferred fault geometry with a seismic reflection profile from deep soundings in the same region.The results provide new constains on the fault geometry and fault evolution shortly after the Wenchuan mainshock.Then,we focuses on the characteristics of seismicity within about 16 months before the Wenchuan earthquake.This study is also based on the data from WSSA.After signal-to-noise ratio detection,175 bulletin events before the Wenchuan earthquake are confirmed as the templates,and 1273 aftershock are also included in the detection.Totally 1640 new earthquakes before the Wenchuan earthquake are detected,up to ~7 times more earthquakes than the template earthquakes.In order to describe the seismicity more precisely,we use the double difference seismic location method to relocate all the detected earthquakes.Finally,we obtain the the evolution and distribution of microseismicity before the Wenchuan earthquake.Based on the above results,the following main conclusions can be drawn:(1)The evolution of the early aftershocks following the Mainling earthquake indicates that the aftershocks mainly distributed along two different NWW direction faults,which located in the Xixingla fault and between the east and west sides of the boundary fault zone in the north of the Namcha Barwa syntaxis.The fault has the clearly segmentation rupture characteristics and the main shock is located in the central part of the aftershocks zone.Combined the results of the focal mechanism solution,we suggest that the seismogenic fault is steep dip to the northeast direction and the segment to the northeast of the main shock could be triggered by the main shock.The overall length of the aftershocks is about 50 km along the fault.The results systematically show the distribution and evolution of aftershocks in the early stage of this earthquake,quantitatively show the process of aftershocks triggering fault activity,and fully show that the combination of waveform matched filtering and double-difference seismic relocation method can be used to quickly and systematically study the temporal and spatial evolution and distribution of seismic activity,and can also excavate important information of fault activity.(2)The distribution of early aftershocks following the Wenchuan Earthquake shows that early aftershocks mostly occurred below 10 km depth and around the peak mainshock rupture.The early aftershocks mostly occurred down-dip to large coseismic slip areas and overlapped with the afterslip.Early aftershocks also show minor along strike migration with time since the mainshock,indicating triggering of afterslip.From the epicentral region to Beichuan where the mainshock slip was primarily reverse faulting,early aftershocks illustrated Yingxiu-Beichuan and Guanxian-Jiangyong as high-angle listric feature,and rooted to a gentle dipping plane at the depth of ~20 km.The aftershocks in the northeast segment of the Longmen Shan faults mostly occurred along a near vertical dipping fault,consistent with the primary strike-slip component of mainshock slip in this region.In addition,difference of early and long-term aftershock distribution indicates significant postseismic deformation along the conjugate Xiaoyudong fault.Our results manifest the deep geometry of the Lomgmen Shan fault zone and the early evolution after the Wenchuan mainshock,which provides additional constrains to the mechanism of plateau uplifting.(3)The results of microseismic distribution before the Wenchuan earthquake show that the microseismics are mainly distributed around the epicenter of the main shock,the Zipingpu reservoir,along the Xiaoyudong fault and at the ends of the Guanxian-Jiangyou fault.The regions of microseismicity mainly coincide with the asperities in the deep part of the Longmenshan fault or in the large co-seismic slip position.Combining with the results of repeated earthquake,co-seismic slip,the change of velocity from the deep of the Longmenshan fault before and after the Wenchuan earthquake,the early aftershocks and the geological research of large regional scale,we speculate that the inconsistency of long-term movement rate on both sides of the Xiaoyudong fault,and its long-term extrusion effect with the intersection of Guanxian-Jiangyou fault,may be the regional dynamic mechanism leading to the occurrence of the Wenchuan earthquake.(4)We find that the focal depth of the earthquakes distributed around the Zipingpu Reservoir are limited to the top 5-6 km.Intensive earthquake activities only ccurr around February 14,2008,and there is almost no earthquake activity in other time period.From the spatial and temporal characteristics of earthquake distribution near the Wenchuan epicenter and the Zipingpu Reservoir,no significant seismicity evidence is observed indicating the impoundment effect of the Zipingbao Reservoir to have triggered or accelerated the occurrence of the Wenchuan earthquake.This study demonstrates that the combination of waveform matched filtering method and double-difference seismic location method can effectively excavate weak waveform signals.Taking Wenchuan earthquake as the key research object,the spatial and temporal evolution of microseismics before and after the Wenchuan earthquake is systematically analyzed by using the above research methods.The deep geometric structure of seismogenic faults,the mechanism of aftershocks and the microseismics before and after Wenchuan earthquake are deeply studied.The distribution characteristics of seismicity provide important evidence for the seismogenic process and post-earthquake evolution of large earthquakes,and also provide important constraints for the uplift mechanism of the Qinghai-Tibet Plateau.