Study On Fault Geometry And Kinematics Of The Wenchuan Mw7.9 Earthquake In Its Initial Rupturing Stage

The Longmen Shan thrust belt is not only the boundary between Sichuan Basin and Qinghai-Tibet Plateau,but also the key part to study the crustal thickness variation,active faults and the potential earthquake disasters of the Tibetan plateau.On 12 May 2008,the Mw 7.9 Wenchuan earthquake occurred on the middle segment of the Longmen Shan thrust belt(LTB).Approximately five years later,on April 20,2013,the Mw 6.9 Lushan earthquake occurred in the southern part of the Longmen Shan fault zone.The distance of the two earthquakes epicenters is about 90 km.Because of the unique seismogenic tectonic environment and the huge loss of life and property,the Wenchuan earthquake causes great concern in China and abroad.Geological and surface surveys show that the maximum vertical displacement of the surface caused by the Wenchuan earthquake reached to 6.5± 0.5m and its created about 240 km surface rupture on the Longmen Shan fault zone with a surface rupture length of about 300 km.The length of the surface rupture along the Yingxiu-Beichuan fault is about 240 km,which was dominated by thrust and with dextral strike-slip component.The pure thrust surface rupture of the Guanxian-Jiangyou fault is about72 km and there is a 6 km long surface rupture of Xiaoyudong fault,which is characterized by thrust and sinistral strike-slip.In order to fully understand the rupture process of the Wenchuan earthquake,a large number of scholars have studied this earthquake and obtained a series of research results.Their results demonstrate that the rupture of the Wenchuan earthquake propagates mainly unilaterally from the hypocenter toward northeast over a distance of more than 270 km and the duration lasts more than 100 s.The entire rupture process can be roughly divided into four sub-events.The focal mechanism of the initial rupture is a pure thrusting,the second stage is mainly thrusting with small strike-slip components,the third stage is thrusting with strike-slip and the fourth stage is strike-slip with thrusting.Although these studies reveal the time-space rupture process of the Wenchuan earthquake,but there are shorten of the detailed study of the initial rupture of theearthquake.For example,1.The source parameters(time and depth of the Wenchuan earthquake)are quite different;2.The details of the rupture process at the initial stage are not studied;3.There are disputes about the seismogenic structural model;4.The types of earthquake nucleation need to be further studied.Therefore,it is still of great scientific significance to study the detailed process of the initial rupture of Wenchuan earthquake.Based on the above problems,this thesis uses P-wave superposition method similar to back projection method to study the fault geometry and kinematics characteristics of the initial rupture process of Wenchuan Mw 7.9 earthquake on May12,2008,China.This paper mainly completes the following work:1.Determination of the initial rupture parameters of the Wenchuan earthquakeA large number of scholars studied the mechanism of the seismogenic mechanism and the epicenter parameters of the Wenchuan Mw 7.9 earthquake and obtained a series of research results.Although these studies have given us a more comprehensive understanding of the focal mechanism of the Wenchuan earthquake,there are large differences in source parameters(eg.,time occurrence and depth of epicenter).The epicenter uncertainty of the Wenchuan earthquake are effect by the difference of the recorded data types and the error of the seismic phase identification.The lateral non-uniformity of the crustal velocity structure in the Longmen Shan fault zone is also an important parameter which can cause the difference of the location results.The near source record can constrain the focal depth and the moment of the earthquake effectively,and the error caused by the velocity model can be further reduced due to the relatively small difference.Therefore,compared with the long-distance(regional and tele-seismic data sets)source records,the location of the source obtained using the near-field data is more reliable.In this study we investigate the initial rupture position of the Wenchuan earthquake from the local seismic array around the Zipingpu Reservoir close to the epicenter,using the back-projection approach and relative time difference of the P wave onset,respectively.The relative time difference of the P wave onset resultshows that the initial rupture of the Wenchuan earthquake is located at 31.016°N ±0.002°,103.386°E ± 0.002°,the focal depth is 8.3 km ± 0.92 km,and the time of the earthquake is May 12,2008.14:27: 58.819 seconds ± 0.52 seconds(Beijing time,the time in the following text is Beijing);The back projection waveform result shows that the initial rupture position of the Wenchuan earthquake is 31.018°N ± 0.003°,103.391°E ± 0.003 °,the focal depth is 8.4 km ± 0.86 km,and the time of the earthquake is 14:27,58.80 seconds ± 0.4 seconds on May 12,2008.2.The Fault Geometry of Wenchuan Earthquake in the Initial rupture stageLongmen Shan thrust belt is about 500 km long and 40-50 km wide.It is mainly composed of Longmen Shan back mountain fault zone along Wenchuan-Maoxian county,Longmen Shan central main fault zone along Yingxiu-Beichuan,Longmen Shan Front Mountain fault along Guanxian-Jiangyou and Longmen Shan front hidden fault.These four faults strike N45 E and inclination NW,and thrust toward Sichuan basin in imbrication.The fault dip angle is 60°-70° at the surface and the dip angle decreases with the increase of depth.Because of the geometry of the Longmen Shan Fault zone,the focal location parameters of earthquakes have a direct impact on its seismotectonic model.The understanding of focal depth of Wenchuan earthquake ~15 km has been widely accepted at present.On this basis,there are two main seismogenic structural models: one research groups think that Wenchuan earthquake is a high dip thrust fault(>50 degrees),while other research groups think that the seismogenic fault of Wenchuan earthquake is a low dip(~33 degrees)thrust fault,relatively.Most of the strong thrusting earthquakes with magnitude 8 or large mainly occur in the subduction zone of the ocean plate boundary,or in the low dip(< 20 degrees)thrust fault zone of continental collision.So why did the Wenchuan Mw 7.9 earthquake occur in the Longmen Shan thrust belt with high dip angle(shallow fault dip angle greater than 50 degrees,depth greater than 30 degrees at 15 km)? The discussion of this scientific problem has certain reference significance for earthquake prediction in the high dipnappe tectonic area in the future.Firstly,the spatial and temporal distribution characteristics of the initial rupture of Wenchuan earthquake are obtained by P-wave back projection method in three-dimensional space,and then the optimal fault model is obtained in the range of inclination NW40-60 degrees and dip angle 20-50 degrees.Finally,the seismogenic structural model of the initial rupture is constructed by synthetically analyzing the geometric characteristics of the internal faults in Longmen Shan thrust belt.The results show that the optimal fault model of the initial rupture of the Wenchuan earthquake is NE48 degrees and NW35 degrees.The analysis shows that the initial rupture is located on the intersection fault plane of Yingxiu-Beichuan fault and Guanxian-Anxian fault at the depth of about 8 km and the occurrence of the earthquake is the result of the interaction of multiple faults.3.Kinematics study of the initial rupture process of Wenchuan earthquakeThe rupture process of the Wenchuan earthquake studies show that the earthquake is dominated by the thrust faults.The rupture propagates nearly 270 km from the epicenter to the northeast along the Longmen Shan thrust fault.The entire rupture process lasts for about 100 s and contains multiple seismic sub-events and the average speed of the rupture process is about 3 km/s.Although,these studies have image a clear overall pattern of the rupture process of the Wenchuan earthquake,no research refer on the fault geometry and kinematics research in the initial stage(0-1 s)of the Wenchuan earthquake.Meanwhile,all of these studies set the initial rupture position base on one of the published source parameter study.However,the epicenter of the Wenchuan earthquake has been controversial,especially in the depth and original time And use the inaccurate source location parameters will have a certain impact on the interpretation of the Wenchuan earthquake rupture process.Therefore,the detailed rupture process at the beginning of the Wenchuan earthquake still requires further research.Here we investigate the very initial rupture of the Wenchuan earthquake(0-1 s),using a back-projection method.The results show that there is a initial rupture processin the Wenchuan earthquake.The rupture is concentrated near the initial rupture areas in 0-0.3 s and then begins to propagate to the both southwestward and the downward next,after that he rupture begins to propagate to both upward and northeastward after and the mean speed of the rupture propagation is about 2.7 km/s.The energy released in the southwest area is stronger than the northeast areas in the initial rupture stage of Wenchuan earthquake.4.Discussion the nucleation process and earthquake genesis of the Wenchuan earthquakeGenerally speaking,earthquakes usually begin with small amplitude vibrations,which often determine the final magnitude of an earthquake.Although the interpretation of the initial rupture process of earthquakes is controversial,the pre-slip and cascade model can be used to explain the earthquake nucleation process.In the study of seismic nucleation types,the source information carried by near source records is analyzed to describe the development process of earthquakes in detail,which can provide some reference for the study of earthquake mechanism.At present,whether the Zipingpu Reservoir impoundment has triggering the occurrence of Wenchuan Earthquake is still controversial.Some scholars think it has no effect,some think it has certain influence,and some think it has obvious triggering effect.Therefore,the study of the nucleation process of the initial rupture of the Wenchuan earthquake can provide a basis for solving this controversial problem.Details of the initial rupture of the Wenchuan earthquake show that the rupture areas concentrates near the initial rupture position firstly with energy increases slowly,then a relatively large new rupture occurs outside the initial rupture point and eventually develops into an earthquake event.The depth range of the whole rupture process is 7.5-9.5 km.From the rupture process of the initial stage of rupture,the nucleation model of this earthquake is satisfy the "pre-slip" model.According to the nucleation model of the initial rupture of the earthquake,combined with the tectonic stress,geological conditions,GPS measurement results before the earthquake,and therelationship between reservoir water storage and seismic activity in the reservoir area before the earthquake,it is concluded that the reservoir water storage of Zipingpu has a significant promoting effect on the occurrence of Wenchuan earthquake.In summary,We study the initial rupturing process of the 2008 Mw7.9 Wenchuan earthquake occurred on the eastern Qinghai-Tibetan Plateau,using the back-projection method and the P-wave phases recorded by a near-source seismic array deployed in the Zipingpu reservoir region.The results show thatthe initial rupture of the Wenchuan earthquake is located at 31.014°N ± 0.003° and 103.391°E ±0.003° with afocal depth of 8.4 km ± 0.82 km,at 14:27:58.800 ± 0.4 s on 12 May2008.The initial rupture is located on the fault plane at the junction of Yingxiu-Beichuan fault and Guanxian-Anxian fault at about 8 km beneath the surface.The strike of the fault is NE48 degrees and the dip angle is NW35 degrees.The rupture process of the Wenchuan earthquake in the initial stage is periodic.The rupture concentrates near the initial rupture point firstly and gradually accumulates energy.A relatively large rupture appears in the southwest direction of the deep initial rupture point at 0.4s and then the rupture begins propagates along the fault plane to both upward and northeastward with an mean speed of about 2.7 km/s.Energy diffuses along both sides of the fault after the rupture occurs,and the rupture in the southwest areas is stronger than that in the northeast areas.The depth of the rupture process is variation from 7.5 to 9.5 km.The behavior of the initial rupture process shows that the nucleation model of the Wenchuan earthquake is satisfy the "pre-slip" model and we support the view that the impoundment of Zipingpu Reservoir has obvious inductive effect on the occurrence of Wenchuan earthquake.In this thesis,we supplemented the study of the initial rupture process of the Wenchuan earthquake and the spatial distribution of rupture points are used to constrain the geometric parameters of the seismogenic faults in the initial stage.Although there are some uncertainties and unavoidable errors,the research ideas and methods in this thesis can provide some reference for the study of the initial ruptureprocess of large earthquakes,the seismogenic structural model,earthquake nucleation and the earthquakes genesis.