Numerical Simulation Of The Listric Thrust Fault During Coseismic Triggering Stage Of The Wenchuan Earthquake

The Wenchuan earthquake occurred in May 12th,2008 at the Longmen Shan fault zone,the junction between Eastern Tibetan and Sichuan Basin,western China.Since then,the Longmen Shan tectonic zone has remained a heat spot for researchers for nearly a decade.The Longmen Shan fault zone has its unique tectonic setting and rupture features during the earthquake,which made this area so important that lots of researchers have showed interest in it and have put forward many different views about the mechanism of the Wenchuan earthquake.This article smmarizes the research results of the numerical simulation of the Wenchuan earthquake and the Longmen Shan area.By using numerical simulation software COMSOL Multiphysics 5.2,this article establishes a two-dimensional finite element numerical simulation model with a listric fault to simulate the inter-seismic and coseismic stage of Wenchuan earthquake to explore the stick-slip effect during the triggering stage of the earthquake.In this thesis,research is conducted as followed:(1)Set up a two-dimensional finite element model across the longmen Shan fault zone with a listric fault.Combining the geological and geophysical materials of the Longmen Shan area and the seismogenic mechanism theories put forward by researchers,to set up a two dimensional numerical model including eastern Tibetan Plateau,Sichuan Basin and the Longmen Shan fault zone.Select appropriate parameters and boundary conditions for each region,and apply the Stribeck friction model to the listric fault interface.(2)Study the relationship between the shape of listric fault and the location of hypocenter.By setting up an artificial switch of lock/unlocked fault state on a continuous listric fault,a conclusion is drawn that the location of hypocenter on a listric fault usually lies where the fault bending the most,which is also where high dip angle part meets low dip angle part.(3)Study the inter-seismic and coseismic displacement distribution.The simulation is conducted with separate stage representing the inter-seismic and coseismic stage of the earthquake,where the results of the first stage is used as the initial value of the second stage.Results show that the listric fault which has high dip angle deep part and low dip angle shallow part has an important role in the energy accumulation process along the fault.During the inter-seismic stage,the strain and stress accumulates at the brittle-ductile transition area of the fault,causing a notable shortening and uplifting of crust at the Tibet side.During the coseismic stage,this unique high dip angle fault could accelerate horizontal displacement transforming into vertical displacement.The maximum cosismic displacement is located at the fault emergence,where the dislocation between hanging wall and footwall can be up to 5m and mainly from the hanging wall.The vertical displacement,which mainly concentrate within 15km around the fault,is greater than the horizontal displacement,which decays slower and has larger distribution range than the former.(4)Study the process and characteristic of stick-slip fault during the coseismic triggering stage.Before the major coseismic displacement,there is a triggering stage lasting about 2 minutes,where small scale of stick-slip happens along the fault.There're approximately 4 steps of this stage.First,tremble happens at the depth of 15km along the fault,the major sliding friction potion and the max value of friction are concentrate at the depth of 10-15km.After about 40s,the friction of low fault is broken through,sliding friction potion occurs at the middle part of the fault.When the trembling period lasts over 90s,the whole fault becomes unstable with alternating sticking and sliding friction and multiple strain and stress concentrate points.This could represent the final breaking though period before the major rupture.