Numerical Simulation Of Coseismic Surface Deformation And Stick-slip Dislocation Process Of Strike-slip Fault

Earthquake may cause huge loss of life and property to human society.Tectonic earthquakes are caused by the sudden dislocation of faults.According to the mode of fault activity,faults can be divided into three categories: normal fault,reverse fault and strike-slip fault.Strike-slip faults are widely distributed around the world.The strike-slip earthquakes are dominant in both quantity,intensity and distribution area in China and its adjacent regions.According to statistics,294 among the relatively complete earthquake sequences above magnitude 5.0 recorded in China since 1970,the strike-slip type,strike-slip type with tilting component,thrusting type with strike-slip component and thrusting type are 48%?24%?17% and 11% respectively.From 1900 to 2015,there were 29 earthquakes with surface rupture in mainland China,the proportion of normal fault,reverse fault,strike-slip fault,normal strike-slip and reverse strike-slip fault are 17%?31%?41%?4% and 7% respectively.These indicate that the strike-slip fault is the main fault in China.China is one of the most earthquake-prone regions in the world.According to statistics,about 33% of the strong earthquakes in the world occur in China.The historical earthquake experience in China shows that the collapse of buildings after earthquake is the direct cause of casualties.As we all know,the disasters caused by strong earthquakes to human beings are merciless and destructive.To avoid or mitigate the loss of such disasters,we can mainly proceed from two aspects: one is seismic fortification.Try to avoid active faults where strong earthquakes may occur.This requires us to work out scientific and reasonable engineering evasion stipulate and improve building quality and fortified standards;another is to improve the ability of earthquake prediction so that strong earthquakes can be prevented in advance and dealt with effectively when they come along.To do this,we need to have a further understanding of earthquake formation and occurrence process,so as to deeply understand the characteristics and laws of seismic activity.Since the introduction of the finite element numerical simulation method into geoscience,it has shown its unique advantages.With the improvement of computer software and hardware technology,the finite element numerical simulation method has been gradually used in many research fields.Based on the above two research requirements,in this study,the finite element numerical simulation method is used to study the surface deformation and rupture characteristics of strong earthquakes by 3D strike-slip faults model,and to analyze the viscous stick-slip motion and the seismic activity characteristics of different influencing factors of 2D strike-slip faults.Some meaningful results and insights have been achieved as following:1.Numerical simulation of coseismic surface deformation of strike-slip faultField investigation after the earthquake shows that the strong deformation and fracture zone caused by the co-earthquake of the strike-slip fault are mainly concentrated in the narrow range on both sides of the fault.In order to quantitative and scientific study on the surface rupture zone characteristics of co-earthquake and its influencing factors,several 3-D finite element models of strike-slip fault is established based on the western section of Xianshuihe fault in Sichuan-Yunnan area and the effect law of dislocations,fault dip,the mode of dislocation,the thickness of the sedimentary layer on surface deformation are discussed respectively.The results show that even if a single strike-slip fault,the surface deformation and fracture width caused by strong earthquake co-seismic dislocation are also affected by many factors.The results are summarized as follows:(1)Based on the pure strike-slip fault model with an inclination of northeast with angle of 80 °,a focal depth of 11 km and a horizontal dislocation of 3.6 m,the strong deformation characteristics and fracture width of the surface are analyzed.The results show that the surface deformation of the strike-slip fault is approximately symmetric single-peak distribution centered on the fault,and the strong surface deformation is concentrated in the 50 m width range on both sides of the fault and the width of the surface rupture is about 32 m.(2)Based on the pure strike-slip fault model with an inclination of northeast with angle of 80 °,we calculated the cases of dislocation from 1.5 m to 9 m with increment of 0.5 m.The results show that the surface deformation peak value increases with the increase of dislocations,and the rupture zone width increases with the increase of dislocations,but the increment decreases gradually and tends to a asymptotic value.(3)Using pure strike-slip fault model with an inclination of northeast and horizontal dislocation 3.6 m,and the fault dip angle is 45 °,50 °,60 °,70 °,80 °,90 ° respectively,the simulation results show that the effect of fault dip on the surface deformation and rupture zone width is that the peak value point of deformation deviates to fault dip direction with decreasing of fault dip.(4)According to normal strike-slip fault and thrust strike slip fault model with an inclination of northeast with angle of 80 ° and horizontal dislocation 3.6 m and vertical dislocation of 0.5 m respectively,the results show that the peak value of deformation of normal strike-slip is the largest,but rupture zone width is the smallest,which is 22 m,the peak value of deformation of thrust strike-slip is the smallest,but rupture zone width is the largest,which is 34 m,while both parameters of vertical pure strike-slip fault are in the middle(the rupture zone width is 32m).The width and the deformation peak value of fracture zone is related to the fault dislocation.The normal strike-slip and reverse strike-slip with dip angle of 80 °,vertical dislocation of 0.5m and horizontal coseismic dislocation of 3.6m are simulated.By comparing the surface deformation distribution of the pure strike slip,normal strike-slip and reverse strike-slip,it is found that the peak value of deformation of normal strike-slip is the largest,but the width of fracture zone is the smallest,which is 22 m;the peak value of deformation of reverse strike-slip is the smallest,but the width of fracture zone is the largest,which is 34 m;the two parameters of the pure strike-slip fault are in the middle.(5)Based on a pure strike-slip fault model with vertical fault,focal depth of 11 km and horizontal dislocation of 3.6 m,the effect of the thickness of loose sedimentary layer on surface deformation is investigated.The surface deformation peak value of the strike-slip fault decreases with the increase of the thickness of the loose sedimentary layer,but the rate decreases gradually.When the thickness of sedimentary layer increases from 5m to 20 m,the rupture zone width increases slowly with the increase of thickness.However the rupture zone width begins to decrease with the increase of thickness when thickness is larger than 20 m.When the thickness of the sedimentary layer is 44 m,the surface deformation can not reach the rupture threshold.(6)Based on a pure strike-slip fault model with vertical fault,focal depth of 11 km and horizontal dislocation of 3.6 m,the surface deformation changes of different soil cover layers and different thicknesses from 10 m to 80 m are analyzed.The results show that when the thickness of different soil cover layers(coarse sand,silt,clay)is the same,the peak value of surface deformation increases successively from coarse sand,silt and clay.And when the thickness of coarse sand is more than 60 m,the horizontal co-seismic dislocation of 3.6 m can no longer form the surface rupture.The thickness of silt and clay is more than 65 m and 75 m respectively.2.Numerical simulation of stick-slip dislocation process of strike-slip faultBased on viscoelastic medium and contact fault model of 150 km length and 30 km width,a century-scale stick-slip motion simulation of a 2D straight strike-slip fault is carried out,and the influence of different parameters on seismic activity of the fault is discussed.The preliminary understandings were obtained as following:(1)Uniform strength strike-slip faultWhen appropriate mechanical parameters and boundary conditions are selected for the model,the uniform strike-slip fault exhibits a alternating status process of stick-slip phenomenon of lock(stress accumulation)-unlock(stress release)-lock(stress accumulation)similar to the experimental results,and active and calm characteristics similar to actual seismicity under long-term structural loading.Prior to the occurrence of a single major event,there was a fault pre-slip phenomenon similar to that observed in the laboratory.The location of large events is relatively concentrated,the period and dislocation are similar,and the characteristics of the earthquakes are characteristic.(1)In model 1,the lateral pressure of upper wall is 1.5 MPa,the sliding friction coefficient is 0.3,the static and dynamic friction ratio is 4,and the loading rate of displacement on both sides of the fault are 4cm/y.Simulation results show the fault has the characteristics of quasi-periodic stick-slip motion.The average period of large event is about 41 years,and the average slip is 1.59 m.The location of the maximum dislocation of the stick-slip event is mainly on the right half part near 121.2km,in which the proportion of large events between 120km~126km is 77.78%,which indicates that strong earthquakes may occur repeatedly in the same area if the external boundary conditions are not changed in the uniform fault.Based on model 1,the influence of different parameters on fault activity is analyzed.(2)When the loading rate of boundary symmetrical displacement on the left and right sides of the fault is 3cm/y,the average period of the large event of quasi-periodic dislocation of the fault is about 51 years,about 10 years longer than that of model 1,and the average dislocation of the large event is 1.45 m,about 0.14 m smaller than that of model 1.The maximum dislocation is mainly located near the 125 km on the right part of the fault,indicating that the occurrence of the strong earthquake is mainly determined by the local geological conditions.The reduction of loading rate leads to increase of time for stress accumulation of fault to reach the endurance limit and the maximum dislocation of a single major event is reduced,and the energy released by earthquake event is reduced accordingly.(3)When the friction coefficient of model 1 is 0.4,the average period of fault events is about 50 years,which is about 9 years longer than that of model 1,and the average dislocation of large events is 1.67 meters,which is slightly larger than that of model 1.It indicates that when the friction coefficient is slightly larger than that of model 1,it can increase the energy accumulation ability of fault plane,increase the cycle of stick-slip fault,and increase the dislocation of single fault.The location of the maximum dislocation is 75% in the right part of the fault and 25% is in the middle of the fault.(4)When the ratio of static and dynamic friction is changed from 4 to 3,the average period of fault dislocation events is about 30 years,which is about 11 years shorter than that of model 1,and the average dislocation is 1.10 meters and 0.49 meters less than that of model 1.The results show that the accumulated shear stress of fault plane is not large due to the decrease of the ratio of static and dynamic friction of fault,the time of fault locking is also shortened,and the recurrence period of large dislocation event is reduced,because of each time energy accumulation decreases.The maximum slip of single event also decreases.(5)When the upper wall lateral pressure in model 1 is 8MPa,it is obviously different from model 1.Almost the whole fault presents quasi-periodic stick-slip dislocation,and the dislocations at different nodes on the fault are almost equal.And there are few small dislocations before and after the major fault events.The average interval of the major events is about 105 years,which is much larger than 41 years of model 1,and the average dislocation of the major events is 5.14 meters,3.2 times that of model 1(1.59 m).The results show that when the lateral pressure increases to a certain extent,the fault stress and energy accumulation capacity increase greatly,the fault is in the state of locking for a long time,and when the stress in the fault reaches the limit of the fault,The energy released by stick-slip instability also increases significantly,and the dislocation also increases significantly.(6)When the displacement rate constraints on the left and right parts of the upper wall in model 1 are different(4 cm/y on the left and 1 cm/y on the right),the distribution of stick-slip events of the fault are no longer as the quasi-periodic characteristics of model 1 and become quite complicated.The dislocation is also small and the event interval tends to increase gradually.(2)Non-uniform strength strike-slip modelThere are two cases of high friction strength and weak friction strength segment in non-uinform model.In order to investigate the influence of the length of non-uniform fault segment,four different models are analyzed in this paper,and the following preliminary understandings are obtained:(1)When model 1 has a 10 km length strong region with friction coefficient of 0.5 in the middle of the fault,the fault presents quasi-periodic stick-slip motion characteristics similar to the results of model 1.Compared with the uniform model 1,the time of occurrence of the first slip event of the fault delays 6 years,the average period of the event increases 6 years,and the average dislocation increases by 0.21 meters.50% of the maximum dislocations occurred in the middle and right part among 84~93km,and it has signs of migration towards the margin near the middle strong area.It shows that the existence of strong 10 km length region makes the fault more favorable to stress and energy accumulation,prolongs the period of earthquake recurrence,increases the dislocation of large event of fault,and increases the energy released by single dislocation event.(2)When there is a 50 km length strong region with friction coefficient of 0.5 in the middle of model 1,compared with the uniform model 1,the first slip event of the fault delays 18 years,the average period increases about 8 years,and the average dislocation increases 0.26 meters.75% of the major events occur in the middle and right part among 80~95km,which is further migrated to the middle strong part of the fault compared with the model with strong 10 km region.It is found that the length of the strong zone in the fault has different influence on the seismic activity of the fault.When the length increases,it is more favorable to the accumulation of the stress and energy of the fault,which makes the locking time of the fault increase.Released energy and dislocation increase when fault is unlocking and sliding.(3)When there is a 10 km length(1/15 of model length)weak region with friction coefficient 0.1 in the middle of model 1,The fault activity is very different from that of the fault with strong zone.Each segment of the fault exhibits different rhythmic characteristics of activity.The quasi-periodic stick-slip faults in the left strong and middle weak sections of the fault are almost identical,but the large dislocation events on the right part of the fault occur 8.6 years later than the left,although the right side of the fault also presents a similar quasi-periodic fault.The period of earthquake recurrence in the left and middle segments of the fault is basically the same as that in the right,and the average dislocation in the right segment is slightly larger than that in the left.It is suggested that the existence of the weak friction coefficient in the fault zone may be an important factor affecting earthquake migration.(4)When there is a 50 km length(1/3 of model length)weak zone with friction coefficient of 0.1 in the middle of the fault,the difference of active rhythm of the left,middle and right segments of the fault is more obvious.The active intensity of each segment of the fault from strong to weak is the right,the left,the middle segment.Compared with model 1,the existence of weak friction coefficient segment(the length of weak segment is 1 / 15,/ 1 / 3 respectively)increases the frequency of seismic activity,but decreases the average dislocation of large event.By comparing the simulation results of weak regions with different lengths,it is found that the influence of the length of weak zones on seismic activity is more complicated.When the length of weak section of fault increases,the influence on the stress accumulation of each segment is increased,the difference between each segment of fault is more obvious,the general level of seismic activity of strong segment is obvious higher than that of weak segment.The increase of weak segment length shortens the period of large event in the left strong segment of fault.On the other hand but increases the period of large events in the right strong segment.(3)Non-uniform model of asymmetric strong and weak region interphaseA asymmetry non-uniform model with 3 weak friction segment of 30 km,60km,30 km and 2 strong frictiom segment of 15 km is designed.The friction coefficient of strong region is 0.6,and 0.15 of weak region,the upper boundary pressure is 1.5 MPa,the static and dynamic friction coefficient ratio is 1.5,and the boundary loading rate of displacement on the left and right sides is 4 cm/y and 2cm/y respectively.The results show that,although each segment of the fault shows the characteristics of active and calm distribution similar to the actual seismic activity,the fault shows obvious piecewise activity.And the stick-slip of the left and right weak segments are respectively influenced by the adjacent strong segments.The strong segments on the fault have controlling effect on the whole fault activity.and the strong area has less stick-slip events and dislocation is larger while the weak area has more stick-slip events and dislocation is smaller.Even the left and right strong segments with the same friction strength have different characteristics of stick-slip activity,their stick-slip dislocation period,The characteristics of activity such as intensity are also different.The results show that the stick-slip motion of the segmental inhomogeneous fault model is extremely complex under the condition of asymmetric boundary loading rate,and the interaction between the fault segments exacerbates the complexity of the fault activity.Based on the simulation of the stick-slip motion of the two dimensional strike-slip fault,the dislocation and stress variation characteristics of the stick-slip process are analyzed,the catalogue of the stick-slip dislocation events of the strike-slip fault and the active and calm phenomena similar to the actual seismic activity are obtained.The process of fault locking-unlocking-locking and energy accumulation-release-accumulation of motion cycles is revealed;The analysis of the influence of different factors on the stick-slip motion provides a theoretical reference for the application of numerical simulation method in fault dynamics and seismic activity research.The main innovations of this study are as follows: 1)Using finite element method,a large scale 3D viscoelastic strike-slip fault model is established,and the distribution characteristics of surface deformation caused by coseismic dislocations of strong earthquakes are simulated.The spatial distribution and the peak value of deformation,which are approximate to the real strong seismic deformation zone are obtained.(2)The effects of dislocation,fault dip angle,dislocation mode,the thickness of the sedimentary layer and different soil media on surface deformation of strike-slip fault are systematically analyzed.It has reference significance for the analysis of deformation zone characteristics of strong earthquake.(3)The 2D viscoelastic straight strike-slip fault model is established,and the dynamic process of stick-slip motion of strike-slip fault is simulated by using fault contact algorithm,and the synthetic seismological catalogue of quasi-period stick-slip instability dislocation of strike-slip fault is obtained.(4)The effects of boundary displacement loading rate,tribological parameters,fault lateral pressure and displacement loading mode on the stick-slip instability of the fault are analyzed.It has theoretical and practical significance for numerical prediction of seismic activity.(5)The influence of the distribution of strong and weak friction segments on the seismic activity characteristics of faults is systematically analyzed,which provides a new idea for the segmentation and seismic risk assessment of complex faults.Some significant results have been obtained by numerical simulation of coseismic surface deformation and stick-slip dislocation of strike-slip faults,but there are still few places to be perfected.For example,the effect of gravity,topography and other factors will make the results more comprehensive in the analysis of coseismic dislocations of strong earthquakes,and the dynamic process simulation of the stick-slip motion of the fault considering more actual 3D model,the geometric complexity,medium heterogeneity and even multi field coupling will be the direction of further study.