| The numerical simulation of the earthquake preparation,occurrence and cycle is helpful for us to understand the physical nature of earthquake preparation and occurrence.contribute to our deep understanding of earthquake phenomena.and contribute to the research of numerical earthquake prediction.The finite element method is very suitable to simulate the seismic rupture process in complex geological conditions(complex fault geometry,complex medium physical structure,complex initial stress field.complex friction constitutive relationship,complex mechanical state distribution and change).However,due to the great difference in the mechanical state of earthquake preparation and earthquake coseismic rupture process,Therefore,the continuous simulation of the cycle of earthquake preparation,occurrence and recurrence brings great challenges to the numerical calculation.Earthquakes take a long time to develop(hundreds or even thousands of years)and occur in a fraction of a second(tens of seconds).The process of earthquake preparation can be expressed by static mechanics equations,while the seismic waves generated by the earthquake can only be described by dynamic equations.The medium is continuous in the process of earthquake preparation,but its continuity changes when the earthquake occurs.In addition,the transient process of earthquake rupture cannot be simulated if the time scale of earthquake incubation is used in the simulation process.On the other hand.to simulate coseismic rupture on time scales(the computational time step is usually 0.001 s or less)requires huge computing resources and huge storage space,which is currently not possible.Therefore,based on the Newmark implicit time integral method.this paper develops a full-dynamic simulation method according to the special requirements of earthquake cycle simulation.This method uses a large time to calculate the step during the earthquake preparation period(up to half a year),but once the fault begins to rupture.the time step decreases rapidly(about 0.001 s),which makes the calculation process stable,robust and convergent.Thus,the continuous simulation of earthquake gestation and occurrence process can be carried out smoothly under the existing computational conditions.Through the case study,it is found that the algorithm inherits the advantages of strong adaptability of the finite element method.At the same time,because it adopts the unconditional and stable implicit time integral and does not change the mechanical equation of state of the model during the operation.the calculation accuracy is high and the stability is good.In particular,in the previous simulation of earthquake rupture,the rupture nucleation area needs to be given in advance.the earthquake rupture can be triggered by human intervention(such as reducing the friction coefficient,etc.),and the initial stress field before the rupture needs to be given.In the simulation of earthquake rupture process,the new algorithm does not need to set the rupture nucleation area,nucleation mode and initial field manually.Instead.the program itself implements it spontaneously by automatically adjusting the parameters.It can be seen that the simulation results of the new method are more consistent with the actual seismic and geological environment.The simulation results can not only give the recurrence interval of strong earthquakes,but also give the detailed mechanical process of fault rupture behavior during each earthquake.In addition,the effects of the friction constitutive relationship and the background loading rate on the incubation period and the recurrence process were also investigated.The free surface of the earth is an important mechanical boundary of the earth medium,which plays an important role in controlling the seismic rupture process,especially the transformation of subshear rupture into supershear rupture.Therefore,the influence of free surface on fault rupture process is simulated in this paper.In the simulation,soft media such as low normal stress in the shallow surface,thick sedimentary overburden and unconsolidated gouge in the fault are considered.In this case,the fault medium near the surface can be considered to be controlled by the frictional constitutive relationship of velocity enhancement.For the sake of comparison,two models are established in this paper.Finite element method is used to simulate the propagation characteristics of spontaneous fault rupture process in the presence and absence of surface friction velocity strengthening layer(VSFL)in the threedimensional half-space model.The simulation results show that the friction velocity enhancement layer(VSFL)inhibits the supershear fracture transition induced by the Earth’s free surface.In particular,the simulation results show that the minimum thickness of the surface friction velocity strengthening layer can completely inhibit the supershear rupture transition is～1.4 km,that is,as long as the thickness of the surface friction velocity strengthening layer reaches 1.4 km,the occurrence of the supershear rupture earthquake can be completely suppressed on the free surface of the earth.At the same time,a large number of domestic and foreign studies have found that the actual thickness of the surface friction velocity strengthening layer in many areas of nature is about 3 km,which is enough to inhibit the generation of supershear fracture.In addition,through the simulation,it is also found that although the barriers,anti-concave-convex bodies and fault step zones on the fault can promote the conversion of sub-shear rupture to supershear rupture,due to the time pause in the process of rupture,the fracture velocity on the whole fault is averaged to sub-shear rupture.In other words,it is difficult to distinguish the occurrence of local supershear rupture from the seismic information received by far-field seismic stations.In order to produce an identifiable super-shear rupture seismic event,the conditions are very harsh,usually the length of the fault is long enough,the geometry of the fault is straight enough,the initial stress level is high enough,the thickness of the surface friction velocity strengthening layer is thin enough.and there are enough seismic stations in the near field,etc.These factors may account for the rarity of supershear rupture earthquakes in nature.Internationally,most of the simulation of earthquake rupture process at present assume that the rupture occurs along the existing fault,but in fact,a new rupture is generated during the earthquake process,that is,the earthquake creates a new fault in the intact crustal rocks.The simulation of new fault rupture involves many scientific problems,such as medium strength,fracture criterion,calculation method and grid resubdivision,which is a great challenge.In this paper,the extended finite element method is used to simulate the fracture propagation of the new fault,and the fracture across the fault order region is simulated.Four comparison models are set up in this paper,which are the step model without fault extension,the step model with fault extension,the step model with parallel intermediate fault and the step model with vertical intermediate fault.By comparing the four models,the influence of the new fault generated by fault propagation and the existing intermediate fault on the process of rupture propagation is studied.The results show that fault propagation changes the geometry of the fault system,and thus changes the strain field and stress field.If the extension length and direction of the fault meet certain conditions,the rupture can continue to spread across the originally uncrossable fault order area,causing greater earthquake disaster. |
PDF Full Text Request