Numerical Simulation On Dynamic Processes Of Earthquakes On Normal Faults

Like compressive tectonics, extensional tectonics, marked by normal faults, isalso common in China and rest of the world. Some areas of extensional tectonics arevery seismically active. In China mainland, some major or even large earthquakesgreater than magnitude8documented by history are associated with normal faults inextensional tectonic setting. Typical examples include the1303Hongdong, ShanxiM8,1556Huaxian, Shanxi M81/4,1739Pingluo, Ningxia M8,1411Dangxiong, TibetM8and1951Bengcuo, Tibet M8. Thus study on earthquakes caused by normalfaulting is an important aspect of seismology.It is widely accepted that earthquakes are attributed to stick-slip motion on faults.Such slip depends on the magnitude of shear stress on the fault plane (including theshear stress produced by gravity) with respect to frictional resistance. When shearstress exceeds frictional resistance, the fault will slide. And as the fault is big enoughto cause a large rupture, a major or great shock will occur. Since the location, timeand magnitude of major quakes are difficult to predict in advance, it is usuallyimpossible to observe the stick-slip and rupturing process on a fault directly. Inaddition to inversion based on geodetic measurements or seismic wave data,numerical modeling is a useful approach to help understand fault mechanism ofearthquakes. So far, there have been many studies in this aspect, of which most arefocused on thrust and strike-slip faults. While the earthquake mechanism related withnormal faults seems to be underappreciated by researchers.In view of the problems aforementioned, this thesis attempts to explore themechanism of the earthquakes caused by normal faults using the finite elementmodeling. It includes two major parts:(1) Simulation of stick-slip on normal faults ontheoretical models.(2) Numerical modeling of major earthquakes in the Sanxi grabenzone.1Simulation of stick-slip on normal faults on theoretical modelsFirst, theoretical models of a normal fault are constructed. By various boundaryconditions, frictional coefficients and dip angles of the fault, this work simulates the slick-slip on the normal fault under extensional stress by the finite element method. Inparticular, this thesis attempts to address the issues that how the extensional rates,frictional coefficient and fault dipping angle influence stick slip on the fault.(1) Effects of extensional the rate on the normal fault1) Under gravity and extensional tectonic stress, a normal fault can generatemajor earthquakes.2) In general, a bigger extensional rate would lead to a shorter interval of majorquake recurrence as well as a larger coseismic displacement（when the extensionalrate is smaller than5mm/a）.3) When the extensional rate is1mm/a, the maximum displacement is2.8m. Theinterval between the two events with>1m displacements is14520years. As theextensional rate increases to2mm/a, the maximum displacement is5.7m. The intervalbetween the two events with>1m displacements is6363years. For an extensional rate3mm/a, the maximum displacement reaches9.2m. The interval between the twoevents with>1m displacements is3409years. With an extensional rate4mm/a, themaximum displacement becomes11m. The interval between the two events with>1mdisplacements reduces to2361years. And an extensional rate5mm/a can generate themaximum displacement as large as9m with interval between the two events with>1mdisplacements only1977years.4) The maximum displacements of small quakes are mostly in upper8km of thesubsurface. Such displacements of large events are generally confined to a depthrange of5-15km. With increasing extensional rates, the depths of maximumdisplacements of large events tend to grow.(2) Effects of frictional coefficients on the normal fault1) In general, with decreasing frictional coefficients, the maximum displacementof the fault becomes larger, and the recurrence interval of major quakes is shortened(when the frictional coefficient is larger than0.4).2) In the case of an extensional2mm/a and a frictional coefficient0.7, the maximum displacement on the fault is3.5m and the recurrence interval of events withdisplacement>1m is8333years. Then, for the values of the frictional coefficient0.6,0.5, and0.4, the maximum fault displacements and recurrence intervals are5.7m,6363years;8.3m,5882years, and14.3m,7692years, respectively.3) The maximum displacements of small quakes are mostly in upper8km of thesubsurface. Such displacements of large events are generally confined to a depthrange of4-15km. With decreasing extensional rates, the depths of maximumdisplacements of large events tend to increase.(3) Effects of fault dipping angles on the normal fault1) When the normal fault dips at about60, the fault is most capable ofgenerating earthquakes. The normal faults with dips45°and75°approximatelysame seismic levels. And the normal fault with dip greater than45°seems to beslightly more active in seismic activity.2) In the conditions of an extensional rate2mm/a and frictional coefficient0.7,with the fault dips60°,45°and75, the resultant maximum fault displacements andaverage recurrence intervals are5.5m,3500years;3.5m,8333years, and2.9m,8333years, respectively.3) With parameters of an extensional rate2mm/a and frictional coefficient0.6,using the fault dips60°,45°and75, the simulated maximum fault displacementsand average recurrence intervals are6.5m,4167years;5.7m,6363years, and3.3m,6500years, respectively.2Numerical simulation of earthquakes in the Linfen basin in ShanxiThis thesis takes the Linfen basin in Shanxi (or fault-bounded depression zone)as an example to simulate stick slip process on the normal fault of such an extensionaltectonics. The modeling calculations reveal stresses, strains, energy transfer, strainaccumulation and release before and after seismic events, which are used to study thesize, frequency, depth, and recurrence intervals of major quakes. By choosing severaltypical events, this thesis analyzes the depth range and surface displacements of the normal fault as well as their influencing factors.(1) Under gravity and extensional stress, the normal faults in the Linfen basin inShanxi can generate major earthquakes.(2) The occurrence of a major quake involves a process of elastic strainaccumulation and release on a causative fault plane. When the fault is slick (quietperiod), shear stress increases gradually, and energy is built up. Immediately beforethe slip (impending time), shear stress grows rapidly, reaching critical state. As thefault slides suddenly (coseismic time), shear stress declines to release stored energy.(3) The maximum displacement on the Linfen basin in Shanxi can be up to14.6m, equivalent to an M8event. The average recurrence interval between the eventswith slip>1m is2971years.(4) Small quakes can happen randomly, of which most are confined to depths4~15km, exhibiting a wave-like migration from deep to shallow and deep again.(5) When a major quake occurs, the larger the displacements of the fault, thedeeper the rupture downward extends. Some ruptures faults with smallerdisplacements do not reach the ground surface.(6) For a normal fault, the displacements on the ground depend on the maximumslip and its depth. A larger maximum value of fault slip would produce largerdisplacements on the surface. While the deeper the maximum slip, the smaller thesurface displacement.(7) In general, a bigger stress drop of the quake will generate a largerdisplacement on the fault.In summary, the primary results of this thesis are as follows:(1) Using the fault contact model and visco-elastic medium, this work hassimulated the dynamic process of stick-slip motion on the normal faults. It isdemonstrated that under gravity and extensional tectonic stress, a normal fault cangenerate major earthquakes. This is a new insight into earthquake mechanisms withrespect to previous studies.(2) By numerical modeling, this thesis studies main influencing factors of stick-slip motion on normal faults. The result shows that with the increasingextensional rate and decreasing frictional coefficient, the maximum displacement onthe fault grows and the recurrence interval of events is shortened. When the dip angleof a normal is around60, it is most capable of generating big quakes.(3) Through numerical simulation, a synthetic catalogue of stick-slip events forthe Linfen basin in Shanxi is established. The stresses, strains, energy transfer, energyaccumulation and release of this tectonic zone are analyzed. The result indicates thatthe maximum fault displacement can reach14.6m (approximately an M8shock), andthe recurrence interval between the events with displacements>1m is2971years.Since there are few previous work on the mechanism of earthquakes causednormal faults, and because of limited software and hardware conditions, this work hassome unperfect aspects. For instance, the analysis model is highly simplified, timestep for calculation is still big, the initial stress field cannot be evaluated precisely,and the state at depth is unclear. These problems remain to be studied further in thefuture.