HR: 0800h
AN: S41A-0944    [Abstracts]
TI: Split Nodes and Fault Zone Models for Dynamic Rupture Simulation
AU: * Dalguer, L A
EM: ldalguer@moho.sdsu.edu
AF: Geological Sciences, SDSU, 5500 Campanile Dr., San Diego, CA 92182 United States
AU: Day, S M
EM: day@moho.sdsu.edu
AF: Geological Sciences, SDSU, 5500 Campanile Dr., San Diego, CA 92182 United States
AB: The accuracy of numerical calculation of the dynamic rupture process of earthquakes mainly depends on the fault boundary condition on the fault where friction is taking place. During rupture the slip is calculated via the equation of motion while the shear stress is controlled by frictional sliding. Such rupture models generally lead to nonlinear, mixed-boundary value problems. The boundary treatment in a numerical method depends in part upon the numerical technique used for the discrete representation of the material. In the present paper we examine two numerical methods. In the first method we use the technique of Day (1982) in which all the components of velocity and stress on the fault are calculated in the same discrete point. In this method, the two sides of the fault share common faces on the fault plane, via split nodes, so that one side of the fault can move relative to the other side; we refer to it as "split model". In the second method we use the staggered-grid velocity-stress FDM formulation, in which the respective components of velocity and stress are calculated on different discrete points. With this formulation the fault is represented with one grid-step (dx) thickness, a method we call the "fault zone" model. For the latter we follow the method proposed by Madariaga et al (1998). To compare these two methods, we solve theoretical dynamic rupture problems of a fault in a homogeneous medium, with the sliding process governed by the slip-weakening friction law. We find that the rupture propagation velocity in the conventional version of the fault-zone model, i.e. with uniform dx, is lower than that in the split model. This appears to be due to the blunting of the stress concentration on the rupture front in the case of the fault-zone model. The rate of stress drop on the rupture zone is faster for the split model, and consequently, the split model tends to accumulate high stress at early time on the rupture front. On the contrary, the fault zone model delays the concentration of stress on the rupture front because the stress drops slowly on the ruptured area. This difference of rupture process suggests that the two methods actually are modeling different fault conditions as a consequence of the different characteristics of the numerical techniques. The split model simulates direct contact between the two sides of the fault that interact through the frictional traction. In contrast, the fault zone model is not simulating direct contact, because the volume of the fault zone plays a role in the dynamics. To obtain further insight into this difference in behavior, we modify the fault zone model by permitting the fault-zone cell dimensions to differ from the rest of the grid. When the fault-zone cell dimension, df, is reduced from dx to dx/2, the rupture velocity in the fault zone model approaches that of the split model. Thus, the behavior of the fault zone model depends upon the grid size. Now the question remains, which method provides a better representation of a real fault? Due to the complexity of real fault zones, either method might be appropriate, depending upon, for example, the amount of distributed shear assumed to occur during rupture. However, it should be kept in mind that the behavior of the fault zone model depends upon what is ordinarily considered to be a purely numerical parameter, dx, and may only approach the behavior of the frictional contact problem when this parameter is considerably smaller than that required for an accurate solution by the split method.
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
DE: 7212 Earthquake ground motions and engineering
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting