Improved Prediction Method for Time Histories of Near-Field Ground Motions with Application to Southern California

We have developed a new method and a computer code to simulate stochastically the kinematic faulting process. To simulate the kinematic rupture we divide the fault of the mainshock into subevents. For each subevent we prescribe the slip history. In our kinematic model each subevent represents a point source with parameters consisting of the local slip amplitude, rupture velocity, and rise timeæall of which are poorly constrained for future earthquakes. In order to allow for our inadequate a priori knowledge we describe these parameters as random variables with probability distribution functions that are bound by estimates of the parameters based on past earthquakes. Dynamic modeling of complex rupture process (e.g. Oglesby and Day, 2002; Guatteri et al. 2003) shows that the areas of large slip correlate with high stress drop, fast rupture velocity and short rise time. But the correlation between rise-time and slip is not so strong as the correlation between rupture velocity and slip (Oglesby and Day, 2002). Based these studies, we assume that the correlation between rupture velocity and slip is about 80 percent and the correlation between rise-time and slip is -50 percent. We have applied this approach to generate kinematics source process for a scenario earthquake on Puente Hills thrust fault. We chose the same the fault geometry as that used by SCEC (https://srb.npaci.edu/cgi-bin/new/mysrb2.cgi, Graves 2003). Near-field synthetic ground motion velocities (up to 1 Hz) are calculated by using a 3D viscoelastic finite difference (FD) algorithm of Liu and Archuleta (2002) and the SCEC 3-D velocity model. This FD code allows for two separate regions: an upper one with finely spaced grid and a lower one with three-times the spacing of the fine grid. This feature allows for a finely spaced grid near the surface where shear-wave velocities are low without carrying the fine spacing to depth where it is not needed. As expected, ground velocities up-dip of the hypocenter (South and West of the fault) are larger due to the average directivity of the rupture. Note the long period late arriving phases due to the deep basin structure and low-velocity near surface material.