Simulating Ground Motions from Geodetic Data for ShakeMaps

Over the past several years, we have developed an automated finite-source analysis procedure making use of data recorded by regional distance broadband stations. The method determines the best fault plane by testing the two possible nodal planes of the regional distance moment tensor. Both line-source and plane-source inversions are performed, and the source parameters from these inversions are used to characterize rupture finiteness and directivity. Near-fault ground motions obtained by integrating the derived slip distribution with near-fault Green's functions can be used to augment ShakeMap. For example, source finiteness information significantly improved the initial ShakeMaps of the 2003 Mw6.5 San Simeon, California, earthquake. Our present work has two primary thrusts: 1) development of a method for the near-realtime inversion of GPS data to independently determine finite-fault geometry and orientation, and slip distribution, and 2) investigation of methods to simulate high-frequency ground motions from the geodetic slip models. In this study, we will present a method for converting slip models obtained from GPS data into kinematic models whose rupture process is governed by the rupture and slip velocities. Preliminary results show that simply assuming a rupture-to-shear velocity ratio of 0.8 and a slip velocity derived from a constant stress drop model performs well. We will demonstrate the approach for the 1994 Northridge earthquake by simulating motions using the Wald et al. (1996) kinematic model, a uniform slip model, and the geodetic slip model of Hudnut et al. (1996). The simulated motions for the geodetic model will be compared to both the kinematic model reference and the data in both the time domain and the spectral acceleration domain. We will also compare the simulations in terms of peak ground velocity ShakeMaps. Finally the results will be characterized in terms of the uncertainty due to the unknown rupture velocity and stress drop.