Source Description of the 1999 Hector Mine, California, Earthquake, Part II: Complexity of Slip History

Abstract

We present a rupture model of the Hector Mine earthquake (M 7.1), determined from the joint inversion of strong-motion records, P and SH teleseismic body waves, Global Positioning System (GPS) displacement vectors, and measured surface offset. We solve for variable local slip, rake angle, rise time, and rupture velocity of a finite-fault model involving multiple segments. The inversion methodology developed in a companion article (Ji et al., 2002) combines a wavelet transform approach with a nonlinear (simulated annealing) algorithm. The final model is checked by forward simulating the Interferometric Synthetic Aperture Radar (InSar) data. Our estimation to the seismic moment is 6.28 × 10^(19) N m, which is distributed along three segments from north to south, releasing 37%, 41%, and 22% of the total moment, respectively. The average slip is 1.5 m, with peak amplitudes as high as 7 m. The fault rupture has an average rise time of 3.5 sec and a relatively slow average rupture velocity (1.9 km/sec) resulting in a 14-sec rupture propagation history. Our approach permits large variation in rupture velocity and rise time, and indicates that rise time appears to be roughly proportional to slip and shorter rise times are associated with the initiation of asperity rupture. We also find evidence for nearly simultaneous rupture of the two northern branches.

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