Dynamic inversion of the 2000 Tottori earthquake based on elliptical subfault approximations

We propose a simplified nonlinear method for the kinematic and dynamic inversion of near-field strong motion data at low frequencies. Using a few elliptical patches we reduce the number of independent parameters of the inverse problem. We apply this method to the dynamic inversion of the Western Tottori (Japan) earthquake (M-w 6.6-6.8) of 6 October 2000. Using unfiltered records we relocated the hypocenter close to 14 km in depth. Fifteen records obtained by the KiK-net and K-NET accelerometer networks were then filtered to the 0.1-0.5 Hz frequency range and integrated to displacement. We compare observed and synthetic records using the L-2 norm. A nonlinear kinematic inversion for the elliptical subfaults is first computed using the neighborhood algorithm (NA). Inversion converges to a slip distribution modeled by just two elliptical patches. We then propose a dynamic inversion method based on the same simple geometrical ideas. Dynamic rupture propagation is computed by finite differences on a coarse numerical grid. Rupture propagation is controlled by a classical slip weakening friction law. Inversion is implemented with the NA for a barrier model. In this model prestress is uniform and rupture propagation is arrested by a simple distribution of barriers. Inversion converges to a model with two elliptical barriers. Synthetics computed for the dynamic inversion fit the observed data, reducing the variance by nearly 60%. By making different assumptions about the rupture process we illustrate the nonuniqueness of the solution to dynamic inversion.