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

Abstract

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 (Mw 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 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.