Abstract

In the present study we investigate the high-frequency (HF) radiation mechanism of the 2000 Tottori earthquake in Japan based on a 3D spontaneous fault rupture dynamic model. We generalize the model of HF radiation of a suddenly stopping circular crack (Madariaga, 1977; Boatwright, 1982) to the radiation from a general 3D rupture in a planar fault, where HF is radiated during gradual changes of rupture velocity at the rupture front. Local rupture velocity changes are expressed as the divergence of local rupture velocity vectors that are derived from gradients of rupture times from the dynamic model. Our numerical model of the Tottori earthquake indicates that rupture velocity changes are largely induced by barriers (locally stronger fault sections) across the fault plane and that HF radiation mainly originates within asperities (large stress-drop regions) in areas where the product of dynamic stress drop and rupture velocity changes is maximum. We develop a strong ground motion simulation methodology that incorporates HF radiation inferred from a dynamic fault rupture model. Using this methodology we investigate the HF radiation of the Tottori earthquake by inverting observed near-source acceleration envelopes of the earthquake. Our inversion results corroborate that HF radiation originates within asperities and show that significant HF radiation represents no more than a 20% of the total asperity area. Our results show that the incorporation of a directivity factor, on the basis of a well-defined physical rupture model to the radiation pattern leads to a significant improvement in fitting of observed ground motions. Our simulated near-source strong ground motions of the Tottori earthquake are also able to reproduce the ω-2 radiation theoretically predicted in 2D dynamic fault rupture models.

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