Abstract
We examine behaviors of close-fault strong motions calculated by the extended stochastic simulation method. Horizontal and vertical peak ground velocities (PGV) calculated by this method behave quite differently from PGV calculated by the attenuation relationships. With decreasing the source distance, horizontal PGV grows monotonically and vertical PGV rapidly decreases, while PGV for both components calculated by the attenuation relationships saturates in close-fault zones. Comparison between the near-field term, the intermediate term, the far-field term in the elastic-wave Green's function and the sum of the three shows that these differences are generated because of a neglect of the near-field term and the intermediate term in the stochastic simulation method. This comparison also suggests the frequency range dealt with by the extended stochastic method should be confined to higher frequencies for shallower earthquakes. More specifically, the frequency range may be above 2-3 Hz for shallow (a few km from ground surface) earthquakes. This is confirmed to be adequate from the comparison close-fault earthquake motions in the Hyogoken-Nanbu Earthquake and earthquake motions simulated by this method.
