Seismic Source and Local Site Effects on Long-Period Ground Motions from the 2003 Tokachi-Oki Earthquake

Abstract eng:
We study characteristics of long-period ground motions from the 2003 Tokachi-oki earthquake (Mj 8.0), a large interplate earthquake, based on spatial distribution maps and attenuation relationships of PGV (peak ground velocity) value for four period ranges; wide band and three narrow bands (central periods of 10, 20, and 30 s). The spatial distribution maps for all kinds of PGV value show azimuth dependence; the PGV values in northern side of the epicenter are larger than those in southwestern side of the epicenter, when compared at a comparable distance. These features result from the radiation pattern of long-period surface waves; that is, the source effect. The attenuation relationships indicate that the PGV values for the wide-band period range are affected by ground motions with periods less than 10 s at distances less than 200 km, while long-period surface waves mainly contribute to them at distances larger than 200 km. The basin site effects generate a patchy pattern in the spatial distribution maps and a large scattering in the attenuation relationships of PGV values for the wide-band and 10 s period ranges. We examine the site effect of the Sarobetsu basin located at the northern tip of Hokkaido based on the observation data and simulations. Finally we conclude that the PGV values from the 2003 Tokachi-oki earthquake are controlled by the radiation pattern of long-period S and surface waves, and various basin site effects.

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