An Attenuation Relationship for Deep-Focus Earthquakes Around the Ogasawara Islands, Japan

Abstract eng:
For an understanding of the ground-motion characteristics of deep-focus earthquakes around the Ogasawara Islands, Japan, first, we investigate the spatial distributions of strong motion indexes for past events and their attenuation relationships. The observed attenuation relationship in East Japan is lower than that in West Japan, especially at short periods, which is expected to be caused by the wave-guide effect of the subducting Pacific plate. In addition, the attenuation relationship in fore-arc of East Japan is lower at short periods than that in back-arc of East Japan because of the heterogeneous attenuation structure of upper mantle in East Japan. The attenuation relationship in the Izu Peninsula and Ogasawara Islands is not consistent with that in fore-arc of East Japan, but with that in back-arc of East Japan or West Japan. This high attenuation observed in Izu-Ogasawara region could be explained by the seismic-wave path perpendicularly propagating through the Pacific Plate. Then, we separately estimate non-geometric attenuation parameters for each deep-focus earthquake. The estimated parameters show that the intrinsic and/or scattering attenuation is weak in order of the paths to fore-arc of East Japan, back-arc of East Japan, and West Japan and Izu-Ogasawara region, and that the intrinsic and/or scattering attenuation along the path to West Japan and Izu-Ogasawara region is far stronger than those along the path to East Japan. In addition, it is suggested that the difference between short-period attenuation characteristics along the path to fore- and back-arcs of East Japan may depend on the event location probably because of the three-dimensional curved shape of the Pacific Plate. Finally, we propose an empirical equation of the attenuation relationship for the deep-focus earthquakes, which is useful to predict strong ground motions for future deep-focus earthquakes.

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