Attenuation Relationship Characteristics of Ground Motions for Deep-focus Earthquakes around the Ogasawara Islands, Japan

Abstract

For an understanding of the ground-motion characteristics of deep-focus earthquakes around the Ogasawara Islands, in this study, we first investigated the spatial distributions of strong motion indexes for past events and their attenuation relationships, and estimated the non-geometric attenuation parameters for each deep-focus event. We found that the observed attenuation relationship differs among each region in Japan: the intrinsic and/or scattering attenuation is weak in order of the paths to the fore-arc of East Japan, the back-arc of East Japan, and West Japan and the region from the Izu Peninsula to the Izu-Ogasawara Islands. The complex distribution of ground motions is expected to be caused by the presence of the subducting Pacific Plate and the heterogeneous attenuation structure of upper mantle in East Japan. In addition, we found that the difference between short-period non-geometric attenuation characteristics along the path to the 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. Considering the observed regional variation of attenuation relationship, we proposed an empirical equation of gourd-motion attenuation relationship for deep-focus earthquakes. We also estimated correction terms for the amplification by deep sediments and shallow soft soils for deep-focus earthquakes. Using the empirical equation of ground-motion attenuation relationship and the correction terms, we carried out the ground motion predictions for past deep-focus earthquakes and compared the results with the observations.