UPPER MANTLE ATTENUATION STRUCTURE BENEATH THEEASTERN HOKKAIDO, JAPAN AND ITS EFFECTS ON STRONGGROUND MOTIONS

摘要

It is especially important for predicting strong ground motion from large intra-slab earthquakes to estimateupper mantle attenuation (Q) structure beneath island-arc regions. We investigate effects of the uppermantle Q structure on strong ground motions based on numerous data observed at dense strong motionstations in the eastern part of Hokkaido, Japan. First we investigate strong motion records and their Swavespectra from intermediate-depth earthquakes. The S-wave spectra at stations on the fore-arc side ofthe volcanic front (VF) are extremely different from those on the back-arc side; the S-wave spectra at theback-arc side stations severely lack in high-frequency contents. Next we estimate the Qs (Q-value for Swave)structure beneath the eastern Hokkaido. The study zone is tentatively divided into three zonesbounded by a coastline and the VF; the three zones mainly correspond to a back-arc side mantle wedge(BAMW), a fore-arc side mantle wedge (FAMW) and a shallow descending plate. We especially usestrong motion data obtained by ocean bottom seismometers for plate Qs estimate. Two different methods,the spectral inversion and the coda-normalization method, are applied to Qs estimate for the FAMW andplate. The Qs-values for these two zones are nearly the same and show strong frequency-dependence. TheQs-values obtained from two methods are nearly the same. We also estimate Qs-values for the BAMW byusing the S-wave spectra observed at stations on the back-arc side of the VF and the source spectraobtained from the spectral inversion. Although the data set for the BAMW is incomplete due to fewrecords and low signal-to-noise ratios of S-wave spectra, we suggest that the Qs-values for the BAMWhave considerably weak frequency-dependence. Finally we examine the attenuation structure of the plate,because the estimated Qs value in this study seems to be small compared with past studies. We assumetwo-layer Qs structure for the plate and obtain high Qs value in the lower layer. We conclude that theanomalous upper mantle attenuation structure beneath the eastern Hokkaido reflects the difference in thefrequency-dependence of Qs-value at high-frequencies (>1Hz). The laterally heterogeneous Qs structureobtained in this study is useful for predicting strong ground motion from large intra-slab earthquakes suchas the 1993 Kushiro-oki earthquake.