Two-dimensional coda Q structure beneath Tohoku, NE Japan

Abstract

Based on the single scattering model, coda-Q values (Qc) are estimated for several frequency bands from the time decay of coda wave amplitude. The Qc value shows a regional variation and frequency dependency of the form Qcαfn, where n is found to be 0.6–1.2. A method for estimating the 2–D Qc structure has been developed to investigate the regional variation of the Qc value beneath Tohoku, the NE part of Honshu, Japan. The Qc value at each grid point spatially distributed is estimated by averaging the observed Qc values for many event-station pairs with adequate weighting factors. The weighting factor at each grid point for an event-station pair is calculated by taking into account the amount of energy contribution to the coda wave from that grid point. The obtained 2-D Qc structure shows some relation to the S-wave velocity structure, the Qs structure and the seismic activity in this area. In general, low Qc regions have a tendency to correspond to the regions with low S-wave velocity, low Qs value and high seismic activity, while high Qc regions correspond to those with high S-wave velocity, high Qs value and low seismic activity. In the regions around active volcanoes and near the coast of the Japan Sea the Qc value is relatively low, whereas it is relatively high near the coast of the Pacific Ocean.