Estimation of the S-Wave Attenuation in the Western Nagano Region from Twofold Spectral Ratio

Abstract

We estimated S-wave attenuation (Q_S^-1) in a wide frequency range between 4Hz and 60Hz using the twofold spectral ratio [Matsuzawa et al. (1989)] in the western Nagano region, Japan, where the 1984 Naganoken-Seibu earthquake (M6.8) occurred and the seismicity is still active. In the region, there are 49 seismic stations in a range of around 10km in diameter and station separation is several kilometers. In this analysis, 156 shallow (depth <10km) events (0.9≤M_W≤2.6) are used. We can effectively reduce the errors of the estimation by using a number of ray paths. We also determined the focal mechanisms of these events and corrected the waveform amplitudes using them. The direct S-wave portions of the seismograms are relatively small in a high frequency range (above 60Hz) at surface stations compared to the lower frequency waves, and contaminated by P-coda waves. Thus, to estimate Q_S^-1 value, we used only the waves whose S/N ratios are greater than 2, where the noise levels are calculated for the time windows just before S-wave arrivals. Obtained Q_S^-1 values show strong frequency dependence below 10Hz, but weak above 10Hz. These values are slightly larger than the ones estimated by Yoshimoto et al. (1998) from the coda-normalization method. This difference is probably owing to the fracture area of the 1984 Naganoken-Seibu earthquake that has strong attenuation.