Ground-motion attenuation, source, and site effects for the 26 September 2003 M 8.1 Tokachi-Oki earthquake sequence

On 26 September 2003 a megathrust earthquake of M 8.1 occurred at the Kurile trench, Northern Japan. We used accelerograms recorded on more than 300 K-NET stations, ranging in distance from 40 to 500 km, to determine source, attenuation, and site effects for the mainshock and four aftershocks with M 7.3, 6.4, 5.9, and 5.5. The analyses are based on linear regression applied to the Fourier spectra data of each event. The ground-motion attenuation for all the events can be modeled using an assumed geometric spreading coefficient b(1) = -1.0 with an associated anelastic attenuation model given by an apparent Q = 135f(0.76). By using dummy variables in the regression, site amplifications relative to National Earthquake Hazard Reduction Program (NEHRP) C sites are determined for D and E sites. Nonlinear site amplification was investigated for the M 8.1 data but was not significant in determining the overall amplification factors. Fourier spectra data were corrected to a reference near-source distance and site condition (hard rock). The source spectrum for each event was compared to that of the theoretical Brune-model spectrum, in order to compute seismic moment and stress drop. Seismic moments obtained in this manner agree with those of other studies. Stress drops range from 100 to 200 bars with no apparent dependence on magnitude. The mainshock stress drop was 120 bars. A similar value (100 bars) was calculated for an interface aftershock. Events with the highest stress drops (near 200 bars) may have been in-slab. events.