Source spectral characteristics of two large intra-slab earthquakes along the southern Kurile-Hokkaido arc

Abstract

In 1993 and 1994, two large earthquakes occurred along the southern Kurile-Hokkaido arc: the 1993 Kushiro-oki earthquake (15 January; Mw=7.6) and the 1994 Shikotan earthquake (4 October; Mw=8.2). These were intra-slab earthquakes that ruptured through a substantial part of the subducting oceanic lithosphere. We study source spectral characteristics of these large intra-slab earthquakes based on strong-motion records. For the Kushiro-oki earthquake, the S-wave spectra on three rock-site stations are directly compared with the theoretical spectra based on the omega-squared source model and the source parameters estimated from long-period seismic waves. The observed spectra are roughly consistent with the theoretical ones at low frequencies (<0.1 Hz), while they depart from the theoretical ones at high frequencies; the observed spectral levels are about 10 times larger than the theoretical ones at frequencies higher than 1.0 Hz. For the 1994 Shikotan earthquake with the long source duration of about 40 s, we cannot directly retrieve the source spectrum from the observed S-wave spectrum, because the S-wave portion is contaminated by the crustal phases and surface waves. This event occurred in the source region of the 1969 Shikotan earthquake (12 August; Mw=8.1; plate-boundary earthquake). We assume that since the complex path effect at a given station is nearly the same for both events, then the S-wave spectral ratio of the 1994 event to the 1969 event at the station represents their source spectral ratio. After careful procedures to standardize the instrumental response for the old and modern records, we calculate the S-wave spectral ratios at three stations and compare these with the theoretical spectral ratio based on the omega-squared source model and relevant source parameters. The observed spectral ratios are nearly the same as the theoretical one at low frequencies, but they are several times larger than the theoretical one at high frequencies. Our study shows that the source spectra of the two large intra-slab earthquakes depart from the omega-squared source model at high frequencies; these events radiate abnormally strong high-frequency (short-period) seismic waves. This conclusion reasonably explains some unique features of the observed ground motions during these events: the much larger peak horizontal accelerations compared with the empirical attenuation relation and the far more extensive felt area for the 1994 event compared with that for the 1969 event.