Time variations in parameters of split S waves from weak local earthquakes beneath eastern Hokkaido

Abstract

Parameters of seismic waves from clusters of local weak earthquakes that occurred at the upper boundary of the seismofocal zone in depth intervals of 40–60 and 70–90 km along eastern Hokkaido are investigated for the period 1998–2003, including the strong (M = 8.0) Tokachi-Oki earthquake of September 26, 2003. Analysis of data indicates that the distribution of anisotropic properties along Hokkaido is inhomogeneous and parameters of split waves (the azimuth of the fast S wave and the time delay between split S waves) are sensitive to variations in the stress-strain state of the medium. Unstable behavior of split wave parameters and increased values of the ratio VP/VS for clusters of events in the areas of the Hidaka Mountains and Nemuro Peninsula imply that the medium is in a mechanically weakened state (in the regime of intense dilatant deformation). On the contrary, the regions beneath the Tokachi and Kushiro plains are more rigid and are characterized by lower VP/VS values and comparatively stable behavior of wave parameters. Anomalous parameters of split waves from events of clusters in areas of different stations correlate with each other and are related to occurrence times of large earthquakes around Hokkaido, which may point to a redistribution of stresses and strains and fluid migration in the subduction zone.