Yoshinobu Motoya

Double seismic zone beneath the middle of Hokkaido, Japan, in the southwestern side of the Kurile Arc

Abstract The vertical section of microearthquakes, determined accurately by using the Hokkaido University network, shows two dipping zones (the double seismic zone) 25–30 km apart in the depth range of 80–150 km beneath the middle of Hokkaido in the southwestern side of the Kurile arc. Hypocentral distribution of large earthquakes ( m b > 4) based on the ISC (International Seismological Centre) bulletin also shows the double seismic zone beneath the same region. The hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone. The difference in seismic activity between the two zones beneath Hokkaido is in contrast with the region beneath northeastern Honshu in the northeastern Japan arc. Composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down-dip extension for the lower zone as is observed beneath northeastern Honshu. For the upper zone, however, the stress field is rather complex and not necessarily similar to that beneath northeastern Honshu. This may be considered to indicate the influence of slab contortion or transformation in the Hokkaido corner between the Kurile and the northeastern Japan arcs.

Spatial distribution of intermediate-depth earthquakes with horizontal or vertical nodal planes beneath northeastern Japan

Abstract The 1993 Kushiro-oki (off-Kushiro) earthquake ( M JMA = 7.8) is one of the largest intermediate-depth earthquakes that have occurred in northeastern Japan. This earthquake is a notable event that has a nearly horizontal fault plane extended from the lower seismic plane of the double-planed seismic zone toward the upper seismic plane. We have examined the spatial distribution of focal mechanism solutions of deep and intermediate-depth earthquakes beneath northeastern Japan concerning this peculiar event. Analyses of 127 new solutions along with existing solutions confirm the previous results on essential features of their spatial distribution. The predominant types of solutions are down-dip compression in the upper plane except beneath eastern Hokkaido, and down-dip extension in the lower plane. P-Axes in the upper seismic plane and T-axes in the lower plane are in the direction of plate convergence. Beneath Hokkaido, however, T-axes show significant deviation from the direction of plate convergence. The stress axes beneath the junction between the northeastern Japan arc and the Kuril arc exhibit slight deviation from those in the adjacent areas, suggesting slab distortion beneath the junction. We next investigate the spatial distribution of events with horizontal nodal planes as candidates for events with similar focal mechanisms to the Kushiro-oki earthquake. The activity of earthquakes with horizontal nodal planes is definitely high in the lower plane, especially beneath Hokkaido. Many of them have similar focal mechanisms to the Kushiro-oki earthquake, indicating that the Kushiro-oki earthquake is not exceptional but is a typical event in the region. However, the fault planes of these events are variable in space; some events are known to have vertical fault planes. There are shallow and deep depth bounds and a southwestern boundary for the events with horizontal fault planes. These facts suggest that the horizontal faulting is probably caused by some regional stress concentration owing to the unbending and the distortion of the segmented slab at the junction.

Waveform Similarity among Foreshocks and Aftershocks of the October 18, 1981, Eniwa, Hokkaido, Earthquake

Digitally-recorded waveforms from foreshocks and aftershocks of the October 18, 1981, Eniwa, Hokkaido, earthquake (M=4.0)are compared. The foreshock activity exhibits the pattern similar to a main shock-aftershock sequence. Four consecutive foreshocks (M= −0.1 to 1.3) that occurred during one hour and a half before the main shock have strongly similar waveforms. An average value of peak cross-correlations for pairs of foreshock seismograms is 0.86. The bandpass-filtered waveforms are well correlated at frequencies up to at least 20 Hz. This implies similar source mechanisms and tight clustering of hypocenters within a radius of no more than 1/4 wavelength, i.e., 70 m or less. Aftershock waveforms are variable from one event to the next, although at least four groups of similar events are found during the aftershock period. The present exercise provides one approach to the identification of a foreshock sequence on the basis of remarkably similar waveforms.