Seismological evidence for the high-velocity zone in the Kuril Trench area from ocean bottom seismometer observations

Abstract

An ocean bottom seismograph was deployed on the seaward side of the Kuril Trench off Hokkaido, Japan, in 5460 m of water in August during the 1975 Joint Soviet-American Tsunami Expedition. During the 7-day record, S-P times were distributed in three groups: 19–24 s, corresponding to aftershocks of the June 10 and June 13, 1975, earthquakes southeast of Nemuro, Japan, and to earthquakes east of Sanriku, Japan; ∼30 s, from south of Erimo Peninsula, Hokkaido; and ∼100 s, from the Izu-Bonin islands. Seven earthquakes, with hypocenters well determined by the land seismic net, are studied in detail. A shallow focus earthquake yields typical oceanic mantle velocities shallower than 50 km in the slab which dips under the Japan archipelago. However, deeper focus earthquakes reveal anomalously high velocities (Vp ≥ 8.50 km/s, Vs ≥ 4.80 km/s) averaged over the upper 230 km in agreement with the models of Utsu and Oliver and Isacks. Two deep earthquakes, whose paths lie in the Pacific Ocean asthenosphere, suggest a velocity 3% lower than that predicted by Jeffreys-Bullen, in agreement with the above models.