Ocean Bottom Seismometer Study of the Kuril Trench Area PDF Download

Author: Junzo Kasahara
Publisher:
ISBN:
Category : Marine geophysics
Languages : en
Pages : 108

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Book Description
An ocean bottom seismograph was deployed on the seaward side of the Kuril Trench off Hokkaido, Japan, in 5460 m of water, in August 1975 during the joint Soviet-American Tsunami Expedition. During the seven-day record, S-P times were distributed in three groups: 19-24 sec, corresponding to aftershocks of the 10 June and 13 June 1975 earthquakes southeast of Nemuro, Japan and to earthquakes east of Sanriku, Japan; approximately 30 sec, from south of Erimo Peninsula, Hokkaido; and approximately 100 sec, 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 Japanese archipelago. However, deeper focus earthquakes reveal anomalously high velocities 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. Spectral analysis of S arrivals suggests Q sub s values of 1000-1500 for nearby earthquakes and 4000-6000 for longer-distance earthquakes, implying an unusual attenuation mechanism for long travel paths, which enhances the high frequencies.