Report on DELP 1984 Cruises in the Middle Okinawa Trough : Part II : Seismic Structural Studies

Multi-channel reflection records across the Central Rift Zone of the Middle Okinawa Trough, near the Natsushima-84 Deep in which an extremely high heat flow was observed, indicate reflections that are probably from the top of a magma chamber. Refraction seismic study using 10 OBS's and four tons of explosives indicated that the crust of the Middle Okinawa Trough has a 5km thick 6.0km/sec layer above a highly attenuating 6.8km/sec layer. Moho was not detected even at epicentral distances greater than 130 km. The crust was considered to be continental but low in Q in its lower part and the uppermost mantle to be anomalously low in both wave velocity and Q. Observations of natural earthquakes, carried out for about one month by 10 OBS's, detected highly active micro-seismicity in the area. Some OBS's recorded particular phases which suspect of reflections from magma sheets as well as particular waves similar to volcanic tremors