Hiroshi Katao

Geophysical observations around the northern Yap Trench: seismicity, gravity and heat flow

Abstract Seismicity, gravity and heat flow measurements were taken around the northern Yap Trench, southwest of the Mariana Trench. Subduction of the Pacific Plate at the Yap Trench is thought to have been interrupted by collision of the Caroline Ridge since the Early Miocene. A large number of shallow seismic events were observed below the inner trench wall by four OBSHs deployed for 10 days in 1986. In contrast, only a small number of earthquakes is reported by the ISC (International Seismological Center) for the same area. Free air and Bouguer gravity anomaly profiles across the northern Yap Trench imply convergence or subduction in the area. The heat flow profile shows similar variation as is observed at other trench systems. However, it differs in two respects, namely, the distance between the high heat flow area and the trench axis is small (90 km), and the highest heat flow occurs in the Parece Vela back-arc basin. These findings together with the available petrological data indicate that current plate subduction is probable in spite of the Caroline Ridge collision. However, simple plate subduction can neither explain the distinctive heat flow profile nor the high and shallow seismicity beneath the inner trench wall. With our present understanding, these features can be explained by very slow subduction which accompanies minor thermal activity and shallower slab assimilation than those of normal subduction.

Crustal structure of the Bonin Trough

Abstract A seismic refraction study using OBSH and explosives was carried out in the Bonin Trough. The P-wave velocity model along the trough axis consists of 4 layers; the velocities, Vp, of these layers are from top to bottom 2.0, 3.6, 6.3 and 7.8 km s−1 respectively. The thickness of the crust is 13 km at the northern part of the trough and becomes thicker toward the south. The negative free-air anomaly pattern in the trough can be explained by thickening of the crust.