CRUSTAL STRUCTURE ACROSS THE MIDDLE RYUKYU TRENCH OBTAINED FROM OCEAN-BOTTOM SEISMOGRAPHIC DATA

In 1988 an extensive geophysical investigation was performed at the mi ddle Ryukyu island are, S. Japan. An aim of the investigation is to ob tain a detailed crustal structure at the trench-arc-back-arc system by various geophysical surveys. In this paper we present fine crustal mo dels obtained from ocean bottom seismographic (OBS) data along two pro files. One profile was acquired from the northwest edge of the Philipp ine sea plate to the Okinawa Island (profile 2-A), and the other profi le, which is perpendicular to profile 2-A, was located on the Ryukyu a re (profile 3). A crustal model of profile 2-A is characterized by a s edimentary wedge and subducting oceanic crust. The sedimentary wedge c onsists of four layers, whose velocities are 1.8, 2.8-2.9, 3.5 and 4.7 -5.0 km/s at the top of each layer. The maximum thickness of the wedge is 9 km at 50 km landward from the trench. Underlying the sedimentary wedge, the model shows subducting oceanic layer 2 and 3 (5.3 and 6.8 km/s). A crustal model of profile 3 indicates three layers of sediment , whose velocities are 1.7-2.0, 2.4-2.8, and 3.7 km/s, above a 5.6-6.0 km/s layer. A low Q value layer at the bottom of the sedimentary laye r is required to explain an observed amplitude reduction versus offset s. Three deep crustal reflection phases are observed along profile 3. These phases are interpreted as the reflection phases from the oceanic crust subducting beneath the Okinawa island. The depth of the subduct ing oceanic moho is deduced to be 26 km at 120 km landward from the tr ench axis by the reflection phases.