Crustal structure across the middle Ryukyu trench obtained from ocean bottom seismographic data

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In 1988 an extensive geophysical investigation was performed at the middle Ryukyu island arc, S. Japan. An aim of the investigation is to obtain a detailed crustal structure at the trench-arc-back-arc system by various geophysical surveys. In this paper we present fine crustal models obtained from ocean bottom seismographic (OBS) data along two profiles. One profile was acquired from the northwest edge of the Philippine sea plate to the Okinawa Island (profile 2-A), and the other profile, which is perpendicular to profile 2-A, was located on the Ryukyu arc (profile 3). A crustal model of profile 2-A is characterized by a sedimentary wedge and subducting oceanic crust. The sedimentary wedge consists 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 layer is required to explain an observed amplitude reduction versus offsets. 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 subducting oceanic moho is deduced to be 26 km at 120 km landward from the trench axis by the reflection phases.