Seismic Structure
Ocean bottom seismography studies of the Ryukyu trench provide insight into the P-wave velocity structure of the area. Several transects (see accompanying images) have been studied, including a profile of the back arc region parallel to the trench, a transect spanning the trench, fore arc and back arc region, and a transect spanning the Ryukyu volcanic arc. With respect to the transect perpendicular to the length of the trench, many distinct velocity layers are imaged. The sedimentary wedge created by subduction has four distinct layers with p-wave velocities of 1.8 km/s, 2.8-2.9 km/s, 3.5 km/s, and 4.5–5 km/s. The wedge reaches a thickness of 9 km at 50 km from the trench. Beneath the wedge are several layers of oceanic crust (see accompanying figures).
Ocean bottom seismography methods combined with earthquake studies can the dip angle of the Philippine Sea Plate along the Ryukyu trench. Whereas the dip in the Northern part of the Ryukyu trench is shallow at shallow depth, reaching only about 11° in the first 50 km, the dip near the middle of the trench is steeper at 40°-50° (at 70 km depth), and the dip in the northern part of the trench at depth (70 km) is inferred from earthquakes describing a Benioff zone to be 70°.
Separate ocean bottom sesimography and multi-channel seismic profile studies provide insight into the seismic structure of the northern end of the Ryukyu trench region. Features of note include a thick (7-12 km) low velocity (4–5 km/s) zone on the landward side of the trench, the existence of subducting paleo-arc crust near the top of the trench in contrast to simple oceanic crust located at the middle of the trench, and a zone in which the Philippine Plate subducts beneath low P-wave velocity material (Vp = 5 km/s) that coincides with the location of the Mw 7.5 1968 Hyuganada earthquake. It has been hypothesized that the above structural heterogeneity, in particular the subducting paleo-arc crust and its associated bathymetric highs, is one reason why earthquakes in this region are not larger i.e. exceeding Mw 8.0. The exact mechanism by which the subduction of paleo-arc crust prevents sufficient stress build up for a larger earthquake is unknown.
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