Editing Ryukyu Trench (section)

== Seismic structure ==
[[Ocean bottom seismography]] methods combined with earthquake studies of the [[Wadati–Benioff zone]] constrain the dip angle of the Philippine Sea plate along the Ryukyu trench. In the Northern part of the Ryukyu trench, the dip of the Philippine Sea plate is shallow at shallow depth, reaching only about 11° in the first 50&nbsp;km, and steeper at deeper depths, reaching 70° below about 70&nbsp;km. In contrast, the slab dip in the central and southern parts of the Ryukyu trench is more gentle, reaching only 40-50° at 70&nbsp;km depth.<ref name=crust />

[[Ocean bottom seismography]] studies of the Ryukyu trench provide insight into the [[P wave]] velocity structure of the area. In the northern part of the trench, several transects 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.<ref name=crust>{{cite journal|last=Kodaira|first=S|author2=T. Iwasaki |author3=T. Urabe |author4=T. Kanazawa |author5=F. Egloff |author6=J. Makris |author7=H. Shimamura |title=Crustal structure across the middle Ryukyu trench obtained from ocean bottom seismographic data|journal=Tectonophysics|date=15 October 1996|volume=263|issue=1–4|pages=39–60|doi=10.1016/S0040-1951(96)00025-X|bibcode = 1996Tectp.263...39K }}</ref> The transect perpendicular to the length of the trench images many distinct velocity layers. The sedimentary wedge created by subduction has four distinct layers with P wave velocities of 1.8&nbsp;km/s, 2.8-2.9&nbsp;km/s, 3.5&nbsp;km/s, and 4.5–5&nbsp;km/s. In the area of this transect, the wedge reaches a thickness of 9&nbsp;km at 50&nbsp;km from the trench. Beneath the wedge are several seismic layers within the oceanic crust.

Separate [[ocean bottom seismography]] and [[reflection seismology|multi-channel seismic]] studies provide insight into the structure of the northern end of the Ryukyu trench region. Features of note include a thick (7–12&nbsp;km) low velocity (4–5&nbsp;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&nbsp;km/s) that coincides with the location of the {{M|w|link=y}} 7.5 1968 Hyuganada earthquake.<ref name=northend /> 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 {{M|w}} 8.0.<ref name=northend /> The exact mechanism by which the subduction of paleo-arc crust prevents sufficient stress build up for a larger earthquake is unknown.