Seismic structure of rifting in the Okinawa Trough, an active backarc basin of the Ryukyu (Nansei-Shoto) island arc\u2013trench system

Azusa Nishizawa,Kentaro Kaneda,Yukari Fujioka,Daishi Horiuchi,Mitsuhiro Oikawa,Chiaki Okada

doi:10.1186/s40623-019-0998-6

Abstract

The Okinawa Trough, to the southwest of Kyusyu, Japan, is an active backarc basin of the Ryukyu (Nansei-Shoto) island arc–trench system caused by the Philippine Sea plate subduction. Unlike other backarc basins around Japan, the Okinawa Trough, ~ 1000 km in length, is unique because the crustal thinning due to backarc rifting is currently in progress in the entire trough. We conducted extensive seismic reflection and refraction surveys to detect detailed variation in seismic structures associated with the rifting tectonics. Seventeen seismic lines were shot, including ten across-trough and seven along-trough lines in the Okinawa Trough. Moho depths estimated mainly from PmP travel times indicate that the crust beneath the trough is thinner than that below the East China Sea shelf and the Ryukyu Island Arc. The shallowest Moho of the across-trough lines was not necessarily detected at the center of the trough, defined as the deepest water depth, but was located beneath the western margin of the northern and middle Okinawa Trough. An M7.1 earthquake occurred in the area on November 14, 2015, and intense aftershock activity was observed along the western margin of the trough. These earthquakes with extension axes of northwest–southeast direction demonstrated that the area is undergoing tectonic rifting. The Moho depth is over 25 km in the northern region of the Okinawa Trough and decreases down to ~ 13 km as it goes to the south, and the thinnest crust of ~ 7 km occurs beneath the Yaeyama Rift in the southern Okinawa Trough. Despite the crustal thinning by the backarc rifting, the P wave velocity models across the Okinawa Trough show that the continental/island arc crust composed of an upper, middle, and lower crusts is continuous between the East China Sea shelf and the Ryukyu Arc. The multichannel seismic profiles along and across the rifts in the southern Okinawa Trough show more volcanic intrusions in the southern side than in the north, which may be related to the position of the volcanic front, which is undetermined in this region.

Highlights

A series of subduction trenches lines up on the Pacific side of the Japanese archipelago, and subductions of the Pacific plate and the Philippine Sea plate through the trenches form trench–arc–backarc systems (Fig. 1)
We acquired 17 seismic lines to study the fine seismic structure related to the active backarc rifting in the Okinawa Trough (OT)
Variation in the crustal thinning in the OT was inferred mainly from the Moho depth distribution based on Moho reflection (PmP) travel times

Summary

Introduction

A series of subduction trenches lines up on the Pacific side of the Japanese archipelago, and subductions of the Pacific plate and the Philippine Sea plate through the trenches form trench–arc–backarc systems (Fig. 1). The Okinawa Trough (OT), a backarc basin in the Ryukyu (Nansei-Shoto) arc–trench system, which. The OT is around 1000 km long, and the water depth generally increases along the trough from ~ 600 m in the north to ~ 2300 m in the south (Fig. 1). The OT is divided into three regions by the westward extensions of the two large bathymetric depressions across the Ryukyu Arc: the Tokara Valley in the north and the Kerama Gap in the south. Sibuet et al (1995) revealed that crustal thinning is different between the northern and southern OT based on seismic refraction and gravity data. Gungor et al (2012) compiled multichannel seismic reflection (MCS) profiles at the northern OT and pointed out that diffuse rifting/

Results

Discussion

Conclusion