High-density surveys conducted to reveal active deformations of the upper forearc slope along the Ryukyu Trench, western Pacific, Japan

Abstract

In this study, which was conducted to investigate present-day geological deformations occurring off the central Ryukyu Arc along the Ryukyu Trench (Okinawa-jima) in the vicinity of Japan, we obtained and examined dense, high-resolution, 2D grid (high-density) multichannel seismic reflection data. The forearc slope of the central Ryukyu Arc lacks a large forearc basin but has a steep narrow slope that dips to the southeast in the direction opposite to the plate convergence. However, while the seismic profiles obtained in our investigation do not reveal any active structures indicative of compressional stress in this convergent subduction zone, recent tectonic deformations are characterized by normal faults that strike perpendicular to the Ryukyu Trench axis. Examinations of high-density geological data show that the recent relative motion of the overriding plate has resulted in an active, arc-parallel extensional field. Furthermore, the results of our observations indicate two stages of extensional stress beneath the forearc slope of the central Ryukyu Arc that resulted from back-arc rifting in the Okinawa Trough.

Relative Sea‐Level Changes Over the Past Centuries in the Central Ryukyu Arc Inferred From Coral Microatolls

“…Arai et al () and Goto et al () also documented the existence of arc perpendicular normal faults in the arc and the fore‐arc domain. Seismic data across the faults show that they cut across Quaternary sedimentary sequences and marine terraces both onshore and offshore.…”

“…The central Ryukyu Islands are affected by numerous active faults either related to back‐arc spreading in the Okinawa Trough or to arc‐parallel extension in the fore‐arc domain (Arai et al, , and references therein).…”

“…It underwent at least two stages of rifting (Kimura, ; Lee et al, ; Letouzey & Kimura, ; Sibuet et al, , ) and is still active today. It is bounded by arc‐parallel en echelon active normal faults, which offset the seafloor by several meters (Arai et al, ; Sibuet et al, , ) and are associated with microseismicity (Sato et al, ). The GPS data allow for estimating spreading rate decreasing northward from 50 to 10 mm/year along the Okinawa Trough (Nishimura et al, ) implying that slip rates may range between a few tenths of a millimeter per year and a few millimeters per year along individual faults.…”

“…Seismic data across the faults show that they cut across Quaternary sedimentary sequences and marine terraces both onshore and offshore. Arai et al () estimated a slip rate of 0.1 mm/year along such normal faults. The islands of Yoron and Okinawa are crosscut by active faults belonging to the later system.…”

“…Another arc‐parallel major normal fault having a several hundred‐meter‐high scarp and belonging to the back‐arc rifting system cuts the seafloor between Yoron and Okinawa and bounds to southeast the Yoron basin (Figure b). It moves at a mean rate on the order of 0.1 mm/year (Arai et al, ; Goto et al, ). Surface deformations related to motion along those crustal faults are limited within a radius similar to the fault depth (at most tens of kilometers; King et al, ).…”