Structure of East China Sea-West Philippine Sea Margin off southern Kyushu, Japan

Abstract

The results from seismic refraction profiles made south of Kyushu, Japan, are presented in a structure section crossing lhe outer East. China Sea shelf, the Okinawa trough, the Ryukyu ridge, the Ryukyu trench, the north Amami basin, the Kyushu-Palau ridge, and the western edge of the Shikoku basin. The foundation of the shelf-trough-ridge province is composed of material of velocity near 5.7 kin/sec. Above this material are others of velocity 3.4-4.6 km/sec that represent acoustic basement. The western edge of the Okinawa trough is a buried ridge of subbasement material located near the edge of the East China Sea shelf. Sediments partly fill the trough, the thickest section being beneath the western side slope, which has the appearance of a prograding sedimentary feature. Beneath the continental slope to the Ryukyu trench a thick sequence of sedimenl; in a deep trough with no surface expression lies on a depressed and thinner than normal oceanic crust. It seems likely that the trough of thick sediments, of which the Ryukyu trench is a part, has a structural continuation off the east coast of Kyushu, suggesting that the present day trench evolved from a formerly more extensive trench system. The north Amami basin and the western edge of the Shikoku basin have oceanic crust, with a slightly thicker than normal layer 2, whereas the Kyushu-Palau ridge between them consists of a far thicker section of upper crustal material, which causes a. downward bulge in oceanic layer 3 and prestonably in the mantle below.

Crustal and upper mantle structure in the Ryukyu Island Arc deduced from deep seismic sounding

“…In the northern part of the arc (outer arc), on the other hand, the velocity of the main crustal layer is 5.6 km s-', and is overlain by materials with velocities of 3.6 and 4.6 km sC1 (Ludwig et al 1973). …”

“…However, most parts of the axial trough of the trench are not deeper than 6500 m. In the north, the trench is divided by the Amami plateau, which is west of the northern end of the Palau Ridge. The shape of the trench in this area is oval, with depths of approximately 5500 m. Ludwig et al (1973) showed that the crust beneath the northern part of the trench is oceanic but with a thinner than normal layer 3. Such a thin layer 3 was also found further south by Murauchi et af.…”

Crust and upper mantle structures of the region between Korea and Taiwan by surface wave dispersion study

“…However, previous seismic studies could not perfectly cover this study area without the distribution of event-station pairs crossing the East China Sea (Ludwing et al, 1973;Leyden et al, 1973;Hirata et al, 1991;Li et al, 2001). Because the event-station pairs are unique to Korean seismic stations, this study provides new information in this study area (Fig.…”

“…Previous studies in this area have used geophysical methods, such as magnetic, gravitational, sonobuoy seismic refraction, and ocean bottom seismometer (OBS) seismic refraction measurements (Ludwing et al, 1973;Leyden et al, 1973;Hirata et al, 1991;Li et al, 2001;Jiang et al, 2002;Lin et al, 2005;Fang and Liu, 2005;Han et al, 2007).…”

Microearthquake activity in the West Philippine Basin and northern Ryukyu Trench region as observed by ocean bottom seismometers at the Amami Plateau.

“…Dredge hauls and drill cores indicate the Daito Ridge to be a remnant island arc (e.g., KLEIN and KOBAYASHI, 1980). Seismic refraction profiling data over the Oki Daito Ridge and the Kyushu Palau Ridge show thick crust with velocities similar to those of continents (MURAUCHI et al, 1968;LUDWIG et al, 1973). It is not known whether these anomalous regions are seismically active or not except the Central Basin Fault.…”