Structure of Southwest Japan Margin off Shikoku

Abstract

Data from seven closely spaced seismic refraction profiles made off the Philippine Sea coast of central Shikoku, Japan, reveal a large thickness of sediment beneath the continental slope that does not slope simply and constantly toward the ocean basin. There is a major ridge in the sediment approximately halfway between the continental shelf and the Nankai trough. The Tosa basin thus developed shoreward of this ridge has been filled by sediment whose surface is a terrace at a depth of 1000 meters. The Nankai trough is a depressed section of oceanic crust within the Shikoku basin. On a revised mapLudwig et al. [1973a] show the general distribution of major structural units off the Philippine Sea coast of southwest Japan by use of seismic reflection measurements combined with other published data. The southwest Japan margin has. a foredeep, the Nankai trough, and a series of intermediatedepth sediment-filled basins that occur off prominent embayments in the coast. This report describes the results of two-ship seismic refraction measurements made across the margin off Tosa embayment, central Shikoku, during a United States-Japan cooperative science program in 1968 (Figure 1). SEISMIC REFRACTION MEASUREMENTS The rather strong drift of the survey ships due to the Kuroshio current made the measurements dincult. The shooting track and drift while each ship was recording on station are shown in Figure 2. Obviously, the refraction profiles are not truly reversed, although we treat them as such 'in the interpretation of the travel time graphs (appendix). The geometry • Lamont-Doherty Geological Observatory contribution 1951.

In the Framework of Deformation in the Nankai Trough

“…The labeled reflectors are the same as shown in Figure 2. B. Interval velocities (from Nasu et al, 1982); linear velocity gradients, derived from interval velocities, refraction velocities (Yoshii et al, 1973), and drilling results; and porosities (percentages), converted from velocities using relationships of Hamilton (1978), across the prism toe.…”

Seismic structure and geomagnetic anomaly in the Nankai Trough related to subduction of the Philippine Sea plate.

“…A fine seismic structure of the oceanic basement beneath the accretionary prism of the area is constructed by the present authors referring to the seismic structures obtained by YOSHII et al (1973), UENO (1984) and HATORI (1985) as in Fig. 2.…”

“…For the (SK) and Nankai Trough (NT). Large scale seismic structure across the trough axis of this area was obtained by YOSHII et al (1973) and the structure obtained by HATORI (1985) experiment conducted in 1986 by R/ V Hakuho-Maru, University of Tokyo, sixteen sets of OBS were deployed along a track (arrow-headed double line, Fig. 3) in water depths between 4300 and 4500 meters.…”

A method of ray tracing in a complicated structure.

“…We compiled the depth contours of the Mohorovicic discontinuity ( Fig. 6(a)) mainly on the basis of explosion seismic studies (HOTTA et al, 1964;YOSHII et al, 1973YOSHII et al, , 1974OKADA et al, 1973OKADA et al, , 1979ONO et al, 1978;KANEDA et al, 1979;ITO et al, 1982). Supplementary information is obtained from studies on natural earthquakes (KIMURA and OKANO, 1980;SHIMAMURA, 1981).…”