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

Abstract

SUMMARY In 1984 an extensive geophysical investigation was conducted in the northernmost part of the Ryukyu Island Arc, south of Kyushu Island. Japan. The aim of this investigation was to obtain new information on the nature of crust and upper mantle at the continental margin through various geophysical measurements (seismic profiling, gravity, magnetism, sea beam and heat flow). This paper presents the crust and upper mantle structure deduced from ocean bottom seismographic profiling conducted along two lines. One line, 190 km long, was taken behind the northernmost part of the Ryukyu Island Arc, parallel to the major tectonic units, i.e. the Okinawa Trough, Ryukyu Arc and Ryukyu Trench. The other, 295 km long, was perpendicular to the tectonic units. Along these profiles, we deployed ocean bottom seismograms spaced 10‐20 km apart. As a controlled source, we used both explosives and an airgun array. The experiment was successful and provided quite important information on the tectonics of a trench‐island arc‐back arc system. The velocity structure obtained for the profile behind the Ryukyu Islands has continental properties. The sediment thickness is 3‐4 km. although it shows lateral variations due to basement undulation. The velocities of the upper and lower crust are 5.8‐6.2 and 6.6‐6.8 km s‐1, respectively. The total crustal thickness decreases southwestward, from 27‐30 to 23‐24 km. This is direct evidence for crustal thinning associated with the process of back‐arc spreading. The velocity structure along the profile from the trench to the island arc clearly shows the subduction, accretion and deformation at this margin. The crust beneath the trench has oceanic properties. The sediment thickness is 1.3‐3.0 km, beneath which the igneous basement shows severe undulation. The total crustal thickness is 7‐8 km. The Pn velocity is slightly less than 8.0 km s‐1. Our seismic data revealed a huge sedimentary wedge located 50‐150 km landward of the trench. The maximum thickness of this wedge exceeds 12 km. The origin of this wedge may be oceanic because the P‐wave velocity in its eastern half is almost comparable to that obtained for the trench area. The velocity structure landward of the wedge is continental and similar to that obtained for the profile behind the island arc.

Variations in seismic velocity distribution along the Ryukyu (Nansei-Shoto) Trench subduction zone at the northwestern end of the Philippine Sea plate

“…Several seismic explorations have been conducted in the Ryukyu region, and they have yielded P-wave velocity (Vp) models of PHS plate subduction-related structures and backarc rifting-related structures (e.g., Hirata et al 1991;Iwasaki et al 1990;Kodaira et al 1996;Nakahigashi et al 2004;Klingelhoefer et al 2012). However, the number of seismic surveys conducted in this area is very small compared to those made in the Nankai Trough and the Japan Trench, which are seismogenic plate subduction zones around Japan.…”

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

“…The maximum depth of the seismogenic zone decreases from ∼300 km in the south to ∼250 km in the north (Gudmundsson and Sambridge, 1998). For studies of the crustal structure of the Ryukyu arc and the adjacent oceanic regions we refer to Iwasaki et al (1990) and Wang et al (2004). Analyses of earthquake focal mechanisms and fault geometries (e.g., McCaffrey, 1996;Fournier et al, 2001;Kubo and Fukuyama, 2003) suggest the existence of localized regions of arc-parallel extension along the arc, but it is unclear to what degree, if any, extension in the crust is coupled to deformation in the mantle.…”

Crustal thickness beneath the Ryukyu arc from travel-time inversion

“…The dark line denotes the volcanic front (Sibuet et al, 1998). MIMAM2: Minami-Daito island; KG: Kerama Gap; TS: Tokara Strait; NCOT: northern-and-central Okinawa Trough; SOT: southern Okinawa Trough; 1984: seismic refraction profiles studied by Iwasaki et al (1990). network were used to investigate the variations in crustal thickness.…”

“…crustal structures and OBS velocity models have been constructed to model crustal thickness and P n velocity for the various parts of the Ryukyu arc and Okinawa Trough. According to the respective models, the crustal thickness and P n velocity for the northern Ryukyu are 26 km and 7.6 km/s, respectively (Iwasaki et al, 1990); for the central Ryukyu arc, 26 km and 7.7-7.8 km/s, respectively (Nakahigashi et al, 2001); for the SOT, 15 km and 8.2 km/s, respectively (Lee et al, 1980); for the SOT, 18 km and 7.8 km/s, respectively .…”