Earth Planets Space, Vol. 58 (No. 8), pp. e33-e36, 2006E-LETTER
Azusa Nishizawa1, Kentaro Kaneda1, Ayako Nakanishi2, Narumi Takahashi2, and Shuichi Kodaira2
1Hydrographic and Oceanographic Department, Japan Coast Guard, Tokyo 104-0045, Japan
2Japan Marine Science and Technology Center, Yokohama 236-0001, Japan
(Received April 25, 2006; Revised July 6, 2006; Accepted July 12, 2006; Online published September 16, 2006)
Wide-angle refraction experiments were conducted to reveal the crustal structure at the transition between the intra-oceanic island arc crust of the mid Izu-Ogasawara (Bonin) arc and the backarc oceanic crust of the Shikoku Basin. The island arc crust consists of an upper crust about 5 km thick with a P-wave velocity <6.0 km/s, a middle crust 5 km thick with a P-wave velocity of 6.0-6.3 km/s, and a lower crust 10 km thick with a P-wave velocity of 6.8-7.2 km/s. The total crustal thickness is about 20 km. The thickness thins to approximately 6 km over a distance of 30 km at the western margin of the Izu-Ogasawara arc (IOA). These features are very similar to those of the northern IOA, which indicates that the crustal structure is relatively constant within 200 km at the northern and mid IOA. The Kinan Escarpment, a 500-km-long fault with a maximum offset of 800 m, characterizes the transition zone between the IOA and Shikoku Basin. The seismic crustal model indicates that the escarpment is a fault which tears the whole oceanic crust along the western margin of the IOA. However, no significant differences exist in the crustal structure on either side of the escarpment, and the Kinan Escarpment seems to be a zone of the structural weakness from its birth.
Key words: Mid Izu-Ogasawara (Bonin) arc, Kinan Escarpment, P-wave velocity structure.