Earth Planets Space, Vol. 58 (No. 2), pp. 113-119, 2006
Eiichiro Araki1, Masanao Shinohara2, Koichiro Obana1, Tomoaki Yamada2, Yoshiyuki Kaneda1, Toshihiko Kanazawa2, and Kiyoshi Suyehiro1
1IFREE, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan
2Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
(Received July 22, 2005; Revised December 2, 2005; Accepted December 3, 2005; Online published February 17, 2006)
We deployed an OBS network in February-March 2005 in the rupture area of the Sumatra Andaman earth-quake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19-22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the sub-ducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120-170 km from the trench axis consist of a surface with a dip of 10-12° dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15-20°. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.
Key words: Sumatra, Andaman, aftershock, seismicity, ocean bottom seismograph (OBS), accretionary prism, plate boundary.