Open Access

Aftershock distribution of the 26 December 2004 Sumatra-Andaman earthquake from ocean bottom seismographic observation

  • Eiichiro Araki15Email author,
  • Masanao Shinohara25,
  • Koichiro Obana15,
  • Tomoaki Yamada25,
  • Yoshiyuki Kaneda15,
  • Toshihiko Kanazawa25 and
  • Kiyoshi Suyehiro15
Earth, Planets and Space200658:BF03353367

Received: 22 July 2005

Accepted: 3 December 2005

Published: 17 February 2006


We deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake 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 subducting 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