Earth Planets Space, Vol. 56 (No. 3), pp. 295-300, 2004LETTER
Masanao Shinohara1, Tomoaki Yamada1, Toshihiko Kanazawa1, Naoshi Hirata1, Yoshiyuki Kaneda2, Tetsuo Takanami3, Hitoshi Mikada2, Kiyoshi Suyehiro2, Shin'ichi Sakai1, Tomoki Watanabe2, Kenji Uehira4, Yoshio Murai3, Narumi Takahashi2, Minoru Nishino5, Kimihiro Mochizuki1, Takeshi Sato2, Ei'ichiro Araki2, Ryota Hino5, Kouichi Uhira6, Hajime Shiobara1, and Hiroshi Shimizu4
1Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
2Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan
3Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan
4Institute of Seismology and Volcanology, Kyushu University, Shimabara 855-0843, Japan
5Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
6Japan Meteorological Agency, Tokyo 100-8122, Japan
(Received December 5, 2003; Revised March 5, 2004; Accepted March 5, 2004)
The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15-20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15-20 km from the aftershock distribution.
Key words: The 2003 Tokachi-oki earthquake, aftershock, subduction, ocean bottom seismometer (OBS).