Skip to main content

Volume 56 Supplement 3

Special Issue: Special section for the 2003 Tokachi-oki earthquake

Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network


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.


  • Ambuter, B. P. and S. C. Solomon, An event recording system for monitoring small earthquakes, Bull. Seismol. Soc. Am., 64, 1181–1188, 1974.

    Google Scholar 

  • Hino, R., S. Ito, H. Shiobara, H. Shimamura, T. Sato, T. Kanazawa, J. Kasahara, and A. Hasegawa, Aftershock distribution of the 1994 Sanriku-oki earthquake (Mw 7.7) revealed by ocean bottom seismographic observation, J. Geophys. Res., 105, 21697–21710, 2000.

    Article  Google Scholar 

  • Hirata, K., M. Aoyagi, H. Mikada, K. Kawaguchi, Y. Kaiho, R. Iwase, S. Morita, I. Fujisawa, H. Sugioka, K. Mitsuzawa, K. Suyehiro, H. Kinoshita, and N. Fujiwara, Real-time geophysical measurements on the deep seafloor using submarine cable in the southern Kuril subduction zone, IEEE Journal of Oceanic Engineering, 27, 2, 170–181, 2002.

    Article  Google Scholar 

  • Hirata, N., T. Kanazawa, K. Suyehiro, and H. Shimamura, H., A seismicity gap beneath the inner trench wall of the Japan Trench as derived by ocean bottom seismograph measurement, Tectonophysics, 112, 293–209, 1985.

    Article  Google Scholar 

  • Hirata, N. and M. Matsu’ura, Maximum-likelihood estimation of hypocenter with origin time eliminated using nonliear inversion technique, Phys. Earth Planet. Inter., 47, 50–61, 1987.

    Article  Google Scholar 

  • Iwasaki, T., H. Shiobara, A. Nishizawa, T. Kanazawa, K. Suyehiro, N. Hirata, T. Urabe, and H. Shimamura, A detailed subduction structure in the Kuril trench deduced from ocean bottom seismographic refraction studies, Tectonophysics, 165, 315–336, 1989.

    Article  Google Scholar 

  • Obana, K., S. Kodaira, K. Mochizuki, and M. Shinohara, Micro-seismicity around the seaward updip limit of the 1946 Nankai earthquake dislocation area, Geophys. Res. Lett., 28, 2333–2336, 2001.

    Article  Google Scholar 

  • Urabe, T., K. Suyehiro, T. Iwasaki, N. Hirata, T. Kanazawa, A. Nishizawa, and H. Shimamura, Aftershock distribution of the 1983 Japan Sea earthquake as determined from helicopter-dispatched OBS observation, J. Phys. Earth, 33, 133–147, 1985.

    Article  Google Scholar 

  • Urabe, T. and S. Tsukada, A workstation-assisted processing system for waveform data from microearthquake networks, Abstracts of Spring Meeting of Seismological Society of Japan, 70, 1991 (in Japanese).

  • Watanabe, H., Determination of earthquake magnitude at regional distance in and near Japan, Zisin 2, 24, 189–200, 1971.

    Google Scholar 

  • Wessel, P. and W. H. F. Smith, New version of the generic mapping tools released, Eos Trans. AGU

  • Yamanaka, Y. and M. Kikuchi, Source processes of the recurrent Tokachioki earthquake on September 26, 2003, inferred from teleseismic body waves, Earth Planets Space, 55, e21–e24, 2003.

    Article  Google Scholar 

  • Yamanaka, Y. and M. Kikuchi, Asperity map along the subduction zone in northeastern Japan inferred from regional seismic data, J. Geophys. Res., 2004 (in press). 76, 329–336, 1995.

    Google Scholar 

Download references

Author information

Authors and Affiliations


Corresponding author

Correspondence to Masanao Shinohara.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Shinohara, M., Yamada, T., Kanazawa, T. et al. Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network. Earth Planet Sp 56, 295–300 (2004).

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Key words

  • The 2003 Tokachi-oki earthquake
  • aftershock
  • subduction
  • ocean bottom seismometer (OBS)