Skip to main content

Volume 57 Supplement 4

Special Issue: Special Section for the 2004 off the Kii peninsula earthquakes (2)

Very low frequency earthquakes excited by the 2004 off the Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism


Anomalous seismic events were observed after the occurrence of the foreshock (Mw=7.2) and the main shock (Mw=7.5) of the 2004 off the Kii peninsula earthquakes. These anomalous events are characterized by very low-frequency energy of around 10 seconds with almost no higher-frequency energy and are considered the same as the very low-frequency (VLF) earthquakes discovered by Ishihara (2003) in some places along the Nankai trough, southwest Japan. The VLF seismic activity is mainly coincident with the aftershock area of the 2004 off the Kii peninsula earthquakes; however a minor activity was also excited in the southern Kii channel area. The VLF seismograms sometimes include higher-frequency wave trains with amplitudes much smaller than that of regular aftershocks. This indicates that VLF earthquakes have different source properties from the regular earthquakes. The centroid moment tensor analysis for one of the larger events suggests that the source depth is very shallow and the focal mechanism is the reverse faulting. These features suggest that the event occurs on the well-developed reverse fault system in the large accretionary prism near the Nankai trough. The swarm activity of VLF earthquakes might be considered as a chain-like occurrence of slips on the reverse fault system and thus the signature of a dynamic deformation process in the accretionary prism.


  • Davis, D., J. Suppe, and F. A. Dahlen, Mechanics of fold-and-thrust belts and accretionary wedges, J. Geophys. Res, 88, 1153–1172, 1983.

    Article  Google Scholar 

  • Fukuyama, E., M. Ishida, D. S. Dreger, and H. Kawai, Automated seismic moment tensor determination by using on-line broadband seismic waveforms, J. Seismol. Soc. Jpn. (Zisin), 51, 149–156, 1998 (in Japanese with English abstract).

    Google Scholar 

  • Ishihara, Y., Major existence of very low frequency earthquakes in background seismicity along subduction zone of south-western Japan, Eos Trans. AGU, 84(46), 2003.

    Google Scholar 

  • Ito, Y., T. Matsumoto, H. Kimura, H. Matsubayashi, K. Obara, and S. Sekiguchi, Spatial distribution of centroid moment tensor solutions for the 2004 off the Kii peninsula earthquakes, Earth Planets Space, 57, this issue, 351–356, 2005.

    Article  Google Scholar 

  • Kawasaki, I., Silent earthquakes occurring in a stable-unstable transition zone and implications for earthquake prediction, Earth Planets Space, 56, 813–821, 2004.

    Article  Google Scholar 

  • Kimura, G., Tectonics of Convergent Plate Boundaries, Univ. of Tokyo Press, Tokyo, 271 pp., 2002 (in Japanese).

    Google Scholar 

  • Kuramoto, S., A. Taira, N. L. Bangs, T. H. Shipley, G. F. Moore, and EW99-07,08 Scientific Parties, Seismogenic zone in the Nankai accretionary wedge: General summary of Japan-U.S. collaborative 3-D seismic investigation, J. Geography, 109, 531–539, 2000 (in Japanese with English abstract).

    Article  Google Scholar 

  • Obara, K. and Y. Ito, Seismic activity of very low-frequency earthquake on the subducting Philippine Sea plate near the Nankai trough, southwest Japan, Eos Trans. AGU, 85(47), 2004.

    Google Scholar 

  • Obara, K., H. Hirose, F. Yamamizu, and K. Kasahara, Episodic slow slip events accompanied by non-volcanic tremors in southwest Japan subduction zone, Geophys. Res. Lett, 31, L23602, doi:10.1029/2004GL020848, 2004a.

    Article  Google Scholar 

  • Obara, K., Y. Haryu, Y. Ito, and K. Shiomi, Low frequency events occurred during the sequence of aftershock activity of the 2003 Tokachi-Oki earthquake; a dynamic process of the tectonic erosion by subducted seamount, Earth Planets Space, 56, 347–351, 2004b.

    Article  Google Scholar 

  • Okada, Y., K. Kasahara, S. Hori, K. Obara, S. Sekiguchi, H. Fujiwara, and A. Yamamoto, Recent progress of seismic observation networks in Japan—Hi-net, F-net, K-NET and KiK-net, Earth Planets Space, 56, xv–xxviii, 2004.

    Article  Google Scholar 

  • Shiomi, K., K. Obara, S. Aoi, and K. Kasahara, Estimation on the azimuth of the Hi-net and KiK-net borehole seismometers, J. Seismol. Soc. Jpn. (Zisin), 56, 99–113, 2003 (in Japanese).

    Google Scholar 

  • Wessel, P. and W. H. F. Smith, Free software helps map and display data, Eos Trans. AGU, 72, 445–446, 1991.

    Article  Google Scholar 

  • Yamasaki, T. and T. Seno, Double seismic zone and dehydration embrittlement of the subducting slab, J. Geophys. Res, 108, 2212, doi:10.1029/2002JB001918, 2003.

    Article  Google Scholar 

Download references

Author information

Authors and Affiliations


Corresponding author

Correspondence to Kazushige Obara.

Rights and permissions

Open Access  This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

To view a copy of this licence, visit

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Obara, K., Ito, Y. Very low frequency earthquakes excited by the 2004 off the Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism. Earth Planet Sp 57, 321–326 (2005).

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Key words

  • Low-frequency earthquake
  • subduction zone
  • Nankai trough
  • accretionary prism
  • broadband seismometer