Skip to main content


A tectonic interpretation of NW-SE strike-slip faulting during the 2004 off the Kii peninsula earthquakes, Japan: Probable tear of the Philippine Sea plate

Article metrics

  • 229 Accesses

  • 10 Citations


The 2004 off the Kii peninsula earthquakes (Mw 7.5 for the main shock) occurred within the subducting Philippine Sea (PHS) plate near its boundary, the Nankai trough, southwest Japan. The rupture mode of the foreshock-main shock-aftershock sequence was complicated, a combination of ENE-WSW striking (almost trough parallel) reverse faulting beneath the trough and NW-SE trending (almost trough normal) strike-slip faulting mostly on the landward side of the former. In this paper, we discuss the tectonic meaning of this NW-SE running strike-slip fault. We examined hypocenter distribution and focal mechanisms of slab earthquakes from October 1997 through September 2004 and confirmed a NW-SE striking tear of the PHS slab beneath the middle part of the Kii Peninsula pointed out by Miyoshi and Ishibashi (2004). According to the Earthquake Research Committee (2004) there is a NW-SE trending structural discontinuity in the PHS crust to the southeast of the main shock epicenter. Putting all features together, we interpret that there is a NW-SE striking fracture within the PHS plate continuously from the Nankai trough region to the slab beneath the Kii Peninsula, and that a partial rupture of this fracture occurred during the off the Kii peninsula earthquakes together with trough-parallel reverse faulting. It should be noted that two disastrous M 7-class slab earthquakes seem to have occurred along this tear beneath the peninsula in 1899 and 1952.


  1. Ando, M., Source mechanisms and tectonic significance of historical earthquakes along the Nankai trough, Japan, Tectonophysics, 27, 119–140. 1975.

  2. Baba, T., P. R. Cummins, and T. Hori, Compound fault rupture during the 2004 off the Kii Peninsula earthquake (M7.4) inferred from highly resolved coseismic sea-surface deformation, Earth Planets Space, 57, 167–172, 2005.

  3. Earthquake Research Committee, Headquarters for Earthquake Research Promotion, On the long-term evaluation of earthquakes along the Nankai trough (September 27, 2001),, 2001 (in Japanese).

  4. Earthquake Research Committee, Headquarters for Earthquake Research Promotion, Seismic activity off-shore southeast of the Kii Peninsula (October 13, 2004),, 2004.

  5. Hara, T., Change of the source mechanism of the main shock of the 2004 off the Kii peninsula earthquakes inferred from long period body wave data, Earth Planets Space, 57, 179–183, 2005.

  6. Hashimoto, M., K. Onoue, F. Ohya, Y. Hoso, K. Segawa, K. Sato, and Y. Fujita, Crustal deformations in Kii peninsula associated with the SE off the Kii peninsula earthquake sequence of September 5, 2004 derived from dense GPS observations, Earth Planets Space, 57, 185–190, 2005.

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

  8. Kikuchi, M., M. Nakamura, and K. Yoshikawa, Source process of the Yoshino earthquake of July 18, 1952, inferred from low-gain seismo-grams, Abstracts of the 2002 Japan Earth and Planetary Science Joint Meeting (CD-ROM), S051–001, 2002 (in Japanese with English abstract).

  9. Miyoshi, T. and K. Ishibashi, Geometry of the seismic Philippine Sea slab beneath the region from Ise Bay to western Shikoku, southwest Japan, Zisin (J. Seism. Soc. Jpn.), Second Ser., 57, 139–152, 2004 (in Japanese with English abstract).

  10. Park, S. C. and J. Mori, The 2004 sequence of triggered earthquakes off the Kii peninsula, Japan, Earth Planets Space, 57, 315–320, 2005.

  11. Sakai, S., T. Yamada, M. Shinohara, H. Hagiwara, T. Kanazawa, K. Obana, S. Kodaira, and Y. Kaneda, Urgent aftershock observation of the 2004 off the Kii Peninsula earthquake using ocean bottom seismometers, Earth Planets Space, 57, 363–368, 2005.

  12. Satake, K., T. Baba, K. Hirata, S. Iwasaki, T. Kato, S. Koshimura, J. Takenaka, and Y. Terada, Tsunami source of the 2004 off the Kii Peninsula earthquakes inferred from offshore tsunami and coastal tide gauges, Earth Planets Space, 57, 173–178, 2005.

  13. Seno, T., The September 5, 2004 off the Kii Peninsula earthquakes as a composition of bending and collision, Earth Planets Space, 57, 327–332, 2005.

  14. Utsu, T., Seismicity of Japan from 1885 through 1925-A new catalog of earthquakes of M ≥ 6 felt in Japan and smaller earthquakes which caused damage in Japan, Bull. Earthq. Res. Inst., Univ. Tokyo, 54, 253–308, 1979 (in Japanese with English abstract).

  15. Utsu, T., Relationships between magnitude scales, in International Handbook of Earthquake and Engineering Seismology, Part A, IASPEI, edited by W. H. K. Lee, H. Kanamori, P. C. Jennings, and C. Kisslinger, pp. 733–746, Academic Press, New York, 2002 (Full manuscript is given in Utsu3FullManuscript.pdf on the attached Handbook CD #1, directory/44Utsu3).

  16. Wessel, P. and W. H. F. Smith, New version of the Generic Mapping Tools released, EOS Trans. Am. Geophys. Union, 76, 329, 1995.

  17. Yagi, Y., On the earthquakes which occurred southeast-off the Kii Peninsula on September 5, 2004,, 2004 (in Japanese).

Download references

Author information

Correspondence to Takayuki Miyoshi.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Miyoshi, T., Ishibashi, K. A tectonic interpretation of NW-SE strike-slip faulting during the 2004 off the Kii peninsula earthquakes, Japan: Probable tear of the Philippine Sea plate. Earth Planet Sp 57, 1115–1120 (2005) doi:10.1186/BF03351891

Download citation

Key words

  • 2004 off the Kii peninsula earthquakes
  • intra-oceanic-plate earthquake
  • Philippine Sea plate
  • slab tear
  • hypocentral distribution
  • focal mechanism