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Earth Planets Space, Vol. 57 (No. 3), pp. 173-178, 2005

LETTER

Tsunami source of the 2004 off the Kii Peninsula earthquakes inferred from offshore tsunami and coastal tide gauges

Kenji Satake1, Toshitaka Baba2, Kenji Hirata2, Shin-Iti Iwasaki3, Teruyuki Kato4, Shunichi Koshimura5, Jun Takenaka6, and Yukihiro Terada7

1Active Fault Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
2Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
3National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan
4Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo, Japan
5Disaster Reduction and Human Renovation Institution, Kobe, Japan
6Seismological and Volcanological Department, Japan Meteorological Agency, Tokyo, Japan
7Hitachi Zosen Corporation, Taisho-ku, Osaka, Japan

(Received November 30, 2004; Revised March 5, 2005; Accepted March 7, 2005)

Abstract: Tsunamis from the 2004 off the Kii Peninsula earthquakes (M 7.1 and 7.4) were recorded on offshore tsunami gauges, a GPS tsunami gauge and eight bottom-pressure gauges, as well as coastal tide gauges located south of Honshu and Shikoku. The maximum amplitudes on the GPS and bottom-pressure gauges were several to ten cm, while those on tide gauges were up to 0.9 m. We first computed tsunami waveforms from the earthquake source models proposed Yamanaka (2004) and Yagi (2004) from seismic waveform analysis, and compared them with the observed waveforms. For the first event (foreshock), both models produce similar waveforms with the observations. For the second event (mainshock), the waveforms computed from the Yamanaka model is closer to the observed waveforms, but there are still discrepancies between the observed and computed waveforms. We then performed tsunami waveform inversions to estimate the water height distributions in the source area. The foreshock source is ~1600 km2 with the maximum water height of 0.2 m. The estimated tsunami source area for the mainshock, ~3600 km2 with the maximum of 0.6 m, extends ~60 km toward northwest and ~40 km southwest from the epicenter along the aftershock distribution, suggesting that multiple faulting was involved in the mainshock.
Key words: The 2004 off the Kii Peninsula earthquake, tsunami source, initial water height distribution, tsunami waveform inversion.


Corresponding author E-mail: kenji.satake@aist.go.jp


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