Aftershock Distribution of the December 14, 2004 Rumoi-nanbu Earthquake (<i>M</i> 6.1) in the Northern Part of Hokkaido, Japan

  • ICHIYANAGI Masayoshi
    Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University
  • MAEDA Takahiro
    Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University
  • YAMAGUCHI Teruhiro
    Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University
  • TAKAHASHI Hiroaki
    Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University
  • KASAHARA Minoru
    Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University
  • SASATANI Tsutomu
    Department of Natural History Sciences, Faculty of Science, Hokkaido University
  • YAMAMOTO Akihiko
    Department of Earth’s Evolution and Environment, Division of Mathematics, Physics, and Earth Sciences, Graduate School of Science and Engineering, Ehime University

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Other Title
  • 2004年 12月 14日に発生した留萌支庁南部の地震 (<i>M</i><sub>JMA</sub> 6.1)の余震活動
  • 2004年12月14日に発生した留萌支庁南部の地震(MJMA6.1)の余震活動
  • 2004ネン 12ガツ ジュウヨッカ ニ ハッセイシタ ルモイ シチョウ ナンブ ノ ジシン M JMA 6 1 ノ ヨシン カツドウ
  • Aftershock Distribution of the December 14, 2004 Rumoi-nanbu Earthquake (M 6.1) in the Northern Part of Hokkaido, Japan

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Abstract

On December 14, 2004, an M 6.1 earthquake occurred in the northwestern part of Hokkaido, Japan. We installed nine temporal seismic stations around the source area immediately after the occurrence and had continued the observation for about two months after the main shock. We determined 823 hypocenters of aftershocks and one dimensional P-wave velocity structure model from the travel time data. It is found that aftershocks are clearly distributed on an eastward dipping plane with a dip angle of about 25 degrees. This plane agrees well with one of the nodal planes of the focal mechanism determined by P-wave first motions in this study. Next, we investigated a position of the main shock relative to the aftershock distribution. The station correction values for permanent stations were determined from the data during the period of the temporary observation. Estimated main shock hypocenter was confirmed to be situated on the aftershock plane. Finally, we estimated three-dimensional P-wave velocity structure based on the temporary travel time data. Relocated aftershocks using the three dimensional velocity structure were mainly distributed along the boundary between the high and low velocity zones.

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