- Summary A sequence of powerful earthquakes struck Mid Niigata Prefecture, central Japan. The hypocenter of the main shock is located at 37.3 N; 138.8E with depth of 13 km and magnitude of 6.8 determined by Japan Meteorological Agency occurred at 5:56 PM JST on October 23, 2004. The intensity is registered 6+ on the 7-grade Japanese intensity scale. Many aftershocks including some M6 class events have been following. Focal mechanisms of those major earthquakes are the reverse fault type with the compression axis oriented to NW/SE that is consistent to the historical solutions of major earthquakes in this area. Aftershocks are distributed along the northeast and southwest direction with a length of about 30km. The maximum acceleration of 1500 gal is recorded at station Ojiya that is the nearest K-NET site from the hypocenter. The recorded maximum acceleration for this earthquake is much greater than that for the 1995 Kobe earthquake. Oct 27: An aftershock with magnitude of 6.1 shook the Mid Niigata area at 10:40AM JST on October 27,2004. The focal mechanism of the aftershock is the reverse fault type with the compression axis oriented to NW/SE that is consistent to the solutions of the main shock. The hypocenter is located to the east of the aftershock area. A hypo map is here.

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