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Earthquake Hazards Program

Tectonic Summary

Magnitude 7.4 NEAR THE SOUTH COAST OF HONSHU, JAPAN
Sunday, September 05, 2004 at 14:57:18 UTC

Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver

The seismotectonics of the region off the south coast of Honshu are dominated by the subduction of the Philippine Sea plate northwestward beneath the Eurasian plate at a velocity of over 40 mm/year. Earthquakes occur at the interface of the two plates and within both the subducting Philippine Sea plate and the overriding Eurasian plate. The sequence of strong shocks of September 2004 off the south coast of Honshu consists principally of reverse-fault earthquakes. The mechanisms of the largest earthquakes as determined from their seismic-wave radiation patterns are not consistent with slip on a shallowly dipping plate interface. Provisionally, we consider that the shocks probably occurred within the strong interior of the Philippine Sea plate as the result of high compressive stresses within that plate.

 


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