Source mechanisms of subcrustal and upper mantle earthquakes around the northeastern Kyushu region, southwestern Japan, and their tectonic implications

Abstract

The source mechanisms of subcrustal and upper mantle earthquakes with magnitudes from 6.0 to 6.8 that occurred around the northeastern Kyushu region have been closely investigated to clarify its tectonic features in relation to the subducting Philippine Sea plate with a laterally bending configuration. Two subcrustal earthquakes that occurred in Suonada and the Bungo channel, which are located close to the leading edge of the subducting Philippine Sea plate, show the mechanism of normal faulting type. These events may have been generated by lateral bending of the oceanic plate. The Kunisaki peninsula earthquake of August 26, 1983 (M=6.8, h=116 km), which was the largest upper-mantle earthquake in this region, shows a reverse fault type mechanism. Comparisons between the observed and synthetic seismograms suggest that the southeastward dipping nodal plane may be the fault plane, and that the rupture propagated northwest-upwards. The seismic moment was estimated to be MO=-1.13×1026 dyn·cm. The focal mechanism of three upper mantle earthquakes inland of Kyushu, including the Kunisaki peninsula earthquake, suggests that they may have been caused under the stress regime of down-dip extension. The tensional stress working southwest-downwards at intermediate depths in the Suonada and inland Kyushu regions may be explained by the gravitational pull acting on the Philippine Sea plate subducting deeper southwestwards.