| Abstract: | The intra-plate seismicity map for southwest Japan, based on fairly complete historical data for the past four hundred years, reveals an inverse correlation between the seismic activity along the island arc and the slip-rate along the Median Tectonic Line during the Late Quaternary. In the eastern part, the tectonic line is geologically inactive but regional historic seismicity has been high. The intra-plate seismic activity is probably related with the well developed mosaic-like conjugate system of strike-slip faults there. Conversely, the historic seismicity has been low in the western part, especially low in an area along the most geologically active segment of the Median Tectonic Line. Since no creep movement has been found there, energy greater than that of the Mino-Owari earthquake of 1891 (M = 8.0) seems to be stored in this seismicity gap. The difference in seismic released energy between the two regions for the last four hundred years would be balanced by the strain energy accumulated in the seismic gap. The fairly uniform strain release is conformable to the idea, proposed on the basis of the trend of maximum compression axes, that the Philippine Sea plate is dragging southwest Japan southwestward along the Nankai trough. |
