Forerunning Activity for Large Earthquake Which Succeeds Disastrous Intraplate Earthquake.

For the six among seven disastrous earthquakes with M7 or larger, which occurred in the Japanese islands since 1926, large earthquakes with M 6.3-6.7 succeeded at the distance of several tens km from the epicenters of the disastrous earthquakes, and moderate forerunning earthquakes with M4.5-5.5 were observed for all of the succeeding large earthquakes near their epicenters. Those forerunning earthquakes seem to be induced by the increase of stress field along the tectonic zone which is brought about by the occurrence of disastrous earthquake in the surrounding regions. The forerunning activity is considered useful for selecting the site where we should make intensive observations to catch precursory phenomena expected to appear immediately before the occurrence of large earthquake.