Seismic Activities in the Nikko-Ashio Area : November 1993～January 1997

The Nikko-Ashio area, the northwestern part of Tochigi prefecture, is one of the most seismically active regions in Japan. We detected many earthquake swarms by routine observation of Earthquake Observation Center, Earthquake Research Institute. This paper reports those seismic activities in the area during the period from November 1993 to January 1997. This area is divided into four active regions. The Ashio region, lying along the Uchinokomori-fault, had the remarkable activity in the period. The earthquake swarm occurred at the bending part of the fault at first on February 1994. In the following three years, the swarms gradually migrated from the north to the south along the lineament of the fault, accompanying many microearthquakes, and then spread toward the east and the west. Finally, those microearthquakes filled up the area of 10x3 km along the Uchinokomori-fault.＜論説