Space-time Variations of Earthquake Occurrence in the Northern Part of Nagano Prefecture

Based on the Hypocenter Data by the Matsushiro Seismic Array System

Abstract

Space-time variations of very-shallow (0-20km) earthquakes occurrence in the northern part of Nagano Prefecture are studied from the hypocenter data by the Matsushiro Seismic Array System (MSAS) and JMA. The very-shallow earthquakes are distinctively characterized by two active zones, or the one from Omachi to Nozawaonsen and the other from Sakakita to Suzaka including hypocenters of the Matsushiro Swarm Earthquake. A significant variation of very-shallow earhtquakes occurrence is recently observed along the Omachi-Nozawaonsen zone. After two aftershock activities from Dec. 30, 1986 in Ogawa and from Feb. 12, 1987 in Nozawaonsen, a very low seismic activity or gap between Ogawa and Nozawaonsen is now observed.