Seismic Activity in the Izu Peninsula Region (1) : October 1975-April 1992

This paper summarizes the history and results of microearthquake observation in the Izu Peninsula region by the Earthquake Research Institute (ERI). ERI started microearthquake observation at Okuno in November 1971. In September 1975, two stations, Kawazu and Yugashima were added and routine hypocenter determination by this local network was started from the next month. As the seismic activities continued, spread and developed, the network has been intensified to monitor it more precisely by adding new stations and improving observation and data processing facilities ; from ink writing drum recording system to digital telemetering and interactive computer processing system. The results of observation for 17 years, October 1975-April 1992, are given here as yearly epicenter maps and brief descriptions about major seismic activities in each year. Major events in this period are : the 1976 Kawazu earthquake (M 5.4), the 1978 Izu-Oshima-Kinkai earthquake (M 7.0), the 1980 Izu-Hanto-Oki earthquake (M 6.7), the 1986 Izu-Oshima eruption with a M 6.1 earthquake, and the 1990 Izu-Oshima-Kinkai earthquake (M 6.5). Since November 1978, active earthquake swarm have occurred repeatedly off the east coast of the peninsula and submarine eruption occurred there during one of such earthquake swarms in 1989.＜論説