The 1997 Izu Swarm Earthquake and the <i>M</i> 5.5 Earthquake Observed Nearby by Using Strainmeters and Tiltmeters Mounted on the Deep Borehole Comprehensive Crustal Activity Observation Device

In the Izu Peninsula located about 100 km southwest of Tokyo, an earthquake swarm occurred in March 1997. The Earthquake Research Institute of the University of Tokyo installed its own developed deep borehole comprehensive crustal activity observation device in a deep borehole drilled in the Izu Peninsula in 1995 to conduct observations. The device was equipped with strainmeters, tiltmeters, seismometers, magnetometers, and a thermometer. The swarm started within about 5 km of the observation point from 0:00 on March 3, 1997. The analysis was carried out based on the hypocenter data by the Japan Meteorological Agency and the continuously observed strain and tilt data. Determined hypocenters of the swarm revealed the existence of an almost rectangular earthquake blank area. From 0:00 to 10:00 on March 3, 1997, microearthquakes of magnitude less than M 2 occurred near 10 km deep around the observation point. These hypocenters moved eastward over time and ended near the edge of the blank area. After that, at 10:00, a vigorous swarm of earthquakes, including earthquakes of M 2 and above, began. From changes observed by the tiltmeters and the strainmeters, it was considered that the microearthquakes were caused by the pressure of magma rising from the depths, and it was estimated that the magma generated the microearthquakes penetrated into the solid medium from near the edge of the blank area and formed an area containing magma (magma intrusion area) connected with magma chamber in the depth. Due to the presence of a pressure source in the solid medium, stress accumulation progressed in the medium, and earthquakes began to happen from the parts where a stress accumulation progressed beyond a fracture limit, resulting in the earthquake swarm. Then in response to changes of magma pressure and state, the number of earthquakes increased, and the strain and the tilt continued to change and accelerate. In this paper, the authors describe the path from magma rising to magma intrusion, the formation process of the magma intrusion area, the three-dimensional structure of the magma intrusion area, changes in earthquake hypocenters, strain and tilt in response to changes in magma intrusion over time. At 23:09, a M 5.5 earthquake occurred at a distance of about 4 km from the borehole observation point and near the east boundary of the magma intrusion area. Three earthquakes of M 3 level occurred near the M 5.5 between 22:15 and the occurrence. Large changes of the principal strains were recorded from around 22:20, and large changes in the area strain continued before the earthquake. The direction of the downward tilt changed and accelerated from around 22:20. Considering both observed and analyzed results, we believe that the comprehensive observation in a deep borehole will make an important contribution to future seismic research.