Deep Borehole Measurements of Crustal Activities Around Tokyo

Abstract

As a part of the Japanese national earthquake prediction program, deep borehole observation of the earth’s crustal activities in and around Tokyo, Japan has been planned by the National Research Center for Disaster Prevention (NRCDP). The first observatory of 3500-m depth was constructed at Iwatsuki City about 30 km ncrth of the center of Tokyo by the end of 1972. Continuous observation of microearthquakes and ground tilt has been carried out since 1973. The second one, the Shimohsa observatory of 2300-m depth, was completed at shonan Town about 30 km east-Northeast of the center of Tokyo at the end of 1977. Observation of microearthquakes and ground tilt is being carried out in 1978. The sensor capsule which is rigidly fixed at the bottom of the hole, contains a three-components set of velocity seismometers, a three-somponents set of accelerometers, a two-components set of thermometers. As expected, background noises at the bottom of the hole which extends to the pre-Tertiary metamorphic rock, are extremely small, and of the order 5-10µ kine, about 1/150 to 1/1000 that at the surface for the period range 1-25 Hz. Therefore, these observatories can detect earthquakes with magnitude greater than 2 (or 3) within 100 (or 200) km radius from the observatory. Also, High sensitive tiltmeter measurements are capable of measuring with a resolution of l/50th of an arc second within 3 degrees. The range of acceleration which can be measured is 5 m gal — 30 gal. Observational data from these observatories are telemetered through the telephone line to the NRCDP in Ibarakiken about 60 km northeast of the center of Tokyo, where observational data of other seismometers and tiltmeters covering the south Kanto and Tokai areas gathered together.