孔井用傾斜計による地殻傾斜観測方式の開発

Abstract

Continuous monitoring of the crustal tilt is one of the most effective approaches for the purpose of earthquake prediction. Drilling a borehole is not so expensive as construction of an underground vault, which is necessary for installation of water-tube tiltmeters and horizontal pendulum tiltmeters, and a borehole station is possible to be made even in a plain area. The National Research Center for Disaster Prevention has not only developed the boreholetype tiltmeter itself but has improved borehole drilling techniques and installation methods for the tiltmeter. The force-balanced pendulum tiltmeter developed for down-hole observation is installed at the bottom of the borehole of which the depth is about 100m. The measuring range is ±2×10-4 radian, the resolution is 6×10-9 radian, and the long term drift is smaller than a few micro-radian per year after half a year since the installation. Clear records of the earth tides demonstrate the high quality of the instrument. Tilt changes of the order of micro-radian caused by precipitation are remarkable in the data of some stations. But, the crustal tilt observation by using borehole-type tiltmeters is expected to be very effective for detecting anomalous tilt changes of the order of 5×10-8 radian per day with a time constant of several hours to several tens of days.