Development of the Crustal Tilt Observation Method Using Borehole-type Tiltmeters

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 x 10-4 radian, the resolution is 6 x 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 x 10-8 radian per day with a time constant of several hours to several tens of days.§1.