ボアホール式計器による地殻変動連続観測

抄録

It is crucial to detect short term precursors by continuous observation of crustal deformation in order to succeed in earthquake prediction. For that purpose we need to establish dense networks composed of high-resolution sensors in the areas concerned. Since these areas often include plain field areas, borehole-type observation system is indispensable. This system has many advantages over the conventional valut-type obser vation system; (a) short period for construction, (b) less affected by surface topography, (c) less affected by precipitation and flow of underground water, (d) negligible disturbance due to the smallness of borehole diameter. We have not only developed a force-balanced pendulum type tiltmeter itself but also has been improving borehole drilling techniques and down-hole installation methods for more than a decade. Tiltmeters are installed in boreholes at depths of about 100m. The measurable range is 400 cradian, the resolution is 0.006pradian, and the long-term stability is better than a few microradian per year. We have 24 stations now in operation in the Kanto-Tokai District. Clear records of the earthtide demonstrate the high quality of our observation method. Recently an anomolous tilt change up to 0.4pradian during 10 days was detected before the earthquake of magnitude 6.0, which occurred in the eastern Yamanashi on Aug. 8, 1983. Detection of three components of horizontal strain is preferable for precise understanding of the strain field. A new type of three-component strainmeter was developed based on the invention by Sakata. At first a pair of the instruments were installed and cemented in boreholes at depths of 160 m at Yasato, Ibaraki. They successfully detected clear strain steps of the order of 10-8 caused by the Off-Ibaraki earthquake (July 23, 1982). Observed data from the two instruments agree with each other very well. They are in good harmony with the theoretical results calculated from a fault model. Since then many strain steps have been recorded. Detectable strain steps are as small as the order of 10-10. Such high performances can be attributed to the fact that the whole system is free from solid friction and less affected by shock and vibration at the time of earthquakes. We are planning to develop an IBOS (Integrated Borehole Obsevation System) in the near future. In this system a borehole tiltmeter and a three-component strainmeter are combined to one unit. This system can simultaneously detect five independent com ponents representing crustal deformation.