BOREHOLE-TYPE TILTMETER AND THREE-COMPONENT STRAINMETER FOR EARTHQUAKE PREDICTION
1986 Volume 34 Issue Supplement Pages S129-S140
Details
1986 Volume 34 Issue Supplement Pages S129-S140
Borehole-type instruments have many advantages over the conventional instruments usually installed in vaults. The primary merit is that the location is not restricted by geomorphic conditions. This decisively helps us to establish an observation network in a plain field area for earthquake prediction. The borehole-type tiltmeter developed is, as a principle, a pendulum tiltmeter with an electric feedback circuit. Tiltmeters are usually installed in boreholes at depths of about 100 m. Now, we have 23 stations in operation in the Kanto-Tokai area, central Japan. The measuring range is 400 microradian, the resolution is 0.006 microradian, and the long-term stability is better than a few microradian per year. This observation system is expected to be very effective for detecting anomalous tilt changes of the order of 0.05 microradian per day with a time constant of from several hours to tens of days. Recently, an anomalous tilt change of up to 0.4 microradian was detected in the vicinity of the focal area preceding an earthquake of magnitude 6.0. The borehole-type three-component strainmeter invented has a dual cylinder of which the narrow clearance is equally divided into three chambers. Two sets of strainmeters of this type were installed in two boreholes at depths of 160 m at Yasato, Ibaraki-ken. They have often detected strain steps caused by big earthquakes. Detectable strain steps can be as small as 10-10. Strain steps observed by the two instruments agree with each other very well, and are in good harmony with the theoretical strain steps calculated on dislocation models. These facts have proven that instruments of this type have high credibility. 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. The future plan is to unify the tiltmeter and the three-component strainmeter to one unit which would be called the Integrated Borehole Observation System (IBOS).
