Observation of Crustal Deformation at the Fujigawa Observatory, Central Honshu, Japan (3)

Abstract

The results of continuous observation of ground strains and tilts made at the Fujigawa Crustal Movement Observatory for the period from July 1970 to June 1980 are described in the present paper. In this period, an interesting phenomenon was observed at the first half of 1976. That is frequent appearance of pulse like shaped strain or tilt changes which have following characteristics. (1) There are four types in pulse shapes. (2) The width of pulses ranges from several to several tens minutes. Extensometers show always extension and borehole type tiltmeters show only one side tilt. (3) The amplitude of pulses ranges from 1X10-9 to 6X10-8. (4) The amplitude ratios among different components of strain- or tiltmeters are almost constant. (5) Borehole dilatationmeter which was installed at Shizuoka by JMA also frequently records sudden contraction in the same period. But none of them occurs simultaneously or has any delayed correlation with these events. (6) No earthquake occurs at the time of these events and also is seen no particular high seismicity around the station in this period. Although the doubt whether these records come from instrumental causes or not remains, it is possible to interprete these phenomena as creep events on the minor fault within the observation vault or those on the Itoigawa-Shizuoka tectonic line which is one of the famous active fault in Japan and lies at only 2 km apart from the Fujigawa Observatory. Main features of the observed secular strain or tilt changes are as follows. (1) As to the mode of horizontal strain accumulation, EW-contraction is predominant from 1970 to 1973. From then, NS-extension is predominant until 1975, and after some stagnant period, the contraction in NW-SE direction becomes predominant from 1978. (2) As to the mode of tilt accumulation, gradual S-down movement is the basic trend, on which the fluctuations in ENE-WSW direction are added. This direction of fluctuations coincides to the direction to which the observation vault is excavated. (3) The modes of strain and tilt accumulations roughly harmonize with the general trends of the crustal movements in the wide area including this ovservatory. (4) The average strain rates and tilting rates are (0.15-3) X 10^-7/year and (0.3-1.4) X 10^-7/year respectively, which are comparable to those known from the geodetic surveys. (5) The results of spectrum analysis of the ground strains and tilts suggest that the state which we are now watching with continuous observations is a time section of the tectonic movements surrounding this observatory. (6) Annual precipitation at this station is almost constantly 3600 mm/year. But in 1978, this value diminish to 2600 mm/year and correspondingly the changes of horizontal strains were quite calm in this year.