Abstract
Distance measurements to estimate ground deformation by use of the AGA Geodimeter-6BL were carried out at intervals of 3 to 4 months at four active volcanoes in Japan: Asamayama, Izu-Oshima, Asosan and Sakurajima. These volcanoes have had frequent eruptions and volcanic earthquakes in historical time. Base lines for measurement are set on 6 lines at Asamayama, 7 at Izu-Oshima, 8 at Asosan and 9 at Sakurajima. Total numbers of surveys for all base lines are counted 184 times during the two years 1975-1976 and 1977-1978.
The accuracy of distance measurement depends on the sum of the observation error (E) and the normal error (e). (E) is the standard deviation of the observation data and (e) is the error due to the setting condition and secular variation of the instrument, weather condition at the observation time, etc.
The total maximum error (Smax) and the total mean error (Smean) are statistically expressed as follows:
Smax=Emax+e=3.5D-1+(1 mm+D×10-6)/D
Smean=Emean+e=3.3D-1+(1 mm+D×10-6)/D
where
D is the length of the base line in km.
The total error for each base line was of the order of 10
-6 strain. On the other hand, the observed strain was of the order of 10
-5 strain at all base lines during the two years. On the other hand, 10
-6∼10
-7 strain was reported in nonvolcanic regions in the same period. It is worth noticing that the strain of the volcanic region is larger than that of other regions.
In this paper, the base line with a variation of strain larger than twice the total error is conveniently named “the significant line”. Such lines are detected at the SSE and the NE foot of Asamayama, at the central cone “Miharayama” and foot of Izu-Oshima, near the Nakadake crater of Asosan, and at the NW to W and S foot of Sakurajima. The significant lines at the W foot of Sakurajima are located in a direction crossing the tectonic line of the “1914 Taisho Eruption”.
Variation of strain of the significant lines has a good correlation with volcanic eruption and or seismic activity in and near the volcano. Expansion of the base line was often observed during an active period of the volcano, while contraction was found during a calm stage. These phenomena suggest that the pressure of the magma reservoir and the vent increases during an active period, so that the diameter of the crater and the width of the tectonic line of the volcano spread outward. On the other hand, the pressure decreases during a calm stage, so that the crater and the tectonic line contract inward.
For the study of the relationship between volcanic activity and ground deformation at the above-mentioned four active volcanoes, it is useful to make the distance survey by the Geodimeter, at intervals of three months or less.
