Hypocenters of volcanic earthquakes in Asama volcano were reexamined and precisely determined using additional four seismic stations that had been installed close to the summit crater from 1996 to 1999. The most significant result obtained in the present study is that the groups of shallow earthquakes beneath the summit crater (B-type and N-type earthquakes), which show broad distribution within the depth of 2.5 km according to the old calculation without the four summit stations, are compactly concentrated into a shallower range within the depth of 600m underneath the west side of the crater. In addition to the conventional method of the hypocenter determination, the double difference method was applied but its application turned out not to have a significant influence on the result. The summit stations were also quite effective in constraining the seismic velocity structure and the P-wave velocity of Asama volcano was determined to be 3.5 km/s so as to minimize the variance between the observed and calculated travel times. Most volcano-tectonic earthquakes (A-type earthquakes) are distributed near the sea level under the west flank of the volcano. The precise hypocenter determination has revealed that there is a seismic gap between the shallow and flank earthquake groups. It is inferred that this seismic gap may represent a magma reservoir because the gap agrees with the location of the pressure source that was estimated from a previous study of the ground deformation.

抄録全体を表示

A medium-sized eruption occurred at Asama Volcano on September 1, 2004 after a 21-year dormancy following the eruption in April 1983. Before the eruption, the number of volcanic earthquakes had increased beginning late 2002. A prominent feature of this seismicity was the appearance of high-frequency BH-type earthquakes beginning in mid-August, 2004, approximately a half-month before the first eruption. Eruptions of similar size occurred on September 23, September 29 and November 14. Preceding these eruptions, changes in ground tilt were recorded on the EW component of a tiltmeter installed 2.5 km NNE of the summit crater. This was the first time that JMA (Japan Meteorological Agency) recorded tilt changes at Asama Volcano before eruptions. Changes in ground tilt were almost simultaneously accompanied with increases in volcanic earthquake occurrence. The increase in volcanic earthquake occurrence and change in ground tilt preceding the eruptions contributed to the precautionary issuance of preparatory volcanic information to the public. In spite of the appearance of preceding phenomena, however, information was insufficient to predict the exact time and size of each eruption. Additional observation and understanding of physical processes may be necessary for the exact assessment of eruptions.

抄録全体を表示

We compared two analytical values for sulfur species in water samples of Yugama, a crater lake of the Kusatsu-Shirane volcano, collected between 1966 and 1995. One analytical method was ion chromatograph, by which concentrations of the sulfate ion (SO₄^2-) were determined. The other was gravimetric analysis preceded by oxidation of the samples, by which total concentrations of sulfur species (T-SO₄^2-) were determined. The concentration differences between the two were in good accordance with the total concentration of poly-thionates reported by Takano and Watanuki (1990) through the whole period studied. This suggested that the concentration difference between SO₄^2-) and T-SO₄^2- can be used to estimate the concentrations of dissolved poly-thionates. In this way we can grasp very easily the secular change of the total concentration of poly-thionates without any special analytical conditions and/or instruments.

抄録全体を表示