Interaction Between Hydrothermal and Magmatic Systems Inferred From Temporal Variations in the Complex Frequencies of Long-period Events at Kusatsu-Shirane Volcano, Japan

The waveform of a volcanic long-period (LP) event is characterized by a superposition of decaying sinusoids. It has been recognized that the complex frequencies (frequency, f and the quality factor, Q) of the decaying sinusoids show significant temporal variations. In this paper, we present a detailed description of temporal variations in f and Q of LP events at Kusatsu-Shirane Volcano to investigate the relation between LP events and magmatic activity. Extensive LP seismicity and systematic temporal variations in the complex frequencies of LP events are identified in 1989, 1991, 1992, and 1993. The complex frequencies of the LP events occurred between 1992 and 1993 were investigated by Kumagai et al. [2002, JGR, 10.129/2001JB000653]. In this paper, we determine f and Q of LP events occurred in 1989, when the renewed eruption activity started in Kusatsu-Shirane Volcano and a swarm of 40 LP events within 25 days occured. Our Sompi spectral analysis of the LP events shows that the frequency decreases from about 3.2 Hz to 1.7 Hz at the middle of the period, and then increases up to about 4 Hz. The Q factor increases from about 30 to 100, and then decreases down to about 20 at the middle of the period. After that, the Q factor again increases up to about 100 and then decreases down to about 30 at the end of the period. We find that the observed temporal variations in 1989 can be reasonably interpreted by oscillations of a crack filled with a water-gas mixture. The frequency of the oscillations of such a crack increases with increasing gas-volume fraction (GVF). On the other hand, the Q factor is around few tens at GVF of 0 and 1, and reaches a maximum value of about 100 in the vicinity of GVF of 0.5. Therefore, the temporal variations in f and Q can be explained by a change of GVF in a water-gas mixture: In the first half of this period, GVF changes from 1 to 0, and then changes from 0 to 1 in the second half of this period. This change may be interpreted as a dry crack gets wet by a seepage of hydrothermal water into the crack in the first half of the period. As heat supply from deeper magma become higher or water supply decreases in the hydrothermal system, the crack again gets dry in the second half of the period. We suggest that the LP activity in Kusatsu-Shirane Volcano may be basically controlled by an interaction between magmatic heat and water supply with possible seasonal variations in the hydrothermal system.