HR: 14:40h
AN: V13D-05 INVITED     [Abstracts]
TI: Temporal evolution of a magmatic dike system inferred from the complex frequencies of very-long-period seismic signals
AU: * Kumagai, H
EM: kumagai@bosai.go.jp
AF: NIED, 3-1 Tennodai, Tskuba, 305-0006 Japan
AB: Very-long-period (VLP) seismic signals showing decaying harmonic oscillations with oscillation periods near 10 s and lasting for 300 s were observed in association with an earthquake swarm that occurred beneath Hachijo Island, Japan. The VLP activity began beneath this island on 20 August 2002. After the heightened VLP activity between 20 and 26 August, in which a maximum daily number of 40 VLP events was recorded on 26 August, the VLP signals were observed until April 2003. Spectral analysis of the decaying harmonic oscillations in the tail of the VLP waveforms identifies two spectral peaks (hereafter referred to as Peaks 1 and 2). The complex frequencies (frequency and quality factor, Q) of Peaks 1 and 2 show systematic trends in the period of the heightened VLP activity between 20 and 26 August. The Q values of both Peaks 1 and 2 show increasing trends from roughly 15 to 35 in this period. On the other hand, the frequencies of Peaks 1 and 2 show opposite trends in this period. The frequency of Peak 1 shows a decreasing trend from 0.1 to 0.09 Hz, while the frequency of Peak 2 shows an increasing trend from 0.128 to 0.136 Hz. After these systematic variations, the frequencies and Q values of the two peaks remain roughly constant, although scatters exist especially in the Q values. Numerical simulations based on the fluid-filled crack model indicate that the observed systematic temporal variations can be reasonably explained by changes in geometry of a crack containing a bubbly basalt. Assuming two crack resonance modes with wavelengths 2L/7 and 2W/5, where L and W are the length and width of a crack, the frequencies and Q values calculated for the crack with gradual increases in the length ranging from 1500 to 4000 m, width ranging from 1000 to 2000 m, and aperture ranging from 0.1 to 0.4 m reproduce the systematic temporal variations observed between 20 and 26 August. This result suggests an expansion of a dike during the heightened VLP activity. The roughly constant frequencies and Q values of the two peaks after these systematic variations suggest that the magmatic dike stably existed beneath Hachijo Island for about eight months. Since such a thin dike solidifies within several days if we only consider conduction of heat, the long lifetime of the dike suggested by the VLP signals implies a circulation of magma between the dike and a possible magma chamber beneath this island.
DE: 8414 Eruption mechanisms
DE: 8419 Eruption monitoring (7280)
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 7260 Theory and modeling
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting