HR: 08:15h
AN: V31B-02    [PDF]
TI: Magmatic Dike Resonances Inferred From Very-Long-Period Signals
AU: * Kumagai, H
EM: kumagai@bosai.go.jp
AF: National Research Institute for Earth Sciece and Disaster Prevention, 3-1 Tennodai, Tsukuba, IBA 305-0006 Japan
AU: Miyakawa, K
EM: miyakawa@bosai.go.jp
AF: National Research Institute for Earth Sciece and Disaster Prevention, 3-1 Tennodai, Tsukuba, IBA 305-0006 Japan
AU: Negishi, H
EM: negishi@bosai.go.jp
AF: National Research Institute for Earth Sciece and Disaster Prevention, 3-1 Tennodai, Tsukuba, IBA 305-0006 Japan
AU: Inoue, H
EM: inoue@bosai.go.jp
AF: National Research Institute for Earth Sciece and Disaster Prevention, 3-1 Tennodai, Tsukuba, IBA 305-0006 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Sciece and Disaster Prevention, 3-1 Tennodai, Tsukuba, IBA 305-0006 Japan
AU: Suetsugu, D
EM: dai@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center, 2-15 Natsushima, Yokosuka, KNA 237-0061 Japan
AB: Hachijo Island is a volcanic island located 280 km south of Tokyo. Renewed activity began on 13 August 2002, when a swarm of volcano-tectonic (VT) earthquakes accompanied by deformation of the volcano was detected. On 20 August, when the VT seismicity was almost waned, a swarm of very-long-period (VLP) events began beneath the island. The VLP signals show simple decaying harmonic oscillations with periods near 10 seconds and lasting for about 300 seconds. We analyze the waveforms of the VLP signals on 4 September. These were observed by two permanent and three temporary broadband seismic stations on this island. Spectral analysis of the decaying harmonic oscillations in the tail of the VLP waveforms identifies two clear peaks at the periods of 7.3 and 11.4 s with quality factors (Q) of about 30. The source mechanism analysis of the VLP signals shows that the moment-tensor solution is dominated by volumetric change components with oscillatory signatures, indicating the resonance of a vertical crack at a depth of 5 km below the northern flank of Mt. Hachijo Fuji. We perform numerical simulations based on a fluid-filled crack model, and find that the basalt-steam mixture with gas-volume fraction around 20 % produces the crack resonance with Q around 30. We further find that the frequency ratio for two longitudinal modes with the wavelengths of 2L/7 and 2L/9, where L is the length of the crack, is similar to the observed frequency ratio. We calculate the synthetic waveform generated by the excitation of a vertical crack containing the basalt-steam mixture, which reproduces the feature of the observed VLP waveform. We may therefore reasonably interpret that the VLP signals originate in the resonance of a dike filled with a basalt-gas mixture, which may be the result of magma intrusion beneath this island as evidenced by the VT earthquake swarm and deformation of the island. The VLP activity continued for about 8 months between August, 2002 and April, 2003. The observed two peaks at around 7 and 11 s were stably observed during this period, although the frequencies showed a slight increasing trend within 5 % change since January, 2003. This suggests that the dike system stably existed beneath Hachijo Island during this interval. The stable existence of the dike system may not be explained by a simple magma cooling process, and implies the magma circulation between the dike and a possible magma chamber beneath the Island.
DE: 7280 Volcano seismology (8419)
DE: 8414 Eruption mechanisms
DE: 8419 Eruption monitoring (7280)
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting