Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
4714142 | Journal of Volcanology and Geothermal Research | 2009 | 11 Pages |
Abstract
A very-long-period (VLP) volcanic event, which occurred on 25 January 2007 beneath Mt Ontake, central Japan, was recorded by nearby broadband seismometers as well as the Japanese nationwide Hi-net and F-net seismic networks. The VLP waveforms show a pulse-like shape with a duration of approximately 1Â min. The VLP event was accompanied by a burst sequence of long-period (LP) events. We carried out waveform inversion to determine the source mechanism and location of the VLP event. Our analysis indicates volumetric changes in an inclined crack (seismic moment of 1014Â Nm) at the VLP source, which is located at 0.6Â km above sea level beneath the summit of Mt Ontake and above dikes determined from GPS data. The source-time functions of the VLP event showed a pulse-like triangular shape, indicating inflation followed by deflation of the crack at the source. We interpreted the result of the waveform inversion as follows. Gases released from the intruded magma heated an aquifer system beneath the summit of Mt Ontake. A break of unstable superheated water in a hydrothermal crack in the system caused vaporization of water and associated crack inflation, which was followed by discharge of steam from the crack resulting in its collapse. These processes caused the VLP event, which may have been a precursor of a small phreatic eruption that occurred in late March 2007.
Keywords
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
Haruhisa Nakamichi, Hiroyuki Kumagai, Masaru Nakano, Makoto Okubo, Fumiaki Kimata, Yoshihiro Ito, Kazushige Obara,