Crustal structure beneath summit crater and flow of volcanic fluid inferred from hypocenter distribution and source mechanisms of low frequency earthquakes at Asama volcano
Abstract
Long period (LP) events called N-type earthquakes are typical phenomena observed at many active volcanoes, such as Asama, Kusatsu-Shirane, Tokachi-dake volcanoes. They are probably related to activities of magma, ground water or volcanic gas and many source mechanisms such as resonance of fluid cracks or spheres are proposed. In this study, we analyze the LP events observed at Asama volcano in Dec. 1-10, 1996, to reveal their source process with the high quality data obtained by the seismic network close to the summit crater of the volcano. We observed 112 N-type earthquakes during the period.The waveforms of these events seem to be a quasi-monochromatic oscillation with gradually decreasing amplitude. The spectrum has a dominant peak at 1.6-7.2 Hz, most of which make a group (Group 0) in which the dominant peak changes from 2.0 Hz to 1.6 Hz gradually, indicating that the scale or the physical properties of the LP source changes gradually if we accept the resonance model. Other groups appear in Dec. 3"|6 (Group 1) and in Dec. 9"|10 (Group 2) in which the dominant peak changes from 7.2 Hz to 1.4 Hz and 4.3 Hz to 1.6 Hz gradually, respectively. This indicates that two or more sources of the N-type earthquakes exist simultaneously. Attenuation factors have a positive but weak correlation with the frequency of dominant peaks. Hypocenters of the events determined by the travel time of the first motion are concentrated within a depth of 300 m underneath the summit crater and are distributed in the shallow part of the region where B- type earthquakes occur. The events of Group 0 are concentrated underneath the southwest side of the crater, and the events of Group 1 and 2 distribute in the east side of the crater.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V51C2052O
- Keywords:
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- 7280 Volcano seismology (8419);
- 8419 Volcano monitoring (7280);
- 8430 Volcanic gases