[SVC47-P17] A study of the temporal change in oscillatory characteristics of Long-period tremor at Aso Volcano, Japan
Keywords:Volcanic earthquake, Hydrothermal system, Boundary wave
In this study, we first examine the temporal variation of dominant periods of LPTs (fundamental mode of around 15 s and first overtone of around 7 s) using the continuous data recorded at broadband seismic stations close to the active crater. The result shows clear temporal change in the dominant periods of LPTs in 2003-2005 and 2014-2015. These two time periods corresponds to the periods in which small phreatic and phereatic/magmatic-hydrothermal eruptions occurred. As to the temporal variation in 2003-2005, as already reported by Ikeda (2005) and Yamamoto (2013), the periods of the fundamental mode and the first overtone show correlated temporal change, and it can be interpreted as compositional and/or thermal change of hydrothermal fluids. On the other hand, in 2014-2015, the period of first overtone is almost constant at around 8 s, while that of the fundamental mode shows relatively large temporal fluctuations between 16 s and 12 s. Such a trend is rather difficult to explain, if we consider the resonance oscillation of a flat fluid-filled crack.
In this study, we therefore examine the oscillatory characteristics of a fluid-filled crack having linearly varying thickness. As a result, it becomes clear that the dispersion of the boundary wave along the fluid-filled crack becomes weaker and thus the ratio between resonance periods of the fundamental mode and the first overtone becomes smaller than the case of a flat crack having constant thickness. This behavior can be understood by considering that the effective thickness of the crack depends on the wavelength of each resonant mode. Based on these results, the different temporal variation of dominant periods of the two resonant modes can be interpreted by depth-dependent thickness of the crack-like conduit which caused by pressurization and/or intrusion of magma at deeper portion of the conduit. Our result suggests that the long-term trend in the state of volcanic fluid systems beneath active volcanoes may be monitored by seismological means.