Hydrothermal signature on episodic deflation/inflation ground tilt at Aso Volcano

Ground deformation in volcanic areas induced by geothermal fluid circulation can reveal useful information about the dynamical processes occurring in the subsurface hydrothermal system. In the present work, we investigate tiltmeter time-series recorded at Aso Volcano during 2011-2016, a time interval during which different phases of volcanic activity occurred. We performed polarization analysis of the data and identified peculiar long-lasting (hours) transients, defined as Very-Long-period Tilt Pulses. The transients were further characterized in terms of waveform cross-correlation, particle tilt pattern, energy, and time distributions. The analyses indicate that such signals, which appear like deflation-inflation (DI) events, are associated with a Poissonian process whose underlying dynamics evolves over time always driven by a Poissonian mechanism. The obtained results have been interpreted in light of the available geophysical, geochemical and volcanological information. In this framework, the Very-Long-period Tilt Pulses may be ascribed to the depressurization/pressurization of the shallow hydrothermal system according to a fault-valve mechanism, which was active with different efficiency throughout eruptive and inter-eruptive phases.