The Response of Repetitive Very‐Long‐Period Seismic Signals at Aso Volcano to Periodic Loading.

Triggering of volcano seismic activity and eruptions by tides, atmosphere pressure, rainfall, and earthquakes have been in constant debate. However, there is limited evidence concerning the triggering of very‐long‐period signals (VLPs), which are closely linked to volcano conduit dynamics. Persistent and repetitive VLP event beneath the Aso volcano, historically termed long‐period tremor (LPT), manifests episodic pressurization and depressurization events in a crack‐like shallow conduit. Here we show that LPT activity display no appreciable spectral peaks associated with major diurnal/semidiurnal tidal constituents or barometric pressure. Instead, passing surface waves of ∼0.01 m/s from the 2011 Tohoku earthquake elevated LPT activity and preferentially increased the likelihood of depressurization events. We suggest that the hydrothermal reservoir near the LPT source behaves like a confined (unconfined) aquifer against short‐period (long‐period) stress. A high stress rate of ∼102 Pa/s is sufficient to enhance the permeability of the conduit plug/wall and preferentially promotes depressurization events. Plain Language Summary: At one of the most active volcanoes in Japan, Aso volcano, long‐period tremor (LPT) was first discovered in the 1930s by Prof. Sassa in Kyoto University and such LPT activity have persisted since. It offers a unique natural laboratory and controlled experiment to systematically explore how periodic loadings trigger or modulate LPT activity in a persistently degassing volcano equipped with a well‐developed hydrothermal system. After carefully considering the background noise level modulated by the local wind and meteorological condition, we find no evidence of modulation or triggering by tides, barometric pressure, or temperature. Systematic analysis of surface wave amplitudes from great earthquakes suggests a common triggering threshold (∼0.01 m/s) shared by LPT (pressure change events) in Aso and volcano‐tectonic earthquakes (brittle failure events) in some geothermal areas, supporting key processes of pressure oscillations and unclogging fractures by dynamic stress. Key Points: Repetitive very‐long‐period signals (VLP) in Aso volcano (aka long‐period tremor, LPT) do not show detectable response against major tidal constituents, barometric pressure and temperatureStrong surface waves from the 2011 Tohoku earthquake (> 0.01 m/s) preferentially modulate depressurization LPT events for several daysResponse of hydrothermal reservoir to pressure change induced by periodic loadings is likely stress‐rate dependent [ABSTRACT FROM AUTHOR]