Real-time and in-situ assessment of conduit permeability through diverse long-period tremors beneath Aso volcano, Japan

Abstract

Long-period signal (LPs, 0.2-2 s) and very long-period signal (VLP, 2-100 s) observed in the shallow volcanic plumbing system are typically repetitive and time-invariant in their location and source mechanism, offering in-situ probes of hot fluid transport over the eruption cycles. While the amplitude and activity of volcanictectonic earthquakes and LP events have been commonly used to infer overpressure within their source region, one missing link is an observable that may permit inference on the change in the permeability of the conduit plug/wall, which can regulate the degree of pressurization, affect the mechanical strength of the surrounding rock, and consequently the likelihood of an upcoming eruption. Here we show that during the 2011-2016 eruption cycle at Aso volcano in Japan, long-period tremor events, a VLP of~15 s period, with opposite waveform polarity can be systematically detected and categorized as pressurization and depressurization events in the same crack-like conduit. We suggest that, depending on the strength of the surrounding rock and the permeability of the crack-like conduit wall/plug, pressurization due to magmatic heat and vaporization is more likely to occur when a less permeable conduit plug/wall can effectively keep the gas inside the crack-like conduit. On the other hand, depressurization is prone to occur if the conduit wall/plug permeability is sufficiently high to allow gas to escape from the conduit. Considering the amplitude of LPT proportional to the conduit overpressure, contrasting energetics of these diverse LPT events allows us to define whether the conduit is prone to pressurization or depressurization, providing a framework to infer how the permeability of the conduit wall/plug may evolve over an eruption cycle.

Episodic transport of discrete magma batches beneath Aso volcano

“…Beneath Aso volcano, an basalt-andesitic volcano in Japan 21 – 23 , repetitive very long-period signals (VLP, ~15 s period) are known to occur in a fluid-filled crack-like conduit within a shallow hydrothermal aquifer 24 – 26 , ~1 km beneath the active Naka-dake first crater 27 – 32 (Fig. 1a ).…”

“…Several VLP families exist in 2011–2016 and they share a largely common source location and geometry but differ somewhat in their resonance periods 32 . VLP families with a positive (negative) initial polarity are categorized as pressurization (depressurization) events in the shallow crack-like conduit 28 , 30 , 32 . The pressurization and depressurization VLPs have been attributed to vaporization and outgassing process, respectively 28 , 32 .…”

“…VLP families with a positive (negative) initial polarity are categorized as pressurization (depressurization) events in the shallow crack-like conduit 28 , 30 , 32 . The pressurization and depressurization VLPs have been attributed to vaporization and outgassing process, respectively 28 , 32 . Most importantly, VLP in Aso is known to occur during the quiescence and active period 28 , 30 , 32 , offering a unique opportunity to methodically explore discrete magma ascents over the entire eruption cycle in 2011–2016, including changes in the crater-lake level, minor phreatic eruptions, the incandescence followed by ash eruption, and Strombolian eruption in the late 2014 33 .…”

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

“…To evaluate variations of conduit plug/wall permeability and rheology, Niu and Song (2020) contrasts the activity of the pressurization event against the depressurization event at a given time, where a period prone NIU AND SONG 10.1029/2021GL092728 to pressurization (depressurization) indicates a lower (higher) permeability in the conduit plug/wall. If the permeability is indeed enhanced by dynamic stress, we expect elevated LPT activity and a greater proportion of depressurization LPT events after great earthquakes, consistent with our observations (Figures 3b-3d).…”

“…We aim to provide observational evidence and understand broadly how VLPs in a persistently degassing volcano respond to periodic loadings ranging from the tidal diurnal period to seismic frequencies. Specifically, we take advantage of the newly available VLP catalog in Aso volcano over the 2011-2016 eruption cycle (Niu & Song, 2020). The repetitive VLP activity during the quiescent and active episodes in Aso volcano provides a unique opportunity to tease out the effect of external triggering/modulation from internal mechanisms.…”