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 volcanic-tectonic 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.