Tracking magma plumbing system overpressure and volume through macroscopic seismic source parameters of repetitive volcanic-seismic events

Microscopic dynamic processes associated with gas-liquid and fluid-solid interaction, as well as the magma/host-rock rheology and tectonic stress, determines the stability of magmatic/hydrothermal system underneath active volcanoes. Specifically, the overpressure in the system largely dictates the timing of upcoming eruptions, whereas the system volume controls the potential magnitude and impact of upcoming eruptions. While quantitative assessment of the system overpressure and volume provides invaluable insights into magma dynamics, eruption forecasting, and hazard mitigation, it is not trivial to constrain these fundamentals.

We devise a generic framework to estimate system overpressure and volume associated with repetitive volcano-seismic events such as VLP and LP.  Following the framework developed by Nishimura (1998), we derive the relationship between macroscopic seismic source parameters (i.e., seismic moment rate and single force), the acoustic properties of the fluid near the seismic source (i.e., sound speed and density), and system overpressure and volume. Macroscopic seismic source parameters can be obtained through waveform modeling and inversion. On the other hand, the acoustic properties of the fluid near the seismic source can be estimated by modeling the VLP/LP resonance peaks (i.e., resonance period and attenuation). Alternatively, the gas fraction obtained from the gas emission (rate) and magma output (rate), as well as local volcanic activities (e.g., hydrothermal or magmatic) could also help evaluate the fluid properties in the context of a variety of mixtures of gas, liquid and solid (e.g., Kumagai & Chouet, 2000).

As a proof of concept, we apply the newly developed framework in Aso volcano where repetitive VLP has been observed since 1930s. We show that the estimated overpressures associated with VLP during the 2014-2016 eruption cycle is on the order of ~1 MPa, generally consistent with the tensional rock strength. The size of pressurized system volume is on the order of ~10⁶ m³, like the magmatic output in the same episode. In this report, after discussing the impact of various assumptions on the estimate of the system overpressure and volume, we will explore a global database to evaluate the system overpressure and volume and discuss relevant microscopic processes that are consistent with these findings.