Episodic transport of discrete magma batches beneath Aso volcano

Abstract

Magma ascent, storage, and discharge in the trans-crustal magma plumbing system are key to long-term volcanic output and short-term eruption dynamics. Petrological analytics, geodetic deformations and mechanical modeling have shaped the current understanding of magma transport. However, due to the lack of observations, how a distinct magma batch transports from a crystal-rich mush region to a crystal-poor pool with eruptible magma remains enigmatic. Through stacking of tilt and seismic waveform data, we find that episodic long-period tremors (LPTs) located near sea level beneath the Aso volcano are accompanied by a synchronous deformation event, which initiates ~50 seconds before individual LPT event and concludes seconds after. The episodic deformation source corresponds to either an inflation or a deflation event located ~3 km below sea level, with a major volumetric component (50-440 cubic meters per event) and a minor high-angle normal-fault component. We suggest that these deformation events likely represent short-lived, episodic upward transport of discrete magma batches accompanied by high-angle shear failure near the roof of the inferred magma chamber at relatively high temperature, whereas their recurrences, potentially composition dependent, are regulated by the brittle-to-ductile transition rheology under low differential stress and high strain rate due to the surge of magma from below, regulating long-term volcanic output rate. The magma ascent velocity, decompression rates, and cumulative magma output deduced from the episodic deformation events before recent eruptions in Aso volcano are compatible with retrospective observations of the eruption style, tephra fallouts, and plume heights, promising real-time evaluation of upcoming eruptions.