Imaging shallow magma storage and fluid pathways beneath Aso volcano using ambient-noise tomography

To investigate the high-resolution three-dimensional internal structure of the large caldera-type volcano Mt. Aso, we deployed a temporary seismometer network and applied ambient-noise surface-wave tomography. We resolve the shallow crustal structure with high spatial resolution and identify an ellipsoidal low S-wave velocity ( Vs ) anomaly at depths of 2.5–4.5 km beneath the Nakadake crater. The Vs within this zone is ~ 1.5 km/s, indicating a highly fractured and pressurized rock, which we interpret as a shallow magma storage zone. Considering the temperature conditions, supercritical fluids could transition to a gas phase beneath the storage zone, leading to volume expansion, fracture opening, and reduced Vs . Furthermore, combined with previously reported temporal variations in seismic velocity and geodetic modeling results, the low- Vs zone could behave as a pressure-sensitive fractured system, potentially connected to the overlying fluid-bearing pathway beneath Aso Volcano. Above the storage zone, we image a narrow, vertically elongated low- Vs structure interpreted as a hydrothermal conduit that likely transports the pressurized fluids to the surface crater and generates very long-period (VLP) tremors.