Abstract
<title>Abstract</title> <p> 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 ( <italic>Vs</italic> ) anomaly at depths of 2.5–4.5 km beneath the Nakadake crater. The <italic>Vs</italic> 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 within the storage zone, leading to volume expansion, fracturing opening, and reduced <italic>Vs</italic> . Temporal variations in seismic velocity, including changes associated with the 2016 Kumamoto earthquake near Mt. Aso, suggest that this zone responds to pressure variations and behaves as a pressure-sensitive fractured system. 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. </p>