Shallow volcanic reservoirs and pathways beneath Aso caldera revealed using ambient seismic noise tomography

Abstract

Imaging the shallow velocity structures beneath Aso caldera is necessary to further understand volcanism at the volcano and in the region. The network for monitoring Aso volcano has been progressively renewed and upgraded with denser and more modern instruments. We used approximately 4 years of seismic data recorded by a network of 25 seismic stations to image S-wave velocity (Vs) structures beneath Aso caldera with seismic noise interferometry. We calculated daily cross-correlation functions (CCFs) of broadband and short-period station pairs separately and then stacked CCFs monthly, to obtain the time-domain empirical Green’s functions and corresponding Rayleigh-wave phase-velocity dispersion curves. Finally, we constructed 1–5-s phase-velocity maps interpolated from nodes spaced 0.05° grid. The maps allow investigating crustal Vs structures between the surface and a depth of 6 km, likely related to shallow volcanic reservoirs and pathways. High velocities are found within the first kilometer of the crust beneath post-caldera central cones. Low velocities in the center of the post-caldera central cones extend from the surface to a depth of 1–2.5 km; we infer that the anomalies mark shallow hydrothermal reservoirs likely replenished by precipitation and hydrographic networks. The prevalence of high velocities below 3 km can be considered as consolidated igneous rock. Low-velocity anomalies identified at depths of 5–6 km beneath the post-caldera central cones could be a manifestation of magma accumulation. The low-velocity belts situated at 2.5–5 km depths are likely pathways for the transfer of hydrothermal fluids, volcanic gases, or melting magma to the surface. The northern part of the caldera shows relevant lateral velocity variations, with low velocities and high velocities predominant in the east and west, respectively. Other low-Vs anomalies appear near the surface to the west and northwest of Aso caldera.