Shear Velocity Evidence of Upper Crustal Magma Storage Beneath Valles Caldera

Abstract

AbstractValles Caldera was formed by large rhyolitic eruptions at ∼1.6 and 1.23 Ma and it hosts post‐caldera rhyolitic deposits as young as ∼69 ka, but the contemporary state of the magmatic system is unclear. Local seismicity beneath Valles Caldera is rare and shear‐velocity (Vs) structure has not been previously imaged. Here, we present the first local Vs tomography beneath Valles Caldera using ambient noise Rayleigh dispersion from a ∼71 km transect of nodal seismographs with mean spacing of ∼750 m. An ∼6 km wide low‐Vs anomaly (Vs < 2.1 km/s) is located at ∼3–10 km depth within the 1.23 Ma caldera's ring fracture. Assuming magma in textural equilibrium, the new tomography suggests that melt fractions up to ∼17%–22% may be present within the upper crustal depth range where previously erupted rhyolites were stored.