Shear Velocity Evidence of Upper Crustal Magma Storage Beneath Valles Caldera.

Wilgus, Justin; Schmandt, Brandon; Maguire, Ross; Jiang, Chengxin; Chaput, Julien

Valles 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. Plain Language Summary: Silica‐rich magma stored in the shallow crust of the Earth can fuel eruptions that pose significant hazards to society. Valles Caldera was created by a large explosive eruption of silica‐rich magma 1.23 million years ago. Numerous smaller volcanic eruptions occurred following the large eruption event (as young as 69 thousand years ago) and the shallow subsurface at Valles remains hot indicating potential for modern magmatic storage at depth. Seismic shear waves are sensitive to the presence of magma and other variations in crustal structure. However, no local shear wave imaging has been conducted at Valles Caldera. Using new seismic data from densely spaced instruments and a technique known as ambient noise tomography, we produced the first shear velocity image beneath Valles Caldera. A region of significant low shear velocity is present in the upper crust beneath the caldera which we attribute to the presence of silica‐rich magma. These new results on shallow magma storage can contribute to assessing volcanic hazards more accurately near Valles Caldera. Key Points: The Valles Caldera magmatic system was imaged with ambient noise Rayleigh tomography using a dense temporary seismic transectA low‐Vs anomaly, Vs < 2.1 km/s is imaged within the caldera's ring fracture at ∼3–10 km depthThe upper crustal magma reservoir beneath Valles Caldera may contain up to ∼17%–22% rhyolitic melt

VOLCANIC eruptions; FRICTION velocity; SURFACE waves (Seismic waves); EXPLOSIVE volcanic eruptions; VOLCANIC hazard analysis; CALDERAS; MAGMAS; CRUST of the earth

Geophysical Research Letters, 2023, Vol 50, Issue 5, p1

0094-8276

Academic Journal

10.1029/2022GL101520