Subsurface Anatomy of the Irazú-Turrialba Volcanic Complex, Inferred from the Integration of Local and Ambient Seismic Tomographic Methods

Irazu and Turrialba are a twin volcanic complex that marks a distinct stop in volcanism along the Central America volcanic arc. We present a new traveltime velocity model of the crust beneath Irazu and Turrialba volcanoes, Costa Rica, and interpret it considering the results of previous ambient noise tomographic inversions. Data were acquired by a temporary seismic network during a period of low activity of the Irazu-Turrialba volcanic complex in 2018-2019. Beneath the Irazu volcano, we observe low P-wave velocities (VP = 5 km s-1) and low velocity ratios (VP/VS = 1.6). In contrast, below the Turrialba volcano, we observe low S-wave velocities (VS = 3 km s-1) and a high VP/VS (= 1.85) anomaly. We found that locations of low VP and VS anomalies (-15 %) correspond well with shear wave velocity anomalies retrieved from ambient noise tomography. At shallower depths, we observe high VP and VS anomalies (+15 %) located between the summits of the volcanoes. Subvertical velocity anomalies are also observed at greater depths, with high VP and VS anomalies appearing at the lower limits of our models. We propose a complex structure of an intermediate magmatic reservoir, presenting multiphase fluid states of a liquid-to-gas transition beneath Irazu and a juvenile store of magmatic fluid beneath Turrialba, while shallow fluid transport provides evidence of magmatic-hydrothermal interactions.