Fiber seismic tomography of the Long Valley volcanic system

High-resolution tomographic imaging of the subsurface structures beneath volcanic systems is fundamental to better assess their hazard and potentially estimate the amount of eruptive materials. However, obtaining such images represents a major challenge for multiple reasons. The significant velocity contrasts commonly present within volcanic systems require accurate wave simulations or traveltime modeling to correctly account for wavefield triplications and ray bending. Moreover, station coverage and temporary deployments usually cannot achieve the requirements needed to meet high-resolution imaging targets. We demonstrate how distributed acoustic sensing (DAS) data recorded on existing telecommunication fiber cables and employed within an accurate and efficient matrix-free Eikonal tomography workflow can overcome these limitations. Specifically, we produce high-resolution tomographic images of the Long Valley caldera system in California, which in recent years has been undergoing significant inflation and seismic unrest. Our results reveal a distinct separation between the large magma chamber at approximately 10 km depth and the shallow crust. We interpret this separation as an upper-crust lid confining the pressurized volcanic fluid released through the crystallization of the magma reservoir over time.  Our study highlights the potential of DAS for advancing volcano science; from providing insights into subsurface structures to monitoring dynamic processing due to fluid and magma movements through these complex systems.