Abstract
Long-term volcanic subsidence provides insight into intereruptive processes, which comprise the longest portion of the eruptive cycle. Ground-based geodetic surveys of Medicine Lake Volcano (MLV), northern CA, document subsidence at rates of ∼−10 mm yr−1 between 1954 and 2004. The long observation period plus the duration and stable magnitude of this signal presents an ideal opportunity to study long-term volcanic deformation, but this first requires accurate knowledge of the geometry and magnitude of the source. Best-fitting analytical source models to past levelling and GPS data sets show conflicting source parameters—primarily the model depth. To overcome this, we combine multiple tracks of InSAR data, each with a different look angle, to improve upon the spatial resolution of ground-based measurements. We compare the results from InSAR to those of past geodetic studies, extending the geodetic record to 2011 and demonstrating that subsidence at MLV continues at ∼−10 mm yr−1. Using geophysical inversions, we obtain the best-fitting analytical source model—a sill located at 9–10 km depth beneath the caldera. This model geometry is similar to those of past studies, providing a good fit to the high spatial density of InSAR measurements, while accounting for the high ratio of vertical to horizontal deformation derived from InSAR and recorded by existing levelling and GPS data sets. We discuss possible causes of subsidence and show that this model supports the hypothesis that deformation at MLV is driven by tectonic extension, gravitational loading, plus a component of volume loss at depth, most likely due to cooling and crystallization within the intrusive complex that underlies the edifice. Past InSAR surveys at MLV, and throughout the Cascades, are of variable success due to dense vegetation, snow cover and atmospheric artefacts. In this study, we demonstrate how InSAR may be successfully used in this setting by applying a suite of multitemporal analysis methods that account for atmospheric and orbital noise sources. These methods include: a stacking strategy based upon the noise characteristics of each data set; pixelwise rate-map formation (π-RATE) and persistent scatterer InSAR (StaMPS).
Highlights
Medicine Lake Volcano (MLV), northern California, is one of several Cascade volcanoes known to have exhibited ground deformation in recent years
Using information from the California Department of Water Resources, we investigate the effects of snow on persistent scatterer InSAR (PSInSAR) by applying StaMPS to a subset of SAR scenes that were acquired during snow-free months
The profiles highlight some variability in the magnitude of deformation when using different interferometric synthetic aperture radar (InSAR) analysis techniques, which we attribute to different interferograms being used by each method, but overall the magnitude of deformation is consistent between the three data sets
Summary
Medicine Lake Volcano (MLV), northern California, is one of several Cascade volcanoes known to have exhibited ground deformation in recent years. The first levelling measurements at MLV were made in 1954 (Dzurisin et al 1991, 2002) and, with the addition of campaign GPS surveys (Poland et al 2006), comprise a geodetic record that spans 50 yr. Few volcanoes have such long geodetic histories, but what is most unique about MLV is that deformation has been recorded at a constant rate of ∼−10 mm yr−1 since measurements began. The side-looking nature of InSAR satellites means that measurements contain a component of both horizontal
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