Michael S Christoffersen

USGS2024

Versatile modeling of deformation (VMOD) inversion framework: Application to 20 years of observations at Westdahl Volcano and Fisher Caldera, Alaska, US

We developed an open source, extensible Python-based framework, that we call the Versatile Modeling of Deformation (VMOD), for forward and inverse modeling of crustal deformation sources. VMOD abstracts from specific source model implementations, data types and inversion methods. We implement the most common geodetic source models which can be combined to model and analyze multi-source deformation. VMOD supports Global Navigation Satellite System (GNSS), InSAR, electronic distance measurement, Leveling and tilt data. To infer source characteristics from observations, VMOD implements non-linear least squares and Markov Chain Monte-Carlo Bayesian inversions, including joint inversions using different sources of data. VMOD's structure allows for easy integration of new geodetic models, data types, and inversion strategies. We benchmark the forward models against other published results and the inversion approaches against other implementations. We apply VMOD to analyze deformation at Unimak Island, Alaska, observed with continuous and campaign GNSS, and ascending and descending InSAR time series generated from Sentinel-1 satellite radar acquisitions. These data show an inflation pattern at Westdahl volcano and subsidence at Fisher Caldera. We use VMOD to test a range of source models by jointly inverting the GNSS and InSAR data sets. Our final model simultaneously constrains the parameters of two sources. Our results reveal a depressurizing spheroid under Fisher Caldera ∼4–6 km deep, contracting at a rate of ∼2–3 Mm 3 /yr, and a pressurizing spherical source underneath Westdahl volcano ∼6–8 km deep, inflating at ∼5 Mm 3 /yr. This and past applications of VMOD to volcanic unrest benefit from an extensible framework which supports jointly inversions of data sets for parameters of easily composable multi-source models.

Alaska↗

USGS2020

Sparse subsurface radar reflectors in Hellas Planitia, Mars

Geomorphological features potentially related to subsurface ice, such as scalloped depressions, expanded craters, pedestal craters, and banded terrain, are present in and around Hellas Planitia, Mars. We present a radar survey of the region using the Shallow Radar (SHARAD) instrument on board the Mars Reconnaissance Orbiter (MRO) to identify candidate subsurface reflectors that may be due to the presence of potentially ice-rich deposits. We found that the majority of radar returns are likely from off-nadir surface topography (“clutter”), arising from the rough topography of the region. There is no widespread radar return from any subsurface interfaces. However, we identify a group of six reflectors adjacent to each other on a plateau in Malea Patera in which we have higher confidence. Landforms associated with a likely ice-rich mantle are associated with the plateau, but the thickness of this mantle does not correspond to the expected depth of the reflectors. However, layers beneath the mantle and marginal pitting at the edge of the plateau are similar to those associated with pedestal craters, which may be ice rich and are a similar thickness to the expected depth of the reflectors. Malea Patera has been interpreted to be a volcanic caldera, so the reflectors may be associated with a volcanic deposit within the plateau, although the evidence for this is inconclusive. Because this radar detection is localized and its origin ambiguous, we cannot use it to make conclusions about the thickness of subsurface deposits in the Hellas region as a whole. The lack of widespread radar reflectors in this region, as compared to the northern mid-latitudes where extensive radar reflections have been mapped, may be due in part to higher surface roughness, which creates radar clutter that may obscure subsurface reflectors. However on the southern rim of the basin and south of the basin, the lack of reflectors may indicate that the possible ice-rich deposits observed geomorphologically in this region are too thin to be resolved by SHARAD, are dielectrically similar to the underlying unit, or have a gradual vertical transition in ice content that is not reflective for the radar. This would imply that recent climate processes may have favored widespread, thick ice deposition or preservation in the northern hemisphere as compared to the southern hemisphere.

Icarus↗