Measuring Long-Term Subsidence of Mount St. Helens 1980 Deposits with Combined InSAR, GNSS, and Forward Modeling

On May 18th 1980, Mount St. Helens erupted with a lateral blast that drastically transformed the landscape north of the volcano. A mix of landslide deposits and pyroclastic density current (PDC) deposits blanketed the north flank of the volcano, and to this day continuous stream erosion in the pumice plain has not yet exposed the entire thickness of these mixed deposits. Thus, the total thickness of the PDC deposits from the 1980 eruptions remains unknown. Using ground deformation measurements to quantify the long-term compaction of the PDC deposits provides a unique opportunity to constrain the deposit thickness, as only initially hot material may deform over multiple decades (thus excluding the avalanche deposits). The thickness of these deposits and their deformation rate over the past decades may provide insights into the characteristics of the debris when it first erupted. Here we study the 1992-2020 ground deformation at Mt. St. Helens with Interferometric Synthetic Aperture Radar (InSAR) data from the ERS-1/2, ENVISAT, ALOS-1/2, and Sentinel satellites. We process all these datasets using the FRinGE time series analysis technique, which combines Persistent and Distributed Scatter methods to improve the density and quality of usable InSAR pixels. This state of the art InSAR processing method offers a chance to revisit Mt. St. Helens' deformation history. We combine InSAR time series with GNSS data to determine the absolute deformation and estimate InSAR uncertainties with noise. Finally, we rely on a forward modeling approach of the observed deformation to estimate the PDC thickness and characteristics. We compare these results to a difference of the pre- and post-eruption Digital Elevation Model of the volcano to estimate how much of the observed topographical changes are related to the PDCs compared to the landslide deposits.