The Complex Relationship between Seismic Velocity and Volcanic, Tectonic, and Environmental Forcings Illustrated by 23 Years of Data at Mt. St. Helens

Mt. St. Helens is an explosively erupting volcano located in close vicinity to major metropolitan centres on the US Westcoast. In recent history, Mt. St. Helens erupted twice, in 2004 and 1980, causing more than 50 fatalities and over one billion USD of damage. Mt. St. Helens is also home to the only advancing glacier in the US, making it a unique site for geophysical measurements. Here, we present a seismic velocity change time-series (dv/v) of an unprecedented length covering the years 1998-2021. We quantify dv/v by applying the method of ambient seismic noise interferometry to waveform data recorded from a combination of various permanent and temporary seismic stations of the Pacific Northwest Seismic Network (PNSN). Due to its ubiquitous nature, ambient seismic noise allows for far denser temporal sampling than, e.g., active source or earthquake coda interferometry. However, source variability related, for example, to volcanic tremor activity affects the results retrieved by this method and can lead to decreased reliability. In this study, we focus on the impact of the complex dynamics at Mt. St. Helens on dv/v specifically by setting it into context with ground deformation, meteorological changes, and volcanic activity with the ultimate goal of unravelling the complex physical relationship between different forcings and the seismic velocity.