SHEAR WAVE SPLITTING OBSERVATIONS BENEATH MOUNT ST. HELENS VOLCANO, WASHINGTON

Mount St. Helens is the most active volcano in the Cascades volcanic arc in terms of seismicity and recent eruptions. However, its location west of the other arc volcanoes, and the geometry of its magmatic plumbing system, remain poorly understood. We measure shear wave splitting of local S and teleseismic SKS waves to characterize seismic anisotropy beneath Mount St. Helens, and infer the strength and orientation of deformation and/or active flow. We use data obtained from the iMUSH (Imaging Magma Under St. Helens) project, for which 70 three-component broadband seismometers recorded continuously for two years (2014-2016). Approximately 100 teleseismic events with M>6 were bandpass filtered at 0.02-0.125 Hz and analyzed for shear wave splitting of SKS phases. Fast direction orientations are consistently oriented NE-SW, with delay times of ~1.0-1.5 s. Stations closest to Mount St. Helens result in more null measurements (i.e., no splitting), likely due to magmatic upwelling beneath the volcano. Similarly, local crustal and upper mantle earthquakes (M>2) surrounding Mount St. Helens were bandpassed from 0.5-1.5 Hz and analyzed for shear wave splitting of direct S phases. Local S arrivals exhibit low signal-to-noise ratios, and shear wave splitting results are highly complex, with the average fast axis orientation differing between closely spaced stations. This work presents a fine-scaled investigation of shear wave splitting beneath Mount St. Helens, and highlights the complexity of the crustal stress-field and possible spatial extent of asthenospheric upwelling.