Rayleigh wave tomography of Mount St. Helens, Washington from ambient seismic noise

Abstract:

Mount St. Helens is the most active volcano of the Cascade range in the Western U.S. Given its recent eruptions in 1980 and 2005, it is clear that magma transport has recently occurred in the shallow crust beneath the volcanic edifice. A dense seismic array was deployed around Mount St. Helens for two weeks in summer 2014. The array was composed of 904 vertical-component 10-Hz geophones distributed within 20 km of the caldera. We cross-correlated all the seismic ambient noise data from this array to measure Rayleigh wave travel times and invert for the seismic shear velocity structure beneath the volcano. Clear Rayleigh waves are observed between 2 to 5 sec period in most directions and the signal is particularly strong in the Southwest-Northeast direction likely caused by ocean waves off the west coast of Washington State. We applied frequency-time analysis to measure phase velocity dispersions for all available station pairs, and we applied surface wave tomography for each period to determine 2-D Rayleigh wave phase velocity maps between 2 to 5 second period. Finally, we inverted these maps for a preliminary 3D velocity model from surface to 5 km depth. The model shows a low-velocity anomaly beneath the center of the caldera. This anomaly could be related to shallow magma storage beneath Mount St. Helens as well as the highly fractured rock of the volcanic edifice. Further analysis of short period surface wave propagation will improve understanding of upper crustal structure beneath Mount St. Helens and how it is linked to supply of silicate melts and volatiles from greater depths.