Validation of Ps-P Tomography for Obtaining 3D Crustal V_P/V_S With Small-N Data Sets: An Application to the Mount St. Helens Magmatic System

The high sensitivity of V_P/V_S to the presence of melt makes images of V_P/V_S structure particularly useful in magmatic systems, but detailed three-dimensional models of V_P/V_S structure in magmatic systems are often restricted to the upper crust where there is a concentration of seismic sources used for imaging. Ps-P tomography is a new technique that has been used to image three-dimensional crustal-scale variations in V_P/V_S in regions with limited seismic instrumentation. We apply the Ps-P tomography technique to a well-constrained, independently imaged magmatic setting, Mount St. Helens, to outline the efficacy and limitations of this imaging approach. Our Ps-P tomography model reveals previously imaged high V_P/V_S upper crustal magma reservoirs beneath active volcanic systems at Mount St. Helens, Mount Adams and the Indian Heaven Volcanic Field and low V_P/V_S anomalies associated with crystallized plutons. Our model also provides new V_P/V_S constraints in the lower crust that reveal a high V_P/V_S anomaly connecting the Mount St. Helens and Mount Adams reservoirs and a low V_P/V_S anomaly associated with lower crustal cumulates or mafic accreted terranes. Decimation tests further show that first order V_P/V_S structure is recoverable using as few as four recording seismometers. These images resemble those of independent, higher resolution images from traditional techniques, highlighting the utility of Ps-P tomography for imaging three-dimensional variations of V_P/V_S throughout the crust, including in data-poor settings or with arrays not designed for structural seismic investigations, such as many volcano monitoring networks.