Validation of <i>Ps‐P</i> Tomography for Obtaining 3D Crustal <i>V</i>_<i>P</i><i>/V</i>_<i>S</i> With Small‐<i>N</i> Data Sets: An Application to the Mount St. Helens Magmatic System

Abstract 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.