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

The high sensitivity of VP/VS to the presence of melt makes images of VP/VS structure particularly useful in magmatic systems, but detailed three‐dimensional models of VP/VS 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 VP/VS 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 VP/VS upper crustal magma reservoirs beneath active volcanic systems at Mount St. Helens, Mount Adams and the Indian Heaven Volcanic Field and low VP/VS anomalies associated with crystallized plutons. Our model also provides new VP/VS constraints in the lower crust that reveal a high VP/VS anomaly connecting the Mount St. Helens and Mount Adams reservoirs and a low VP/VS anomaly associated with lower crustal cumulates or mafic accreted terranes. Decimation tests further show that first order VP/VS 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 VP/VS throughout the crust, including in data‐poor settings or with arrays not designed for structural seismic investigations, such as many volcano monitoring networks. Plain Language Summary: There are a variety of ways that seismic waves can be used to infer the three‐dimensional distribution of physical properties of the Earth including parameters like composition, temperature and the presence of partially molten rock (magma), but there are some parts of the Earth that are more difficult to see than others. The lower crust is one region of the Earth that is both important and difficult to image, but many traditional imaging tools require extensive and expensive data collection to see it. However, a technique called receiver function analysis, which highlights the recording of waves that convert from compressional (P) waves to shear (S) waves at physical boundaries in the Earth, can be used to see the entire crust at relatively low cost. This paper tests a method for imaging three‐dimensional crustal structure, including the lower crust, using measurements of those converted waves. Importantly, the paper tests this method at a data‐rich setting, Mount St. Helens in Washington state, USA, to illustrate how well it can be used in other data‐poor settings. The results show that the method, called Ps‐P tomography, works well and can be applied to data‐poor settings with confidence despite known limitations in resolution. Key Points: We use Ps‐P tomography to image the relative VP/VS crustal structure in the region surrounding Mount St. Helens, Washington state, USAPs‐P tomography model is correlated with previously imaged structure around the volcano, highlighting the accuracy of the techniquePs‐P tomography reveals VP/VS structure throughout the crust and is shown to be effective with as few as four available seismic stations [ABSTRACT FROM AUTHOR]