Seismic Imaging with the Mount St. Helens Node Array

The 900 instrument Mount St. Helens nodal array recorded continuous data for approximately two weeks in the summer of 2014 and was deployed contemporaneously with the active source component of the iMUSH experiment (imaging Magma Under St. Helens). Two distinct imaging methodologies are applied to node data 1) reverse time imaging (RTI) is used to automatically detect and locate microseismicity occurring beneath the array and 2) the depth and seismic amplitude of the crust mantle boundary (Moho) are investigated using reflection imaging (common mid-point stacking). Reverse-time source imaging is applied to the 10 km region directly beneath the volcanic edifice where most of cataloged seismicity occurred during the experiment. These efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the Pacific Northwest Seismic Network [Hansen and Schmandt, 2015]. Earthquake locations resolve a narrow, ≤1 km wide, vertical lineament of seismicity which extends from the surface to 4 km depth directly beneath the summit crater and is consistent with the historical distribution of seismicity from Waite and Moran [2009]. This feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface. Additionally, the RTI method is modified to image two deep long-period events that were recorded by the array. For these events, energy from the S-wave coda is included in the back-projection and provides increased depth resolution for hypocenter determination. Moho structure is investigated using the near-offset (< 30 km) PmP recordings that were generated by the iMUSH active source shots. Arrivals are enhanced using the short-termaverage over long-term-average signal processing technique and the resulting data are migrated using a 3D velocity model. Significant lateral variations in the Moho reflectivity are observed; high amplitude PmP energy is observed from shots originating from the east whereas shots from the west display little-to-no PmP energy. A Moho amplitude map shows that boundary between the regions of high and low reflectivity occurs directly beneath Mount St. Helens. We attribute the lack of PmP west of the volcano to the presence of a cold serpentinized mantle wedge which is consistent with the forearc location of Mount St. Helens.