Deep Long-Period Earthquakes Beneath Mount St. Helens: Their Relationship to Tidal Stress, Episodic Tremor and Slip, and Regular Earthquakes

Volcanic deep long-period earthquakes (DLPs) have been observed for a long time but remain poorly understood. In this study, 22 catalog DLPs were taken as templates to search for repeating events at Mount St. Helens. During the period from 2007 to 2016, we have detected 277 DLPs. Three templates are isolated DLPs, while all other templates produced matches, identifying loci of repeated activity. Overall, the DLPs show no significant correlation with either the timing of the subduction zone tremor and slow slip west of Mount St. Helens, or the shallow seismicity, or distant earthquakes. Temporal analysis shows an elevated rate of DLPs at times of compressional volumetric tidal stress, suggesting a possible association with magmatic and/or fluid activity. We also observed variable S wave polarization of the DLPs in the most productive DLP source region, indicating source mechanisms are not identical despite the waveform similarity that allowed their detection.