Deep Long-Period Earthquakes

A potential additional source of insight for Mount St Helens could come from mining the continuous passive array data for signals of anomalous earthquakes in and around the magmatic system. Numerous seismic events are recorded at Mount St Helens, so many that, along with Mount Rainier, it provides the majority of earthquake detections within the Pacific Northwest Seismic Network (PNSN). One enigmatic source that has occurred on occasion at Mount St Helens is deep long- period events (DLPs; Nichols et al. 2011). DLPs are events that occur at mid-to-lower crustal depths beneath/near volcanoes that are deficient in high frequency energy. They have been observed at a number of volcanoes globally, including Mount St Helens (9 events since 1980, generally located 5-10 km southeast of Mount St Helens at 25-30 km depth).

Various explanations for their occurrence have been put forward since they were first recognized at Mount Pinatubo in 1991 (White 1996), including magma movement (Power et al. 2004) and fluid migration along pre-existing faults. Such explanations have largely been speculative, however, because DLPs generally have been recorded on volcano seismic networks consisting of a handful of short-period vertical-component instruments. Were DLPs to occur during deployment of the passive array, they would unquestionably become the best recorded DLPs in the world. Event locations, depths, and possibly source mechanisms through moment-tensor inversions would be much better constrained by the broadband array, and would provide much fodder for further investigations of these intriguing events.