Deep long-period earthquakes under Mount St. Helens captured with dense recordings by iMUSH

Abstract:

Mysteries abound regarding the mechanism generating deep-long-period earthquakes (DLPs). DLPs are most commonly associated with the process of magma ascent from a deep source to a crustal reservoir, and plausible ideas include dehydration embrittlement, sluggish faulting, gurgling flow of magmatic fluids, and cooling of relic magma conduits.

By good fortune, at least four DLPs occurred since the imaging Magma Under St Helens (iMUSH) experiment began in late June 2014. The DLPs were captured by 70 broadband seismometers in the passive array, and several were also recorded by the 3500 short-period seismometers deployed for the active experiment.

These lower crust/upper mantle events were 20-35 km deep, offset less than 15 km from the crater, and have the low-frequency, long-duration reverberative waveforms, and lower crust/upper mantle locations characteristic of DLPs. One DLP had numerous bursts across ~100s, and two others consisted of two bursts within a minute. These are similar to the 19 DLPs seen beneath Mount St. Helens (MSH) previously [Nichols et al., 2011, JVGR]. We will also use these DLPs as templates in the search for others that are too small to be found otherwise.

DLPs at MSH occur beneath the St. Helens Seismic Zone, proposed to be the block boundary between the Southern Washington Cascades Conductor and Siletzia rocks to the west. This actively-slipping and weak structural boundary could enhance the ability of magmatic fluids to reach the surface, and the co-located DLPs provide evidence for such fluid migration.

We plan to investigate the frequency content, time evolution, spatial location, and clustering of DLPs under Mount St. Helens to shed light on the underlying physics and implications for shallower activity.