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Deja Vu: Small Earthquake Swarm at Mount St. Helens Does Not Indicate Future Eruptive Activity

May 4, 2017

As of May 5, the PNSN has located 47 earthquakes near Mount St. Helens since the seismic network was restored on April 21, 2017.

As many in the Pacific Northwest can attest to, the winter of 2017 has been a rough one.  Deep snow in the high country buried volcano monitoring sites and caused loss of telemetry and/or power. These problems reduced the Mount St. Helens seismic network, operated jointly by the Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismic Network (PNSN), to roughly half its normal operating capacity. The consistently bad weather prevented CVO and PNSN staff from performing any mid-winter repairs.

A clear weather day on April 21, permitted CVO and PNSN personnel to visit Mount St. Helens and restore the seismic network to nearly full capacity. Immediately after repairs were made, the PNSN began locating small earthquakes at relatively high rates (1 earthquake every few hours) under Mount St. Helens. The damage to seismic stations reduced the ability of the seismic network to locate small-magnitude earthquakes, at least somewhat.

Further analysis has revealed that many of the earthquakes look similar to each other, a common feature of swarms at Mount St. Helens and a sign that the events are occurring in close proximity.  Using data from stations operable all winter, CVO scientists used the repeating characteristic of the earthquakes located since April 21 to track down when the swarm started. The result? There is good evidence that the uptick began as early as April 16 and definitely was occurring as of April 18.

As of May 5, the PNSN has located 47 earthquakes near Mount St. Helens since the seismic network was restored on April 21. Utilizing the similarity of earthquakes, we can detect well over 100 earthquakes that are part of this swarm.  Most earthquakes have depths between sea level and 3 mi (5 km) below sea level (approximately 2-7 km below the surface).  This is consistent with depths of earthquakes occurring since 2008, which are thought to be in response to recharge in the magmatic system.  Earthquake rates, though relatively high compared to background, are still only 1 earthquake every few hours, a rate that is consistent with past small swarms since 2008.  All earthquakes are volcano tectonic in character (no detected low-frequency or long period earthquakes) and the maximum magnitude thus far is a M1.3.  There is no detectable deformation or gas signal associated with this swarm.

Similar swarms occurred at Mount St. Helens in March-May 2016 and November 2016.  Both swarms had repeating earthquakes, average rates of 1-2 earthquakes/hour, and most earthquakes with magnitude below M1.5. 

The similarity of swarms at Mount St. Helens leads us to believe that similar processes cause them, and they are likely tied to magma recharge first detected in 2008. However, pinpointing that exact process is difficult. Some possible mechanisms include a spontaneous release of brine from the pressurized magma chamber into the crust above, a pulse of magma into the magma reservoir that transferred stress into the crust above, or just the breakage of a new pathway of fluid flow that was previously blocked by precipitated minerals. There are several reasons why it is very unlikely that this swarm is a precursor to imminent eruptive activity at Mount St. Helens—it is similar to ones in the past that did not lead to surface activity; it consists of very small earthquakes occurring at relatively low rates; there are no other geophysical indicators (deformation, tilt, gas) of unrest.

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