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Something brewing at Mt Rainier? 23 September 2009

Posted by admin in natural hazards, United States, volcano monitoring.
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Probably not, but there has been an earthquake swarm under Mount Rainier which has attracted some interest. This graph from the Pacific Northwest Seismic Network shows recent seismicity at the volcano, up to 22 September 2009, indicating an increase in the number of earthquakes occurring between around 1.5 and 4.5 km depth:

Mount Rainier seismicity 08/08 - 09/09

(The fact that the latest quakes are shown in red doesn’t mean anything alarming his happening, by the way. The graph happens to show the most recent month in red and earlier months in green, which is a bit unfortunate as it gives a misleading impression of increasing crisis.)

This is certainly interesting, but the Cascades Volcano Observatory doesn’t think there’s anything to be concerned about (update 20 September 2009, 19:53 UTC):

Recent Observations: A swarm of small, shallow earthquakes occurred beneath the summit of Mount Rainier on Sunday, September 20, 2009, starting at ~0920 PDT. According to the Pacific Northwest Seismic Network (PNSN), the largest event at 0945 PDT has a preliminary magnitude of M 2.2 with a depth of 1.8 km, located 1 km NNE of the summit. The swarm also featured many small events that were too small to be located, and lasted for several hours before gradually waning. Such swarms have happened several times in the past decade. One example is the swarm on November 7, 2004, that included a M 3.2 event. We consider the September 20, 2009, swarm to be within the realm of normal activity at Mount Rainier.

Rainier is an active volcano with a dynamic hydrothermal system, and these quakes may just be the result of fluid shifting about in the depths. If the number of earthquakes continues to increase and they move upwards, of course, there’ll be something to worry about, as that would mean magma ascending the conduit. Rainier, which last erupted in 1894, is a dangerous volcano with an alarming range of hazards to throw at the Pacific Northwest should it decide to erupt. In the meantime Mount Rainier is closely watched, and we can be sure that if anything untoward happens the CVO will tell us about it. That’s the wonder of volcano monitoring.

Global Volcanism Program: Rainier – information about Rainier (1201-03-) from the GVP
CVO: Rainier – more Rainier information from the Cascades Volcano Observatory
Mount Rainier Recent Earthquakes – Pacific Northwest Seismic Network page for recent Rainier seismicity (click here for Rainier webicorders)

The Volcanism Blog