Historical Seismicity

This map view of well recorded historical seismicity at Mount Rainier since 2007 (gray circles) with seismicity since Sept. 11 plotted as filled red circles. Location of seismic stations are indicated by arrows. At bottom, a cross-sectional view looking north shows earthquake depths. Black outline is a west-east elevation profile of Mount Rainier. Circles are sized with respect to earthquake magnitude. Data are from the Pacific Northwest Seismic Network.

Over the last week more than 20 earthquakes have been located by the Pacific Northwest Seismic Network at Mount Rainier.

In a typical week Mount Rainier experiences about two “located” earthquakes, so this represents a modest increase over background rates, according to the USGS-Cascades Volcano Observatory and Pacific Northwest Seismic Network.

These swarms are a common and occurrence at active volcanoes such as Mount Rainier. While interesting, most never result in surface changes, according to the United States Geological Survey.

These earthquakes are inferred by scientists to be caused by processes occurring in Mount Rainier’s hydrothermal system. The hydrothermal system is the region beneath the volcano containing hot mineral-rich water; one manifestation of this system is the boiling-point fumaroles that are found at the volcano’s summit.

Similar to pipes in geothermal plants, cracks transporting water away from a hot source may seal shut as the water cools and loses its dissolved minerals. Earthquakes are created when sufficient fluid pressure builds behind these seals to fracture them.

“For the current swarm, earthquake location, depth, and size are all consistent with background seismicity; the only thing that is different is the event rate,” according to the USGS.

The seismicity uptick started late Sept. 11 with a swarm of five earthquakes located 0.5 to 1.5 miles to the southeast of the summit area. These earthquakes were shallow, small and in an area that has not had a lot of previously recorded seismicity. Beginning Sept. 13, earthquakes were also detected about 0.6 miles to the northeast and southeast of the summit, in areas where earthquakes typically occur at Mount Rainier. Depths for these events were 0.6 to 1.2 miles below sea level, which is also typical for background seismicity at the volcano, and event magnitudes were small (maximum magnitude 1.2)

23 earthquakes have occurred since Sept. 11, with event rates of up to eight located earthquakes per day.

“Although this is higher than the normal seismicity rates at Mount Rainier, it is not unprecedented,” according to the USGS.

Over the past 10 years there have been three previous periods with similar or higher event rates (September 2009, April 2015 and May 2016). Current event rates are similar to those seen in April 2015 and May 2016, but are far smaller in rate, energy release, and total number of earthquakes than what was seen in September 2009, when a swarm featuring hundreds of located earthquakes occurred over a three-day period.

The most likely scenario for the current swarm is that elevated earthquake rates will continue for days to a few weeks before slowly decaying to background rates of seismicity. Each swarm provides seismologists with more perspective on the personality of the volcano and aids in our ability to assess hazards in the future. Scientists at the USGS‒Cascades Volcano Observatory and Pacific Northwest Seismic Network will continue to monitor the situation and provide updates as conditions warrant.

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