Earthquakes occur in Mount Rainier's hydrothermal system.

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While the seismicity represents a temporary uptick in activity, Mount Rainier remains at normal, background levels of activity.

A minor earthquake swarm was detected beneath Mount Rainier on December 11, 2019. Located just south of the summit at depths of up to 1 mile, there were 10 located earthquakes in the swarm, the largest of which was a Magnitude 1.1. The small magnitude earthquakes were too weak to be felt at the surface.

There has been no detectable surface deformation or volcanic gas signal associated with this swarm, so while the seismicity represents a temporary uptick in activity, Mount Rainier remains at normal, background levels of activity.

Earthquake swarms are not uncommon at this Cascade Range volcano. A minor swarm of 8 located earthquakes occurred November 28-29, 2019, and 5 were located at Mount Rainier in late October 2019. Other swarms occurred in September 2018, September and October 2017, and April 2016.

Compared to earthquakes with a single large mainshock and many lesser aftershocks, a swarm is a sequence of many earthquakes that occur at above-normal rates and do not have an obvious mainshock (an earthquake that is much larger than the others). While researchers continue to investigate the cause of swarms, most often volcanic swarms are caused by fluids (dominantly water) interacting with faults.

As shown in the graphic, fluids from the magmatic system beneath the volcano rise through existing cracks and weaknesses in the crust. Along with rainwater and ice/snow melt, these fluids combine to create a hydrothermal system within the volcano. When pressurized fluids move along faults in the shallow subsurface, they sometimes generate small magnitude earthquakes and earthquake swarms.

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