A swarm of small earthquakes started on Sunday night at Oregon’s Mount Hood and continued through Monday.
Rates have reached as high as 20 earthquakes per hour, peaking between 6 and 7 a.m. on Monday, according to the U.S. Geological Survey.
"Swarms are not uncommon in the Mount Hood area, which typically experiences one or two per year that last for several days to weeks," USGS Volcanoes wrote on Facebook.
Studies of past swarms in the Mount Hood area have concluded that they likely are occurring on pre-existing regional faults and are best thought of as regional tectonic earthquakes, rather than earthquakes directly linked to magmatic processes.
This is in contrast to the recent uptick in seismicity underneath Mount St. Helens, which USGS believes is caused by magmatic processes (specifically magma recharge).
Starting on March 14, a number of small earthquakes have occurred beneath the Mount St. Helens volcano, at a depth between 1.2 to 4 miles. The earthquakes have low magnitudes of 0.5 or less with the largest a 1.3.
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