Swarm of dozens of earthquakes recorded under Mt. Hood

Swarm of dozens of earthquakes recorded under Mt. Hood

Mt. Hood. Image credit: Richard Stovall via USGS Volcanoes

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.

The Pacific Northwest Seismic Network has so far located nearly 40 earthquakes; many more events have occurred that are too small to be located.

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"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.

Here's three things to know about the swarm:
  • USGS says the swarms on Monday were located two to three miles south of the summit of Mount Hood at depths of two to three miles below sea level. The large event was a magnitude 1.8.
  • Monday’s swarm is very typical for Mount Hood, because it is located several miles away from the summit vent – it is rare to see swarms occur directly beneath the summit. 
  • The most energetic swarm recorded to date occurred in June and July of 2002, which featured a magnitude 4.5 that was broadly felt throughout the Government Camp area.

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.

Mount St. Helens’ catastrophic eruption happened 36 years ago this week (May 18, 1980). Hood's last eruption was in 1907.