Just months after a similar swarm impacted the volcano, a fresh swarm of earthquakes is rocking Mount Hood, Oregon’s largest volcano. In the last 30 days, 41 earthquakes struck around the slopes and summit of Mount Hood; in just the last 24 hours, 33 earthquakes occured. Most of the earthquakes in the last 24 hours have struck the southern flank of the stratovolcano. Of all of the earthquakes, the strongest was a 2.5 which struck earlier today.
The U.S. Geological Survey’s (USGS) Cascades Volcano Observatory (CVO) monitors volcanoes in Washington, Oregon, and Idaho. While they’ve classified Mount Hood as a “very high threat potential”, CVO has not changed the volcano alert level nor the aviation color code at the volcano. As such, the current volcano alert level is simply “NORMAL” and the aviation color code is “GREEN.”
In a statement about the earthquake swarm, USGS wrote, “Several tens of small earthquakes have occurred near the summit of Mount Hood. This is not a volcanic event. This earthquake swarm began late on Sunday October 17. So far, the largest magnitude was a M2.5; depths are 1-3 km below sea level (3-5 km below the surface). No earthquakes have been reported as felt. Swarms at this location and depth are common at Mount Hood, and do not indicate a change in the volcanic hazard. Past swarms have lasted several days to a couple of weeks.”
In their last regional update CVO also said, “All volcanoes in the Cascade Range of Oregon and Washington are at normal background levels of activity. These include Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams in Washington State; and Mount Hood, Mount Jefferson, Three Sisters, Newberry, and Crater Lake in Oregon.” They added that earthquakes located at Mount Rainier, Mount St. Helens, and Mount Hood over the past week are consistent with background levels of activity at each volcano.
An earthquake swarm, according to the USGS, is a sequence of mostly small earthquakes with no identifiable mainshock. Swarms are usually short-lived, but they can continue for days, weeks, or sometimes even months. They often recur at the same locations. Most swarms are associated with geothermal activity. Swarms are usually not tied to aftershocks. Aftershocks are a sequence of earthquakes that happen after a larger mainshock on a fault. Aftershocks occur near the fault zone where the mainshock rupture occurred and are part of the “readjustment process” after the main slip on the fault. Aftershocks become less frequent with time, although they can continue for days, weeks, months, or even years for a very large mainshock.
Mount Hood is Oregon’s largest volcano and the state’s tallest mountain. Formed by a subduction zone on the Pacific Coast in the Pacific Northwest, it’s located about 50 miles east-southeast of Portland. Mount Hood stands a tall 1,240 feet high; the high elevation is home to 12 named glaciers and snowfields. While USGS characterizes the volcano as “potentially active”, the mountain is considered informally dormant for now.