While southern California continues to rattle and shake from an earthquake swarm that’s produced more than 1,000 earthquakes over the last 7 days, a fresh swarm is now impacting Oregon’s largest volcano: Mount Hood. In the last week, 67 earthquakes have struck the southern flank of the stratovolcano. Of all of the earthquakes, the strongest was a 3.9 which struck shortly before 9 pm last night.
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 3.9 earthquake and the overall swarm, CVO wrote, “The swarm was preceded by several earthquakes in the hour prior to the M 3.9, and tens of aftershocks have occurred with event rates declining in a manner typical of mainshock-aftershock sequences. The mainshock characteristics and location are consistent with past swarms in the Mount Hood area, including a M 4.5 on June 29, 2002, that was located ~1 mile east of the M 3.9. Aftershocks will likely continue for hours or days, some of which may be felt.”
CVO also said, “At this time, seismologists at the Cascades Volcano Observatory and Pacific Northwest Seismic Network do not believe this swarm signifies a change in volcanic hazard at Mount Hood, but will continue to monitor the swarm and will issue further updates as the situation warrants.”
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.