A minor earthquake swarm at Mount Hood began on December 8, 2023. As of December 13, the Pacific Northwest Seismic Network located 19 earthquakes:

• The earthquakes are clustered to the southwest of Mount Hood’s summit at depths between 3.4 and 4.6 miles (5.5 and 7.5 km) below sea level.
• The earthquake magnitudes are small. The maximum magnitude was a M1.5, which occurred on December 8. The earthquakes are too small to be felt at the surface.
• Three small earthquakes (the largest a M1.5) have been located near the summit of the volcano, a common observation for flank swarms at Mount Hood.
• The earthquakes occurred in short bursts, with a few per minute for brief time periods, followed by a lull in activity that lasted for many hours.

Small earthquake swarms in this area suggest movement along an existing fault. At Mount Hood, an active zone of normal faults extends south and north of the volcano, as described by the Oregon Department of Geology & Mineral Industries in the USGS Field Trip Guide to Mount Hood. Previous swarms of earthquakes at Mount Hood are similar to this one, are characterized by normal faulting, and have orientations that roughly parallel the regional fault zone.

Monitoring stations installed at Mount Hood in 2020 help geophysicists characterize these earthquakes. The monitoring stations have broadband seismometers (in addition to other sensors) that can detect tiny earthquakes smaller than magnitude 1.0. Adding three new stations creates a more robust network so geophysicists can extract more information about each earthquake.

For example, the new stations, and improvements to existing stations, allow more polarity observations for smaller earthquakes. This refers to the pattern scientists observe during the arrival of seismic waves at a station. Polarity (up or down motion) provides clues about the faulting motions that produced the earthquake, called a focal mechanism. These data help the USGS understand the current swarm in the context of past activity and are one of the main lines of evidence for characterizing this recent flank activity as movement along a tectonic fault.

USGS geophysicists do not believe the earthquakes are precursors to future volcanism at Mount Hood but will continue to monitor the volcano and provide updates as needed. There is no change in alert level or color code at Mount Hood. The volcano remains at normal, background level of activity.