A swarm of small earthquakes began under Mount Rainier’s summit early on July 8, with seismicity first detected at 01:29 LT (08:29 UTC). The Pacific Northwest Seismic Network (PNSN) and the USGS Cascades Volcano Observatory (CVO) have since recorded hundreds of earthquakes in the region.
The earthquakes are located at depths between 2 and 6 km (1.2–3.7 miles) beneath the summit. The largest earthquake thus far was M1.7 at 04:52 LT (11:52 UTC). The rate of activity peaked at several events per minute during the initial hours of the swarm but no earthquakes have been felt at the surface.
According to CVO, there is no indication of magmatic movement or surface deformation. No changes have been observed in infrasound signals or volcanic gas emissions. As a result, the Volcano Alert Level remains at Green (Normal), and the Aviation Color Code is also Green.
Hydrothermal activity, rather than magmatic intrusion, is believed to be the primary driver of the swarm. Such swarms occur once or twice annually beneath Mount Rainier.
On average, about 9 earthquakes per month are typically located within 5 km (3 miles) of the summit. This current swarm is one of the more significant in recent years but remains within historical patterns. The most comparable event occurred in 2009 when over 1 000 earthquakes were recorded within three days, with the largest reaching magnitude 2.3.
CVO and PNSN continue to monitor the situation with a full suite of ground-based seismometers, infrasound sensors, GPS stations, and webcams.
At this time, there is no evidence suggesting an increased likelihood of volcanic eruption.
