MOUNT RAINIER, WA — A swarm of small earthquakes was detected beneath Mount Rainier early Tuesday morning, according to the U.S. Geological Survey’s Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismic Network (PNSN) at the University of Washington.

The swarm began at 1:29 a.m. PDT on July 8 and included several hundred small quakes, the largest measuring magnitude 1.7. Earthquakes have been located at depths ranging from approximately 1.2 to 3.7 miles (2 to 6 kilometers) below the surface.

While earthquake swarms are not unusual at Mount Rainier, CVO scientists note that the current activity involves more events than typically observed during these episodes. Most swarms occur once or twice per year with fewer quakes. The last notably large swarm occurred in 2009 and lasted three days, with events reaching up to magnitude 2.3.

Researchers believe such swarms are often caused by the movement of hydrothermal fluids interacting with existing faults beneath the volcano’s summit, rather than by magma movement.

Monitoring instruments, including GPS stations, have shown no signs of ground deformation. In addition, no other changes have been observed at the volcano.

Mount Rainier remains closely monitored by a network of seismic, infrasound, GPS, and webcam stations. CVO and PNSN officials say they will continue to track activity and issue updates if conditions change.