Alert level for Mount St. Helens remains stable despite recent reports of “recharge”

The reflector

Mount St. Helens has experienced smaller earthquakes in recent months, with activity similar to that observed in 2023.

The Cascades Volcano Observatory stated in a press release that no significant changes were observed in other monitoring parameters and that there is currently no change in alert levels as Mount St. Helens activity remains at normal background levels.

From February 1 to June 17, 2024, about 350 earthquakes were recorded at Mount St. Helens by the Pacific Northwest Seismic Network. Over 95% of the earthquakes were below magnitude 1.0 and are too weak to be felt on the surface.

The number of earthquakes recorded per week peaked at 38 per week earlier this month, the press release said. The strongest earthquake during this period had a magnitude of 2.0 and occurred on May 31. The earthquakes occur at an average depth of about 5.7 kilometers below sea level, or about 7.4 kilometers below the crater floor, the press release said.

“Short-term increases in earthquake rates are common at Mount St. Helens and are considered part of the background seismicity,” the Cascades Volcano Observatory explained in the press release. “These last two periods of increased seismicity (2023 and 2024) represent the largest short-term increases in earthquake rates since the end of the last eruption in 2008. However, longer sequences with more events occurred in 1988-1992, 1995-1996, and 1997-1999.”

None of the sequences in the 1980s and 1990s led directly to outbreaks.

Low-magnitude earthquakes beneath Mount St. Helens at depths well below sea level are associated with pressurization of the magma transport system. One cause of this pressurization is the arrival of additional magma, a process called recharge, the release said. Mount St. Helens is fed by magma that forms near the crustal base at a depth of about 25 kilometers. Magma slowly rises through the lower crust and collects in a reservoir about 4 to 10 kilometers below sea level. Recharge events can occur when magma intrudes into this upper reservoir and increases stress, causing earthquakes, the release said.

Mount St. Helens has historically experienced high rates of seismic activity interpreted as new formation, and other volcanoes are known to go many years without erupting. There have been no significant changes in other monitoring parameters such as ground deformation, volcanic gas or heat emissions, and no change in hazards at Mount St. Helens as a result of this activity, the Cascades Volcano Observatory said.

The United States Geological Survey’s Cascades Volcano Observatory operates the monitoring network at Mount St. Helens in conjunction with the Pacific Northwest Seismic Network, the press release said. The USGS and PNSN will continue to monitor the volcano closely and issue alerts as needed. No changes have been observed at other volcanoes in the Cascade Range, the press release said.