Over the past several months, small magnitude earthquakes have been occuring at Mount St. Helens, the activity is similar to 2023 observations.
The Cascades Volcano Observatory stated in a press release that no significant changes have been observed in other monitoring parameters, and there is no change in alert levels at this time as Mount St. Helens remains at normal, background levels of activity.
Since Feb. 1 to June 17, 2024, approximately 350 earthquakes have been detected at Mount St. Helens by the Pacific Northwest Seismic Network. Over 95% of the earthquakes have been less than magnitude 1.0 and are too small to be felt at the surface.
The number of earthquakes detected per week reached a peak earlier this month, at 38 quakes per week, the release stated. The largest earthquake in the time frame was a magnitude 2.0 that occurred on May 31. Earthquakes are occurring at a median depth of about 3.5 miles (5.7 kilometers) below sea level, which is approximately 4.6 miles (7.4 kilometers) below the crater floor, the release added.
“Short-term increases in earthquake rates are common at Mount St. Helens and are considered part of background seismicity,” the Cascades Volcano Observatory stated in the release. “These last two periods of elevated seismicity (in 2023 and 2024) represent the largest short-term increase in earthquake rates since the last eruption ended in 2008. However, longer duration sequences with more events occurred in 1988-1992, 1995-1996 and 1997-1999.”
None of the sequences in the 1980s and 1990s directly led to eruptions.
Small magnitude earthquakes located beneath Mount St. Helens at depths well-below sea level are associated with the pressurization of the magma transport system. One cause for this pressurization is the arrival of additional magma, a process called recharge, the release stated. Mount St. Helens is fed by magma that forms near the base of the crust at depths of about 16 miles (25 kilometers). Magma slowly rises through the lower crust and accumulates in a reservoir about 2.5 to 6 miles (4 to 10 kilometers) below sea level. Recharge events can occur when magma enters this upper reservoir and increases stresses, leading to earthquakes, the release stated.
High rates of seismicity, interpreted as recharge, have been observed in the past at Mount St. Helens, and other volcanoes are known to continue for many years without an eruption. There have been no significant changes in other monitoring parameters such as ground deformation, volcanic gas or thermal emissions, and no change in hazards at Mount St. Helens as a result of this activity, the Cascades Volcano Observatory stated.
The United States Geological Survey’s Cascades Volcano Observatory jointly operates the monitoring network at Mount St. Helens with the Pacific Northwest Seismic Network, the release stated. The USGS and PNSN will continue to monitor the volcano closely and issue notifications as warranted. No changes have been observed at other Cascade Range volcanoes, the release added.
