Uptick in earthquake activity at Mount St. Helens remains within background levels (July 15, 2023-Present)
Mount St. Helens
Over the past three months, seismicity at Mount St. Helens has been elevated but remains within the normal range of background seismicity. Most of the earthquakes have been less than magnitude 1.0 and too small to be felt at the surface. No significant changes have been observed in other monitoring data and there are no signs of an imminent eruption.
At Mount St. Helens, over 400 earthquakes have been located by the Pacific Northwest Seismic Network since July 15, 2023. In late August to early September, about 40-50 earthquakes were located per week, and more recently, the number has been about 30 located earthquakes per week. To compare, since 2008, on average about 11 earthquakes have been located per month at Mount St. Helens.
The largest earthquake in this recent period was a magnitude 2.4 that occurred on August 27, 2023. Most of the events have been less than a magnitude 1.0. Earthquake depths are between 2 and 6 km (1.2 and 3.7 miles) below sea level, which is approximately 4 to 8 km (2.5 to 5 miles) below the crater floor.
No changes have been detected in ground deformation, volcanic gas or thermal emissions at Mount St. Helens. No changes have been observed at other Cascade Range volcanoes.
Short-term increases in earthquake rates are common at Mount St. Helens and are considered part of the background seismicity. The current seismicity represents 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 1980’s and 90’s directly led to eruptions.
Small magnitude earthquakes located beneath Mount St. Helens at depths well below sea level are generally thought to be associated with pressurization of the magma transport system. One cause for this pressurization is the arrival of additional magma, a process called recharge. Mount St. Helens is fed by magma that forms near the base of the crust at depths of about 25 km (~16 miles). Magma slowly rises through the lower crust and accumulates in a reservoir about 4‒10 km (~2.5‒6 miles) below sea level. Recharge events can occur when magma enters this upper reservoir, increasing stresses that lead to earthquakes.
High rates of seismicity, interpreted as recharge, have been observed in the past at Mount St. Helens and at other volcanoes and can continue for many years without an eruption.
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