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October 30, 2023

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. 



Mount St Helens late-2023 earthquake heat map.
Comparison of current seismicity to previous seismicity.  Upper left: Map of Mount St. Helens with a grayscale representing a digital elevation model.  Earthquakes interpreted as recharge between 1987 and 2004 are plotted as a heatmap of earthquake density.  Areas with a lot of earthquakes are in blue, while areas with fewer earthquakes are shown in oranges and browns. The earthquakes associated with the current episode are shown in orange.  Bottom left: Cross-section of the volcano looking north.  The topographic outline is shown as a black line at the top of the plot. Top right: Cross-section of the volcano looking west.  Other symbology is the same as other plots.
Mount St Helens time vs depth plot 1988-2023
Summary of earthquakes at Mount St. Helens during non-eruptive periods.  Top: Events per week (upper graphic), earthquake depths between 1987 and Sept. 2004 (lower graphic), and histogram (on right) showing earthquake depths.  Earthquake circles are sized according to magnitude. Previous time periods of high earthquakes rates are denoted as light red boxes.  These time periods have been previously interpreted as recharge. Bottom: Events per week (upper graphic), earthquake depths between 2008 and the present (Oct 2023) (lower graphic), and histogram (on right). Symbols and lines are the same as above. Orange dots and areas denote the current activity.

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