Mount St. Helens seismicity elevated but within the range of background levels (February to June 2024)
Mount St. Helens
Over the past several months, small magnitude earthquakes have been located at Mount St. Helens. The seismicity is similar to what was observed at this volcano from July to December 2023. No significant changes have been observed in other monitoring parameters and there is no change in alert levels at this time. Mount St. Helens remains at normal, background levels of activity.
Since February 1, 2024, approximately 350 earthquakes have been located 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 located per week reached a peak in early June, at 38 events per week. The largest earthquake has been a magnitude 2.0 that occurred on May 31, 2024. Earthquakes are occurring at a median depth of about 3.5 miles (5.7 km) below sea level, which is approximately 4.6 miles (7.4 km) below the crater floor.
Short-term increases in earthquake rates are common at Mount St. Helens and are considered part of background seismicity. 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 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 16 miles (25 km). Magma slowly rises through the lower crust and accumulates in a reservoir about 2.5 to 6 miles (4‒10 km) below sea level. Recharge events can occur when magma enters this upper reservoir and increases 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. There have been no significant changes in other monitoring parameters (ground deformation, volcanic gas or thermal emissions) and no change in hazards at Mount St. Helens as a result of this activity.
The USGS Cascades Volcano Observatory jointly operates the monitoring network at Mount St. Helens with the Pacific Northwest Seismic Network. 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.
From February 1 to June 17, 2024, approximately 350 earthquakes were located at Mount St. Helens by the Pacific Northwest Seismic Network. Over 95% of the earthquakes were less than a magnitude 1.0 and too small to be felt at the surface. The number of earthquakes located per week appears to have reached a peak in early June, at 38 events per week. USGS graphic. Data courtesy of the Pacific Northwest Seismic Network. Background image of Mount St. Helens from the North Fork Toutle River taken June 16, 2024, by L. Westby.
Earthquakes located at Mount St. Helens from 2008-2024, a non-eruptive period. This activity is consistent with normal, background levels. Top: Earthquake events located per week. The orange color at the far right denotes earthquakes from February to June 2024. Bottom: Earthquake depths below sea level (bsl) in kilometers. The orange-filled circles denote seismicity from February to June 2024. Larger circles represent higher magnitudes. USGS graphic. Data courtesy of the Pacific Northwest Seismic Network.
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Comparison of February-June 2024 seismicity to previous seismic swarms (1987-2004). 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 more earthquakes are in blue (deeper hues) while areas with fewer earthquakes are shown in oranges and browns (lighter hues). The earthquakes associated with the current episode (February-June 2024) are shown as orange-filled circles. Larger circles represent higher magnitudes. 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. USGS graphic. Data courtesy of the Pacific Northwest Seismic Network. |
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