An increase in seismic activity beneath Mount St. Helens that started on July 15, 2023 continued this week, reports the Cascades Volcano Observatory. Seismologists were able to locate nineteen earthquakes this past week a few miles below the volcano’s summit bringing the total for this earthquake swarm to 465. All of these earthquakes have been too small to be felt by people on the surface.
Earthquakes are common at active volcanoes as the magma that feeds them moves underground causing small disruptions in the surrounding rock. Earthquake swarms that don’t lead to an eruption are also common. The observatory noted in a recent article that Mount St. Helens experienced numerous long-lasting swarms in the late 1980s and 1990s that weren’t directly associated with volcanic eruptions.
Since Mount St. Helens’ most recent eruption ended in 2008 the volcano has averaged eleven earthquakes per month. The ongoing swarm has so far peaked around 40-50 earthquakes that were able to be located weekly in August and September but the number has decreased since then. Despite this being above average, it is within what the volcano observatory terms “background levels.”
Other Western U.S. volcanoes have experienced earthquake swarms without incident in recent years. I reported on one at Mount Hood in 2021 and one in the Yellowstone area in 2017 was briefly a hot topic for tabloids and websites that rely on clickbait-type headlines.
Volcanologists also consider ground deformation, the amount and chemical content of volcanic gas emitted by the volcano, and thermal emissions to determine the risk of eruption. All of these perameters remain unchanged at Mount St. Helens since the start of the swarm in July. A notable recent example of an eruption that included these changes was the Kīlauea eruption in 2018 leading to the evacuation of nearby residents and later distruction of some of their homes.
Mount St. Helens is likely to erupt again eventually. Hazards such as pyroclastic flows, deep ashfall, and large rocks falling are limited to within a few miles of the volcano itself. Lahars, which are rapid mudflows resulting from volcanic debris mixing with melted glacial ice, may occur on the Toutle and Kalama Rivers depending on how violent the eruption is and how much ice is present.
Some hazards are present even without a volcanic eruption. Earthquakes, of course, are one of these. Debris flows and small lahars can also occur without an eruption. Small debris flows triggered by heavy rain were observed around Mount St. Helens over the past week. One in May cut off access to significant portions of the national monument surrounding the volcano, including the Johnston Ridge Observatory.
Volcanism in the Cascades is produced by the Juan de Fuca Plate offshore being forced, or subducted, under the North American Plate. As the water-laden rock from below the Pacific Ocean sinks beneath North America, it lowers the melting point of rock at the bottom of the continent causing it to melt and then rise toward the surface. Mount St. Helens is the most recent Cascade volcano to erupt. The second most recent is California’s Lassen Peak which experienced an eruptive period from 1914 to 1921.
The Cascades Volcano Observatory in Vancouver, Washington monitors volcanoes in Washington, Oregon, and Idaho and publishes weekly updates on activity in the region. The California Volcano Observatory keeps tabs on volcanic hazards in California and Nevada with monthly updates. In total, the USGS operates five volcano observatories. With the amount of monitoring equipment on the Cascade volcanoes, it is extremely unlikely that an eruption will occur without warning.
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