Prior to the 1980 eruption, Mount St. Helens formed a conical, youthful volcano sometimes called the Fuji-san of America. During the 1980 eruption the upper 400 m of the summit was removed by collapse of the slope, that left a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens is a very young volcano and only about 40-50,000 years old.
It has been the most active volcano in the Cascade Range during the past 10,000 years, and the modern edifice was constructed during the last 2,200 years, when the volcano produced basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century were witnessed by early settlers.

Mt. St. Helens volcano (Washington): small earthquake swarms no signs of worries

Tue, 10 May 2016, 11:03

11:03 AM | BY: T

Earthquakes under Mount St. Helens volcano during 1988-2016; magmatic recharge swarms are marked, along with the most recent earthquake swarm. (image: USGS / Cascade Volcano Observatory via Eruptions Blog)

Since the beginning of 2016, a swarm of small earthquakes has been occurring at under the volcano, suggesting that another phase of magma recharge is currently taking place.
The recorded quakes have been at shallow depths between 2-7 km under the summit and ranged in magnitudes from 0 (or less) to 3 on the Richter scale, i.e. they are all tiny. Only rarely, they exceeded magnitude 2 and only a very few might have been felt by persons being very close.
USGS volcanologists and seismologists interpret the vertical area where these quakes occurred as the area where a small amount of fresh magma has been rising into the volcano's plumbing system (a magma chamber). Magma rises, pressurizes surrounding rock to make space and causes fracturing = tiny quakes.
The current swarm is only one of a series of similar swarms that have taken place since 1988, including some much more energetic ones (such as during 1998-99). Most such earthquake swarms, not only at Mt. St. Helens, but at almost any volcano, are NOT followed by eruption, or at least not immediately. In the case of Mt. St. Helens, the large earthquake swarm during 1998-99 preceded the small effusive activity during 2004-2008 by 5 years.

For the time being, USGS keeps the volcano at alert level green (=normal); the observed seismic activity is too small to justify a raise in alert and is best seen as part of the normal activity during a dormant phase of the volcano. When the volcano will erupt again (it is almost certain that it will), is impossible to predict on the basis of these quakes alone. It might be years or even centuries.

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Mount St. Helens Is Recharging Its Magma Stores, Setting Off Earthquake Swarms (Eruptions Blog)

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Mt. St. Helens volcano (Washington): small earthquake swarms no signs of worries

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