The level of earthquake activity at Mount St. Helens has been gradually increasing over the past several months and accelerated during May. Rates of activity have gone from an average of ~60 well located events per month last winter to 165 events in May. Most of these recent earthquakes are very small with only 3 events larger than magnitude 2. The largest earthquake was on May 1 with a magnitude of 2.2. These earthquakes are occurring in two clusters directly beneath the lava dome in the crater. One cluster is in the depth range of 2 to 5 km below the dome and the other is in the depth range of 7 to 9 km. Almost no events have been located in the very shallow region of 0-2 km below the dome. None of these earthquakes are low-frequency, volcanic events that typically occur as precursors to major eruptions.
This increased earthquake activity seems to be similar to that which occurred in 1995, although the current activity is now more energetic. The 1995 activity lasted for several months in the summer and fall, had a maximum earthquake rate of 95 events per month, and resulted in no volcanic activity. A similar increase in earthquake activity in the Mount St Helens system occurred in 1989-91. However, at that time there were also a number of very shallow earthquakes accompanied by a series of sudden steam explosions. These explosions were small eruptions of steam and gas that ejected rocks and dust (ash) from cracks in the dome. Rocks were thrown up to 1 km (1/2 mile) from the dome and ash clouds reached altitudes up to 6 km (20,000 feet) and locally deposited a dusting of ash downwind in a few cases. Some explosions melted snow in the crater and generated small lahars that flowed onto the Pumice Plain.
Because increased earthquake activity within the deep St. Helens system may reflect increased pressure at depth, it is possible that the current earthquake activity may eventually lead to renewed volcanic activity. However, it is unlikely to do so without significant additional precursory activity.
We continue to monitor the situation closely and will issue additional information statements should they be warranted.
Maps and temporal plots of the Mount St. Helens activity may be found on the WORLD WIDE WEB at URL:
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