Small earthquake swarm at Mount St. Helens March 11, 2019

On March 11, around 7:16 pm local time, a magnitude 1.1 (M 1.1) earthquake occurred at Mount St. Helens (MSH), followed by about 40 smaller earthquakes over the next 25 minutes. Earthquake depths ranged from 0.8 to 1.7 km (0.5 to 1.0 miles) below sea level. Seismicity rates were back to normal within a half-hour.

The M 1.1 event had complex waveforms that could indicate small-scale underground movement of hydrothermal fluids or gas. In contrast, waveforms for the smaller earthquakes only had rock-breaking signatures. Events similar to the M 1.1 have been observed before at MSH, most recently in June 2017. As with prior similar events, there was no evidence of any gas or steam emission at the surface on March 11.

One very important piece of evidence supporting the no-emission interpretation came from an infrasound array installed in 2018 inside the crater of Mount St. Helens. "Infrasound" measures sound waves that are so low-frequency (< 20 Hz) that the human ear can't hear them. At volcanoes, infrasound can be created by rockfalls, avalanches, lahars, and explosions. The purpose of the new infrasound array is to enable CVO scientists to listen to sounds created within the crater of Mount St. Helens in real-time.

The March 11 event was the first real test of this new infrasound array, and the instruments performed well. And they detected… nothing. The absence of an infrasound signal gave CVO scientists a strong independent piece of evidence that no explosion had occurred in association with the March 11 event. Infrasound arrays are fairly new to the Cascades, and their importance in CVO's interpretation of the March 11 event is just one more example of how diverse instrumentation networks on volcanoes can enhance the monitoring capabilities of the USGS.