Seismologists deploy largest number of seismometers ever at Mount St. Helens—a dense array designed to detect tiny quakes.

Between August 19 and 22, seismologists with the USGS-Cascades Volcano Observatory, University of New Mexico, University of Oregon, University of Wisconsin-Madison, Cornell and Northwestern worked together to install 140 temporary seismometers at Mount St. Helens. Instruments were placed on top of the lava dome that erupted in 2004-2008, as well as the 1980-86 lava dome, the 1980 crater floor, and around the volcanic cone. The goal of the month-long deployment is to capture small magnitude volcanic earthquakes and learn more about the shallow plumbing system beneath the crater floor.

The project represents the largest number of seismometers ever placed on a U.S. volcano. The seismometers, which resemble an insulated big-mouth thermos with spikes on the bottom, weigh only six pounds, are self-contained, and are easy to deploy. The seismometers store data on a small internal computer and have enough battery power to operate for about one month.

"The goals of the project are to more precisely locate and characterize the small-magnitude volcanic earthquakes that routinely occur at St. Helens. We also want to be able to more reliably discriminate volcanic earthquakes from rockfalls off the crater wall, which have a similar seismic signature in many cases," said Wes Thelen, a seismologist with the USGS-Cascades Volcano Observatory. "Once we collect the data and have a better idea about what is occurring in the shallow subsurface, we will be able to compare those signals with signals recorded on our permanent network stations to recognize and identify earthquake sources when they happen again."

The data will augment the results of the recently completed iMUSH (imaging Magma Under St. Helens) experiment. The equipment used in iMUSH looked "deeper" than a mile (2 km), whereas this project looks at shallow earthquakes that occur between the surface and 2 km. A number of earthquakes and rockfalls have already occurred since the instruments were deployed.

Mount St. Helens is the most seismically active volcano in the Washington and Oregon Cascades. In an average month about 20 events are located by the Pacific Northwest Seismic Network, with the number going far higher during eruptive periods. Seismologists have also tracked several shallow earthquake swarms at Mount St. Helens since the eruption ended in 2008, most recently in May of 2017. Generally, swarms consist of tens to hundreds of earthquakes with magnitudes less than M1.5 and depths between 1 and 4 miles (2 to 7 kilometers) below the surface. These swarms are believed to be associated with the ongoing magma recharge of the plumbing system beneath Mount St. Helens, but are not an indication that an eruption is imminent.

Crews will return to Mount St. Helens on September 20-22 to retrieve the equipment and collect the data. Use this link to learn more about Monitoring Instruments and Data at Mount St. Helens.