An earthquake swarm began near Mount Hood Monday night ~7 km (4.3 miles) south-southeast of the summit in the White River Valley, an area that commonly exhibits seismicity. It started October 9 at 19:51 UTC (12:51 PDT local) with the largest earthquake a M2.8 at 2:45 UTC on October 10 (1945 PDT on October 9). 14 earthquakes have been located by the Pacific Northwest Seismic Network (PNSN) through October 11, all with depths of 7-8km (4.3 - 5 miles) below sea level.
Such swarms are common in the vicinity of Mount Hood, with the last swarm occurring in May 2016 (largest event M 2.9). Summaries of past swarms near Mount Hood can be found on the PNSN website and on the Mount Hood monitoring webpage.
Over the last week, more than 20 earthquakes have been located by the Pacific Northwest Seismic Network at Mount Rainier. In a typical week, Mount Rainier experiences about two "located" earthquakes (to be locatable, earthquakes need to be large enough to be well-recorded on a sufficient number of seismic stations (at least four)), so this represents a modest increase over background rates.
Swarms are a common and expectable occurrence at active volcanoes such as Mount Rainier. While interesting, most never result in surface changes. The most likely scenario for the current swarm is that elevated earthquake rates will continue for a few days before slowly decaying to background rates of seismicity.
The seismicity uptick started late September 11 with a swarm of five earthquakes located 1-2 km (0.5 to 1.5 mi) to the southeast of the summit area. These earthquakes were shallow (up to 2 km (~1.5 mi) above sea level), small (maximum magnitude was a M1.6), and in an area that has not had a lot of previously recorded seismicity. Beginning September 13, earthquakes were also detected about 1 km (~0.5 mi) to the northeast and southeast of the summit, in areas where earthquakes typically occur at Mount Rainier. Depths for these events were 1-2 km (~0.5 to 1.5 mi) below sea level, which is also typical for background seismicity at the volcano, and event magnitudes were small (maximum magnitude 1.2).
In total there have been 23 earthquakes since September 11, with event rates of up to eight located earthquakes per day. Although this is higher than the normal seismicity rates at Mount Rainier, it is not unprecedented. Over the past 10 years there have been three previous periods with similar or higher event rates (September 2009, April 2015 and May 2016). Current event rates are similar to those seen in April 2015 and May 2016, but are far smaller in rate, energy release, and total number of earthquakes than what was seen in September 2009, when a swarm featuring hundreds of located earthquakes occurred over a three-day period. For the current swarm, earthquake location, depth, and size are all consistent with background seismicity; the only thing that is different is the event rate.
These earthquakes are inferred by scientists to be caused by processes occurring in Mount Rainer's hydrothermal system. The hydrothermal system is the region beneath the volcano containing hot mineral-rich water; one manifestation of this system is the boiling-point fumaroles that are found at the volcano's summit. Similar to pipes in geothermal plants, cracks transporting water away from a hot source may seal shut as the water cools and loses its dissolved minerals. Earthquakes are created when sufficient fluid pressure builds behind these seals to fracture them.
Each swarm provides seismologists with more perspective on the personality of the volcano and aids in our ability to assess hazards in the future. As always, scientists at the USGS Cascades Volcano Observatory and Pacific Northwest Seismic Network will continue to monitor the situation and provide updates as conditions warrant.
Use this link to learn more about Monitoring Mount Rainier.
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.