A minor earthquake swarm was detected beneath Mount Rainier on December 11, 2019. Located just south of the summit at depths of up to 1 mile, there were 10 located earthquakes in the swarm, the largest of which was a Magnitude 1.1. The small magnitude earthquakes were too weak to be felt at the surface.
There has been no detectable surface deformation or volcanic gas signal associated with this swarm, so while the seismicity represents a temporary uptick in activity, Mount Rainier remains at normal, background levels of activity.
Earthquake swarms are not uncommon at this Cascade Range volcano. A minor swarm of 8 located earthquakes occurred November 28-29, 2019, and 5 were located at Mount Rainier in late October 2019. Other swarms occurred in September 2018, September and October 2017, and April 2016.
Compared to earthquakes with a single large mainshock and many lesser aftershocks, a swarm is a sequence of many earthquakes that occur at above-normal rates and do not have an obvious mainshock (an earthquake that is much larger than the others). While researchers continue to investigate the cause of swarms, most often volcanic swarms are caused by fluids (dominantly water) interacting with faults.
As shown in the graphic, fluids from the magmatic system beneath the volcano rise through existing cracks and weaknesses in the crust. Along with rainwater and ice/snow melt, these fluids combine to create a hydrothermal system within the volcano. When pressurized fluids move along faults in the shallow subsurface, they sometimes generate small magnitude earthquakes and earthquake swarms.
Follow the link for Activity Updates for Cascade Range volcanoes, including Mount Rainier.
Exposures in valleys surrounding Mount St. Helens reveal records of diverse geologic processes including debris avalanche, lahar, huge water wave on a nearby lake, pyroclastic density currents (surge and flow), tephra fall, lava flow, growth of domes, and past glaciation. This new field guide provides detailed information for 28 self-guided field trip stops, many of which explore effects of the several catastrophes that constituted the May 18, 1980 eruption.
Follow the link to download Field trip guide to Mount St. Helens, Washington—Recent and ancient volcaniclastic processes and deposits.
Other Mount St. Helens field guides include:
Additional field-trip guides for selected volcanoes and volcanic landscapes of the western United States can be downloaded from the USGS Publications Warehouse.
On the afternoon of July 8, 2019 a swarm of small earthquakes started near Mount Hood, Oregon. As of 11:00 AM PDT on July 9, the Pacific Northwest Seismic Network has located more than 30 earthquakes, all occurring about 1 mile ENE of Government Camp and about 5 miles south of the Mount Hood summit. The earthquakes are relatively shallow (2-3 miles) and are likely too small (maximum magnitude 2.1) to be felt.
Swarms in this area have occurred multiple times over the past two decades, most recently in 2014, with the largest event being a M 2.9 on September 14, 2001. The largest event ever recorded near Mount Hood was a M 4.5 on June 29, 2002, at a location 3 miles south of the summit. Based on similarity to past seismic sequences near Mount Hood and on past studies of seismicity in the Mount Hood area, we infer that these earthquakes are occurring on tectonic faults and are not directly related to volcanic processes occurring beneath Mount Hood.
Over the past month, more than 70 small earthquakes occurred beneath Mount St. Helens, the largest of which was a Magnitude 1.4 on June 30, 2019. Ranging in depths from 1 to 5 miles (2 to 8 km) below sea level, the earthquakes are too small to be felt at the surface.
The current pattern of seismicity is similar to swarms detected at Mount St. Helens in March 2019, May 2017, November-December 2016, March-May 2016, and in 2014. The activity is likely the result of small-scale underground movements of hydrothermal fluids or gas — a sign that Mount St. Helens remains an active volcano.
There is no detectable surface deformation or volcanic gas signal associated with this swarm. While the swarm represents a temporary uptick in activity, Mount St. Helens remains at normal, background levels of activity.
For more information, see the Activity Updates for Volcanoes in CVO Area of Responsibility and Earthquake Monitoring at Mount St. Helens.
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.