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.
Since 1980, there have been 120 eruptions and 52 episodes of notable volcanic unrest at 44 U.S. volcanoes. When erupting, all volcanoes pose a degree of risk to people and infrastructure. However, the risks are not equivalent from one volcano to another because of differences in eruptive style and geographic location.
The USGS assesses active and potentially active volcanoes in the U.S., focusing on history, hazards and the exposure of people, property and infrastructure to harm during the next eruption. The assessment uses 24 factors to obtain a score and threat ranking. The findings are in the newly published 2018 Update to the U.S. Geological Survey National Volcanic Threat Assessment.
Eleven of the eighteen very high threat volcanoes are in Washington, Oregon, or California, where explosive and often snow- and ice-covered volcanoes can project ash or lahar (debris flow) hazards long distances to densely populated and highly developed areas. These include Mount St. Helens, Mount Rainier, Mount Hood, Three Sisters, Newberry, Mount Baker, Glacier Peak and Crater Lake (in Washington and Oregon), and Mount Shasta, Lassen and Long Valley (in California). The threat ranking is not a list of which volcano will erupt next. Rather, it indicates how severe the impacts might be from future eruptions at any given volcano.
The volcanic threat assessment helps prioritize U.S. volcanoes for research, hazard assessment, emergency planning, and volcano monitoring. It is a way to help focus attention and resources where they can be most effective, guiding the decision-making process on where to build or strengthen volcano monitoring networks and where more work is needed on emergency preparedness and response.
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