May 18 marks the 36th anniversary of the catastrophic eruption of Mount St. Helens. The eruption remains a seminal historical event; studying it and its aftermath revolutionized the way scientists approach the field of volcanology. Not only was the eruption spectacular, it occurred in daytime, at an accessible volcano, in a country with the resources to transform disaster into scientific opportunity, amid a transformation in digital technology. Lives lost and the impact of the eruption on people and infrastructure downstream and downwind made it imperative for scientists to investigate events and work with communities to lessen losses from future eruptions. Follow the signs and symptoms of volcanic unrest that led to the May 18, 1980 eruption in the volcano activity update archives. Sign up with the Volcano Notification Service to receive news of current activity and join the discussion on USGS Volcanoes Facebook.
At Mount Hood, a swarm of small earthquakes was detected May 15-16, 2016. Studies of past swarms have concluded that they likely are occurring on pre-existing regional faults and are best thought of as "tectonic" earthquakes rather than earthquakes directly linked to magmatic processes.
The earthquakes in this swarm are located 2-3 miles south of the summit of Mount Hood at depths of 2-3 miles below sea level. The largest event was a magnitude 1.8. Earthquake rates reached as many as 20 earthquakes per hour, peaking between 6-7 am on May 16 before decreasing later in the day. The Pacific Northwest Seismic Network (PNSN) located nearly 60 earthquakes; many more events occurred that were too small to be located. This swarm is very typical for Mount Hood because it is located several miles away from the summit vent – it is rare to see swarms occur directly beneath the summit.
Swarms are not uncommon in the Mount Hood area, which typically experiences one or two swarms per year that last for several days to weeks. The most energetic swarm recorded to-date occurred in June-July of 2002, which included a magnitude 4.5 that was broadly felt in the Government Camp area. The current swarm is much, much smaller than the 2002 swarm, both in terms of earthquake size and in number. A paper published in 2005 by J. Jones and S.D. Malone studied Mount Hood swarms in great detail; read more in a PNSN 2012 blog.
Beginning March 14, 2016, a number of small magnitude earthquakes have occurred beneath Mount St. Helens, at a depth between 2 and 7 km (1.2 to 4 miles). Over the last 8 weeks, there have been over 130 earthquakes formally located by the Pacific Northwest Seismic Network and many more earthquakes too small to be located. The earthquakes have low magnitudes of 0.5 or less; the largest a magnitude 1.3. Earthquake rates have been steadily increasing since March, reaching nearly 40 located earthquakes per week. These earthquakes are too small to be felt at the surface.
The earthquakes are volcano-tectonic in nature, indicative of a slip on a small fault. Such events are commonly seen in active hydrothermal and magmatic systems. The magma chamber is likely imparting its own stresses on the crust around and above it, as the system slowly recharges. The stress drives fluids through cracks, producing the small quakes. The current pattern of seismicity is similar to swarms seen at Mount St. Helens in 2013 and 2014; recharge swarms in the 1990s had much higher earthquake rates and energy release.
No anomalous gases, increases in ground inflation or shallow seismicity have been detected with this swarm, and there are no signs of an imminent eruption. As was observed at Mount St. Helens between 1987-2004, recharge can continue for many years beneath a volcano without an eruption.For more information, see the Activity Updates for Volcanoes in CVO Area of Responsibility and Earthquake Monitoring at Mount St. Helens.
On March 14, 2016, the seismic network at Mount St. Helens began detecting small magnitude earthquakes at a depth of 3–4 km beneath the crater. Twelve earthquakes have been formally located and the local seismic network detected at least 100 earthquakes too small to be recorded on enough seismometers to calculate a location. Many of the earthquakes have similar seismic signatures, suggesting they are occurring in the same area as the located earthquakes. According to the Pacific Northwest Seismic Network, the largest earthquake over a four-day period was a Magnitude 0.7, an event that would not be felt even if you were standing on the surface above it.
These types of volcano-tectonic earthquakes beneath Mount St. Helens are likely associated with the recharge of the volcano. After the 2004-2008 eruption, subtle inflation of the ground surface and seismicity indicate that the magma reservoir beneath Mount St. Helens is slowly re-pressurizing, as it did after the conclusion of the 1980-1986 eruption. This is to be expected and it does not indicate that the volcano is likely to erupt anytime soon. Re-pressurization of a volcano's magma reservoir is commonly observed at volcanoes that erupted recently, and recharge can continue for many years without an eruption. For more information, see the Activity Updates for Volcanoes in CVO Area of Responsibility and Earthquake Monitoring at Mount St. Helens.
Join us July 25–29, 2016, for a 5-day educator workshop at Mount Rainier. The workshop features informative talks on Cascade volcanoes and volcanic processes, ideas for classroom activities, hikes into the field, and tips for organizing school field trips to visit the volcano. There is no charge for this workshop and camping is available to participants. Registration information is at the Mount Rainier Teacher Professional Development webpage.