In the early morning hours of September 23, 2004, a swarm of small-magnitude earthquakes about half a mile below Earth's surface marked the reawakening of Mount St. Helens after 18 years of eruptive quiescence. Steam and ash explosions on October 1 were followed by three years of lava extrusion that formed a new dome inside the crater. The lava dome pushed Crater Glacier aside, causing it to flow rapidly toward the front of the 1980 breach; flow continues today.
Scientists at the USGS-Cascades Volcano Observatory and its partners used many techniques during the 2004-2008 eruption to monitor the volcano, including interpretation of seismicity, ground deformation, thermal imaging, time lapse photography and lava sampling. Because of its location, easy access, and varied styles of eruptions, Mount St. Helens has become our 'go-to' volcano for development and testing of monitoring devices and techniques. Lessons learned at Mount St. Helens have been shared with researchers around the world to better understand volcano behavior, assess hazards and potential impacts, and provide timely warnings of future events.Read more in USGS Professional Paper 1750, chronicling the renewed eruption of Mount St. Helens (2004-2006). Follow the link for a brief recap of 2004-2008 events.
Every year, Mount St. Helens gets an average of 162 inches of rain and about 40 feet of snow. So where does all the water go? Some of it enters rivers and streams that originate on the volcano and some of it enters the groundwater system. USGS researchers used the Earth's naturally-occurring electromagnetic energy, specialized instrumentation and data processing techniques to find groundwater at Mount St. Helens, learning more about where it flows.
Using a geophysical method called Controlled-Source Audio-Magnetotellurics, researchers located two aquifers at Mount St. Helens. There is a deep conductor (the top of which is at a depth of about 1600 ft), interpreted as the regional aquifer that mounds up beneath the edifice. There is also an overlying separate, relatively thin but much more conductive body at a depth of about 100-160 ft that appears to be recharged by meltwater from Crater Glacier and heated by hot rock beneath the 2004-08 dome. The higher ionic content of the shallow aquifer may come from dissolved volatiles from the cooling dome and hydrothermal alteration. The CSAMT soundings extending north of the volcanic to the upper North Fork Toutle River show that these aquifers remain separate but eventually merge farther to the northwest into a single conductor (or groundwater system).
Knowing more about groundwater at Mount St. Helens is useful in understanding the hydrologic system and in interpreting seismicity. This research develops a baseline for Mount St. Helens that can be compared to future years to understand more about the evolving groundwater system and its effect on the volcano.Read more at Where is the Hot Rock, and Where is the Groundwater—Using CSAMT to Map Beneath and Around Mount St. Helens.
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.