As many in the Pacific Northwest can attest to, the winter of 2017 has been a rough one. Deep snow in the high country buried volcano monitoring sites and caused loss of telemetry and/or power. These problems reduced the Mount St. Helens seismic network, operated jointly by the Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismic Network (PNSN), to roughly half its normal operating capacity. The consistently bad weather prevented CVO and PNSN staff from performing any mid-winter repairs.
A clear weather day on April 21, permitted CVO and PNSN personnel to visit Mount St. Helens and restore the seismic network to nearly full capacity. Immediately after repairs were made, the PNSN began locating small earthquakes at relatively high rates (1 earthquake every few hours) under Mount St. Helens. The damage to seismic stations reduced the ability of the seismic network to locate small-magnitude earthquakes, at least somewhat.
Further analysis has revealed that many of the earthquakes look similar to each other, a common feature of swarms at Mount St. Helens and a sign that the events are occurring in close proximity. Using data from stations operable all winter, CVO scientists used the repeating characteristic of the earthquakes located since April 21 to track down when the swarm started. The result? There is good evidence that the uptick began as early as April 16 and definitely was occurring as of April 18.
As of May 5, the PNSN has located 47 earthquakes near Mount St. Helens since the seismic network was restored on April 21. Utilizing the similarity of earthquakes, we can detect well over 100 earthquakes that are part of this swarm. Most earthquakes have depths between sea level and 3 mi (5 km) below sea level (approximately 2-7 km below the surface). This is consistent with depths of earthquakes occurring since 2008, which are thought to be in response to recharge in the magmatic system. Earthquake rates, though relatively high compared to background, are still only 1 earthquake every few hours, a rate that is consistent with past small swarms since 2008. All earthquakes are volcano tectonic in character (no detected low-frequency or long period earthquakes) and the maximum magnitude thus far is a M1.3. There is no detectable deformation or gas signal associated with this swarm.
Similar swarms occurred at Mount St. Helens in March-May 2016 and November 2016. Both swarms had repeating earthquakes, average rates of 1-2 earthquakes/hour, and most earthquakes with magnitude below M1.5.
The similarity of swarms at Mount St. Helens leads us to believe that similar processes cause them, and they are likely tied to magma recharge first detected in 2008. However, pinpointing that exact process is difficult. Some possible mechanisms include a spontaneous release of brine from the pressurized magma chamber into the crust above, a pulse of magma into the magma reservoir that transferred stress into the crust above, or just the breakage of a new pathway of fluid flow that was previously blocked by precipitated minerals. There are several reasons why it is very unlikely that this swarm is a precursor to imminent eruptive activity at Mount St. Helens—it is similar to ones in the past that did not lead to surface activity; it consists of very small earthquakes occurring at relatively low rates; there are no other geophysical indicators (deformation, tilt, gas) of unrest.
Mount Rainier National Park is a unique classroom, rich in resources for observing geologic change. Join us July 17–21, 2017, for a 5-day educator workshop in the Park, where we will explore the diverse and dynamic processes that have shaped the volcano and share new classroom ideas that will engage middle school students. There is a $20 fee for this workshop and free camping is available to participants. Registration information is at the Mount Rainier Teacher Professional Development webpage.
Four swarms of small magnitude earthquakes were detected beneath Mount St. Helens beginning November 21, 2016. No anomalous gases or increases in ground inflation have been detected and there are no signs of an imminent eruption.
During a week's time, there have been over 120 tiny earthquakes, most too small to be formally located by the Pacific Northwest Seismic Network. The earthquakes are magnitudes 0.3 or less; the largest has been a magnitude 0.5. Most of the earthquakes are occurring in the shallow volcano plumbing system about 1-2 miles below sea level. These earthquakes are too small to be felt at the surface.
The current pattern of seismicity is similar to swarms most recently seen at Mount St. Helens in March-May 2016, and in 2014 and 2013. The magmatic system is likely imparting its own stresses on the crust around and above it, as the system slowly recharges. The stresses drive fluids through cracks, producing the small quakes. Subtle evidence of recharge has been observed since 2008 and can continue for many years. It is a sign that Mount St. Helens remains an active volcano.
For more information, see the Activity Updates for Volcanoes in CVO Area of Responsibility and Earthquake Monitoring at Mount St. Helens.
The western Columbia Gorge has been long recognized as an area susceptible to landslides. Abundant rainfall, steep terrain, geologic structure and erosion by the Columbia River combine to create topography capable of ground movement. Yet dense forests have hampered efforts to accurately map old and currently active landslides and to fully understand the scope of this hazard.
A new study uses lidar to map and characterize known and previously unrecognized landslides in the western Columbia Gorge, Washington. Lidar is a remote-sensing technique that provides images of terrain from which vegetation and structures can be digitally "erased" to show the underlying bare ground. Formerly hidden by forest, lidar reveals telltale landslide indicators such as scarps, cracks and ridges, slope depressions, bulges and toes.
The imagery shows that landslides cover about two thirds of the 222 square km (86 square mi) map area. Two of the largest landslides in the map area—the Bonneville and Red Bluffs landslides, averaging about 75 m (250 ft) thick with runouts of 6-7 km (~4 mi)—failed catastrophically and slid rapidly to the river within the last 600 years; the Bonneville landslide temporarily dammed the Columbia River and formed the "Bridge of the Gods" known from Native American legends.
Research shows that these landslides have complex movement histories and have been active over thousands of years; some have moved recently or are currently moving. Another such landslide rapidly sliding into the Columbia River today could have a catastrophic impact on downstream communities and on the transportation and energy-distribution infrastructure of the Pacific Northwest.
The publication, Landslides in the western Columbia Gorge, Skamania County, Washington, is available online.