Small magnitude earthquakes detected 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.
Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fuji-san of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens was formed during nine eruptive periods beginning about 40-50,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to 2200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older St. Helens edifice, but few lava flows extended beyond the base of the volcano. The modern edifice was constructed during the last 2200 years, when the volcano produced basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century originated from the Goat Rocks area on the north flank, and were witnessed by early settlers.
March 23, 2016.Mount St Helens , United-Sates of America :
Sources : CVO, GVP.
Photo : Cocomagnanville