CVO issued a Notice of Volcanic Unrest for Mount St. Helens on 26 September. They reported that increased activity started when a swarm of very small, shallow earthquakes (less than M 1) began on the morning of 23 September. The swarm peaked by about mid-day on 24 September and slowly declined through the morning of the 25th. The character of the swarm then changed to include more than 10 larger earthquakes (M 2-2.8), the most in a 24-hour period since the last eruption in October 1986. In addition, the character of some of the earthquakes suggested the involvement of pressurized fluids (water and steam) or magma. The events continued through 27 September at shallow depths (less than 1.6 km) below the lava dome that formed in the crater between 1980 and 1986.

As of the 27th, seismicity had slowly increased throughout the day and the largest earthquake recorded was about M 1.5. CVO crews installed global positioning system (GPS) equipment to monitor any ground movement on the lava dome, the crater floor, and the volcano's slopes. Preliminary results from a gas flight on the 27th revealed that no magmatic gas was recorded around the lava dome.

Geological Summary. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fujisan 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 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of 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.

Source: US Geological Survey Cascades Volcano Observatory (CVO)