Report on St. Helens (United States) — February 1987
Scientific Event Alert Network Bulletin, vol. 12, no. 2 (February 1987)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Seismicity and deformation at background
Please cite this report as:
Global Volcanism Program, 1987. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198702-321050
St. Helens
United States
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Since the last dome-building event in October, seismicity and deformation have remained at background levels. Fourteen surface-type events (mostly rockfalls from the crater walls) were recorded in January, compared with nine in December. Three low-frequency earthquakes were recorded, up from one in December.
Geological Summary. Prior to 1980, Mount St. Helens was a conical 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 breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it 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. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.
Information Contacts: D. Swanson, CVO; C. Jonientz-Trisler, University of Washington.