Report on St. Helens (United States) — August 1986
Scientific Event Alert Network Bulletin, vol. 11, no. 8 (August 1986)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Earthquake swarm and small plume emission
Please cite this report as:
Global Volcanism Program, 1986. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 11:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198608-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Deformation, seismicity, and rates of SO2 emission remained at background levels through early September. Deformation of the lava dome did not exceed 1-3 mm/day and no significant changes were detected by tiltmeters. SO2 emission was measured at 55 plus or minus 15 t/d on 13 August and 35 plus or minus 5 t/d on the 19th, normal values for periods between extrusion episodes. Seismicity also remained at background, with an approximately equal number of low-frequency and medium- to high-frequency events. Previous periods between extrusion episodes had generally been dominated by low-frequency seismicity.
A swarm of tiny earthquakes began late 12 September, with rates reaching 800-1,000 events/day. Maximum focal depths were 2-3 km below the crater, and most were probably shallower. The swarm continued until the onset of seismicity characteristic of steam-and-ash emission at 0939 on 15 September. A white plume rose from the crater to ~3.5 km altitude, and the associated seismicity continued for about an hour. Seismicity then declined to background, although seismometers recorded another few minutes of apparent, but unwitnessed, gas emission starting at 2047 that evening. A tiltmeter on the W side of the dome indicated inflation for 1-2 days before the 15 September plume emission. This apparent inflationary tilt stopped after the morning plume emission, and no other tiltmeters or deformation measurements showed any significant changes.
Geologic Background. 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.
Information Contacts: D. Swanson and S. Brantley, CVO; C. Jonientz-Trisler, University of Washington.