Report on St. Helens (United States) — January 1986
Scientific Event Alert Network Bulletin, vol. 11, no. 1 (January 1986)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Brief earthquake swarm and H2 pulse
Please cite this report as:
Global Volcanism Program, 1986. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 11:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198601-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Seismicity, deformation rates, and SO2 emissions remained at background levels through early February. Deformation on the dome averaged <1 mm/day; two successful SO2 flights in January showed an average emission of 68 plus or minus 22 t/d. Seismometers recorded 27 surface events, all attributed to rockfalls. There was no seismic evidence of energetic steam emission.
On 30 December at 1600, seismometers in the crater began to record a sequence of earthquakes centered on the lava dome. For the next 1.75 hours, one hundred individual earthquakes were recorded above a "tremor-like" background signal with frequencies 3-4 times higher than that of harmonic tremor. Approximately an hour after the onset of seismicity, a gas emission event was detected by the atmospheric hydrogen sensor located on the dome. H2 was released in one large pulse lasting about an hour, followed by several smaller pulses.
Scientists at CVO interpret the seismicity coupled with H2 degassing as evidence for sudden contact of groundwater with magma. The resulting pressure change may have produced microfractures in overlying crustal material, which in turn allowed the release of gas to the atmosphere. The background "tremor-like" signal (acoustic emission) is believed to be associated with moving gases rather than the moving magma associated with harmonic tremor.
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: K. McGee, E. Endo, E. Iwatsubo, and S. Brantley, CVO; C. Jonientz-Trisler, Geophysics Program, University of Washington, Seattle, Washington 98195 USA.