No eruptive activity has occurred since measureable downslope movement of the newest lobe of the composite dome stopped 4 November. However, seismicity beneath the crater and deformation within the crater began in late February, and the USGS expected an eruption by late March.

Only two shallow earthquakes large enough to be detected by more than one instrument of the Mt. St. Helens net occurred 1-23 February. Seismic activity increased on 24 February. Since then, seismographs have recorded about five very small (negative magnitude) events per day centered below the crater at 5-12 km depth, and 10-12 somewhat larger (but usually less than M 1) shocks per day at 1-3 km depth. No earthquakes have been located in the zone between the groups of shallow and deeper foci, and no harmonic tremor has been recorded. The deeper earthquakes were the first since 1980, when they often followed (but never preceded) explosive activity.

Deformation within the crater began at about the same time as the increased seismicity. Outward movement of the N-crater rampart started in late February and had increased to 2.4 cm/day by early March. Horizontal expansion of the lower portion of the composite lava dome increased to 1.6-1.7 cm/day by early March, apparently accompanied by uplift, but conditions prevented measurements of targets on the upper section of the dome. Movement also occurred along the small crater-floor thrust faults, produced by past expansions of the dome, but snow made quantitative monitoring difficult. A dry tilt station about 300 m N of the dome showed inflation during each of three measurement intervals (6-24 February, 24 February-5 March, and 5-8 March), but a tiltmeter 700 m farther N detected no inflation, suggesting to USGS geologists a shallow source near the first tilt station. In contrast, simultaneous inflation was measured at these two sites before the last extrusion episode October-November 1981. No expansion of the edifice as a whole had occurred as of early March. Measurements of SO₂ emission have been infrequent because of poor weather. The rate of SO₂ emission had increased in early February and reached 240 t/d on the 10th. Poor weather prevented further measurements until 21 February. Since then SO₂ emission has remained at background levels of 80-120 t/d.

On 12 March, the USGS and University of Washington issued a joint extended outlook advisory stating that an eruption was likely within the next 3 weeks. Another dome-building episode was rated as the most probable eruption type, but because of the changed seismic pattern the advisory noted that explosions or lava flows were possible.

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. Dzurisin, J. Ewert, D. Swanson, K. Cashman, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.