During March, seismicity, rates of SO₂ emission, and deformation remained at background levels. Maximum deformation rates on the dome were 1-2 mm/day, measured across the graben formed during the last extrusive event in May-June 1985. An average of 20 plus or minus 5 t/d of SO₂ were measured in March during three overflights. Seismicity was also at background levels, although 17 small, but deeper than usual, earthquakes were located between 29 January and 1 April. These medium- to high-frequency events had a depth range of 3-8 km and epicenters trended NNW to SSE across the crater.

On 16 April at 1717, a minor gas-and-ash emission event sent a plume to 5.4 km above the dome. An airplane pilot reported that the cloud moved NE from the volcano. A smaller plume rose < 2 km above the crater rim the next day at 1428. Beginning early 15 April, there had been several occasions on which brief bursts of seismicity were associated with minor (1-2 mm) dome strain episodes (measured by continuously-recording strainmeter across a zone of cracks just N of the graben), tilt excursions of a few µrads, and increases in the signal from the dome gas sensor, which detects H₂, H₂S, and SO₂. Earlier (and weaker) episodes of instrumental activity were accompanied by poor weather that prevented observation of associated emissions. Occasional deeper earthquakes, like those located in February and March, continued through mid-April, and there was no significant change in the volcano's seismic energy release. No general increase in the rate of swelling of the dome was detected before the emission of the steam-and-ash plumes.

No episodes of gas-and-ash emission had been seen since mid-May 1985, shortly before the last lava extrusion episode (SEAN 10:05).

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 and J. Sutton, CVO; C. Jonientz-Trisler, University of Washington.