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.

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 S. Brantley, CVO; C. Jonientz-Trisler, University of Washington.