Increasing seismicity and deformation culminated in the extrusion of a new lobe onto the composite lava dome during the night of 21-22 October.

Seismicity and tilt had returned to background levels after an earthquake swarm that started on 12 September and ended after a small steam-and-ash emission 3 days later (SEAN 11:08). Another small seismic swarm, accompanied by minor tilting on the dome's W flank, was recorded on 30 September and 1 October. Seismicity declined and tilt leveled off at about the same time on 1 October, but no associated plume emission was detected.

Late on 3 October, seismicity and tilt on the dome's W flank began to increase again, with some seismic events reaching about M 2. As in the earlier seismicity/tilt episodes, tiltmeters on the dome's S and E flanks detected no deformation, while the N-flank instrument detected smaller changes than were recorded on the W flank. The activity culminated in a large rockfall from the overhanging N face of the dome's May 1986 lobe. The hot rockfall moved down the dome's N flank to ~250 m beyond its base, covering the W side of the deposit left by the large rock avalanche of May 1986. An associated surge melted plastic, lightly charred a wooden antenna post, and deposited several centimeters of hot sand- and silt-sized material several hundred meters beyond the toe of the rockfall. A substantial quantity of dust from the rockfall was entrained in a vertical column, then drifted to the S and SW. Light ashfalls were reported in Vancouver, Washington, 80 km SW. Seismicity dropped to background more than an hour before the rockfall. Tilt reversed at about the time the rockfall occurred, remained flat for a few hours, then inflation resumed at an initially slow but accelerating rate. SO₂ emission, at background rates of 20-30 t/d 3, 11, and 22 September, increased slightly to 65 plus or minus 5 t/d on 6 October, the day after the rockfall, declining to 45 plus or minus 5 t/d two days later.

Seismicity began to build again about 8 October. After a slight decrease between 6 and 9 October, the rate of outward movement at targets on the W flank of the dome increased to 2.5 cm/day between measurements on 9 and 14 October, an order of magnitude above background, and to 15-16 cm/day on the W and NW flanks by 16 October. Inflationary tilt also continued to accelerate, reaching 400 µrads/day by 15 October. A strainmeter across a crack on the W part of the dome's summit (the September 1984 lobe) measured rates of opening of ~1 mm/day, and new cracks were visible in the same area. Seismicity, slightly elevated on 13 October, was at moderate levels by the 15th. Rates of SO₂ emission remained at background levels of 15-25 t/d.

On 16 October, the USGS and the University of Washington issued an advisory notice stating that an episode of rapid dome growth was likely to begin within the next 3 weeks. Until 18 October, as the number of earthquakes increased, the ratio of low-frequency and medium- to high-frequency events remained roughly the same. Beginning late on the 18th, the number of low-frequency events began to increase rapidly. As seismicity increased to high levels, a 19 October notice updated the expected start time for rapid dome growth to 2-10 days. Many new cracks and rockfalls were sighted on the W part of the dome on 20 October. Rates of SO₂ emission, 50 t/d on the 20th, increased to 550 and 375 t/d during two flights on 21 October. Vigorous seismicity continued until mid-afternoon 21 October, when discrete earthquakes suddenly stopped, replaced by several hours of a constant tremor-like signal of much lower amplitude. Deformation rates on the NW side of the dome reached 2.5 m/day on the 21st, but more rapid deformation was probably occurring on the W side. Earthquakes resumed late that night as inflationary tilt at the summit began to flatten gradually.

There were no night observations of the dome, but after sunrise a new lobe was visible just W of the dome's summit, within the area that had been most rapidly deforming. The lobe emerged from an elongate spreading center oriented slightly W of N. It continued to grow through the day, and by 1600 was roughly 250 m in longest dimension (approximately N-S) and 40 m high. Seismicity diminished during the day, and by evening was at only moderate levels. There was no evidence of explosive activity associated with the extrusion. A large thrust fault and many smaller thrusts severely disrupted a wide area of the snow- and ice-covered talus on the SW and WSW crater floor, extending 250-300 m from the base of the dome to the crater wall. Displacements may have been as much as several meters. Thrust faulting had not affected that part of the crater floor since [1982]; significant crater floor thrusting had last been observed, over a smaller area, in May 1985.

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, D. Dzurisin, J. Sutton, and Patrick Pringle, CVO; C. Jonientz-Trisler, University of Washington.