Friday, 01 October 2004

Mount St. Helens Notice of Volcano Alert, October 2, 2004

A notice of Volcano Alert (Alert Level 3) was issued today at 2:00 p.m. PDT

Immediately after the small steam emission at 12:15, seismic activity changed from principally rock breakage events to continuous low-frequency tremor, which is indicative of magma movement. We are increasing the alert level to Volcano Alert the highest alert level indicating that an eruption could be imminent.

The cause and outcome of the accelerating unrest is uncertain. Explosions from the vent could occur suddenly and without further warning. During such explosions the dome and crater floor are at greatest risk from ballistic projectiles, but the rim of the crater and flanks of the volcano could also be at risk. Explosions would also be expected to produce ash clouds that rise several to tens of thousands of feet above the crater rim and drift downwind. Currently wind forecasts from the National Weather Service, combined with eruption models, show that ash clouds will move to the northwest. If ash emissions are large, drifting ash could affect downwind communities. Minor melting of the glacier could trigger debris flows from the crater that are large enough to reach the Pumice Plain. There is very low probability that downstream communities would be impacted by these hydrologic events.

We continue to monitor the situation very closely and will issue additional updates as warranted, whether activity escalates or returns to background levels.


Mount St. Helens Update, October 1, 2004, 7:00 P.M.

Current status is Alert-Level 2-Volcano Advisory

The increasingly energetic seismic swarm of the past week culminated in a small 25-miunute-long eruption around noon today from a vent just south of the lava dome. The vent opened in a portion of the glacier that had become increasingly crevassed and uplifted over the past few days. This deformation was probably driven by piston-like uplift of a portion of the lava dome and crater floor. The eruption sent a steam and minor ash plume to an altitude of about 10,000 ft. It drifted southwestward accompanied by minor ashfall in areas close to the volcano. Seismicity dropped to a low level for several hours after the eruption, but is gradually increasing with earthquakes (maximum Magnitude about 3) occurring a rate of 1-2 per minute. We infer that the system is repressurizing. As a result, additional steam-and-ash eruptions similar to today’s could occur at any time.

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Friday, 01 October 2004 20:22:19 (Pacific Standard Time, UTC-08:00)
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