Mt. St. Helens Update Sunday 10/3/04

[Windy]

U.S. Geological Survey, Vancouver, Washington
University of Washington, Pacific Northwest Seismograph Network, Seattle, Washington

Mount St. Helens Update, October 3, 2004, 7:40 P.M.

Current status is Volcano Alert (Alert Level 3); aviation color code RED

At 2 pm yesterday, we increased the alert level to Volcano Alert due to a change in the character of seismic signals (50 minute long tremor) that occurred immediately after a steam emission at 12:16 P.M., recognition of ongoing uplift of the crater floor, and reports of sulfur gas odor. In addition, we believe that there is a significantly increased probability that gas-rich magma is moving toward the surface. After another period of tremor starting at 2:57 A.M. this morning, seismicity returned to discrete earthquakes. Seismic activity decreased gradually until about 2:00 P.M. then increased again, reaching levels comparable to those prior to steam and ash eruptions. M3 earthquakes are occurring at a rate of about one every 5 minutes. All earthquake locations remain shallow.

Yesterday’s gas sensing flights detected significant concentrations of carbon dioxide north and west of the dome. No significant levels of sulfur gasses were detected. Hydrogen sulfide odors detected by helicopter crews are attributed to steam emissions. These low levels of hydrogen sulfide are likely the result of boiling of the hydrothermal system.

Results from GPS measurements indicate no significant deformation of the outer flanks of the volcano. However, visual observations and photographic analysis show large-scale uplift (10’s of meters) of part of the glacier and a nearby segment of the south flank of the lava dome. This suggests rise of magma to shallow levels. Additional steam and ash eruptions could occur at any time. There is also an increased probability of larger magnitude and more ash-rich eruptions.

Today field crews took additional thermal images of the dome and crater and conducted gas sensing, infrared and geologic observation flights. Deformation crews retrieved data from GPS instruments and lowered a new GPS station from a helicopter onto the dome. In addition, two telemetered microphones are now operating to detect explosions.

Wind forecasts from the National Oceanic and Atmospheric Administration (NOAA), combined with eruption models, show winds this evening will be from the east and southeast and any ash clouds would drift to the west and northwest.

We continue to monitor the situation closely and will issue additional updates and Alert Level changes as warranted.

Press conferences will continue to be held at the Headquarters office of the Gifford Pinchot National Forest. The morning press conference is at 9:30 AM.

[tallywx]

Harmonic tremor as of 9:55 p.m. PDT...

http://www.pnsn.org/WEBICORDER/GREEN/HS ... rrent.html

[Windy]

Harmonic tremor as of 9:55 p.m. PDT...

http://www.pnsn.org/WEBICORDER/GREEN/HS ... rrent.html

I think that Webicorder is just saturated. The seismo out of Longmire and Cedar Flats are better. Doesn't look like a harmonic tremor at the moment (though I'm definately not a geologist! )

[tallywx]

Yeah, I think you're right. I forgot to check Longmire...

http://www.pnsn.org/WEBICORDER/GREEN/LO ... 00400.html

Clear difference between the current pattern and a harmonic tremor, as seen between the 12:00 and 13:00 hour back on the 2nd...

http://www.pnsn.org/WEBICORDER/GREEN/LO ... 00212.html