Earthquake seismicity remains the frontline of our monitoring tools. Mount St. Helens is tracked by the Pacific Northwest Seismic Network, a regional system managed collaboratively by the University of Washington and the U.S. Geological Survey. A variety of seismometers, nine of them within 10 km of the new dome, record the numerous tiny earthquakes associated with the rise of magma. These data are transmitted in real time to offices at Seattle (University of Washington, UW) and Vancouver (Cascades Volcano Observatory, CVO).

Distribution of seismometers around Mount St. Helens. Seismometers (yellow squares) are distributed around Mount St. Helens. JRO is seismometer at the Johnston Ridge Observatory, north of the crater. SHW is westside station mentioned in text. Numerous other stations lie beyond the area of the map.

The most common seismic signal since mid-October has been the waveform shown by the record sampled by the SHW seismometer on November 13, 2004. Very small magnitude earthquakes (Mag < 1.5), with their characteristic P- and S-wave arrivals, occur at rates of 1-4 per minute. They probably correspond to small rock-breakage events as the new dome is extruded through its wall rocks onto the surface of the crater floor. If someone were sitting on the dome, they might experience the largest of these earthquakes as dull tiny thuds.

Waveform example from SHW seismic station.

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Deformation

Ground deformation at Mount St. Helens occurs when magma forces its way upward toward the surface. As lava is extruded, the Earth's surface is also modified by the enlarging dome or lava flow. The extrusive changes are monitored using classic visual observing techniques and photography, now furthered by round-the-clock digital cameras. The U.S. Forest Service maintains a webcam at its Johnston Ridge Observatory visitor center, about 8.5 km north-northwest of the crater. Images are refreshed every fifteen minutes. On clear nights, incandescence from the budding new dome is visible on this camera.

U.S. Forest Service Mount St. Helens VolcanoCam. Main image is reduction of image seen November 26, 2004. Inset is night incandescence from October 10, 2004.

CVO also operates a fixed-site camera, a telemetered digital camera that was installed October 10. It is located northeast of the crater mouth, on Sugar Bowl, about 2.2 km from the center of activity. A 2-megapixel image is acquired and transmitted every 2.5 minutes during daylight hours and every hour at night (with a longer exposure to look for glow from hot rock). These images have proven useful for tracking the changing profile of the growing dome (see image below). The new dome now stands taller than the 1980-86 dome. As the new dome's height increases and its sides steepen, the hazard of small collapses also increases. A simple graphic shows the time-progressive sequence of dome growth.

Sugar Bowl DomeCam. Sequential Sugar Bowl DomeCam profiles from October 10 through November 10, 2004, traced from images.

The CVO Sugar Bowl DomeCam was engineered and built by USGS staff at the Hawaiian Volcano Observatory. So far we have received over 20,000 images showing growth of the new lava dome and area of uplift, steam and ash emissions, and general conditions in the crater. These images aren't available for public viewing in real time owing to substantial logistical problems still being addressed.

	[Click image to enlarge]
Sugar Bowl DomeCam. The Sugar Bowl DomeCam is located on the northeast flank of Mount St. Helens and was installed October 10, 2004. The image on the left shows the camera setup, with Mount St. Helens' dome in the background. USGS Photo by Gene Iwatsubo, October 10, 2004. The animated GIF image on the right shows dome growth spanning October 10 to November 21, 2004.

In order to understand the movement of magma at depth, small ground movements are monitored by Global Positioning System (GPS) receivers located at some distance from the volcano. Unlike the vivid changes seen at the growing dome, these far-field changes typically are imperceptible to human observers over most time scales. Several GPS receivers surround the volcano a few kilometers from the crater. All GPS data are transmitted in real time to the Cascades Volcano Observatory, where they are evaluated for changes in activity. The far-field receivers show no detectable changes in the Earth's surface, which indicates that the magma source is quite deep, or that any magma moving upward from a shallow source is being replaced from below.

Other GPS receivers lie within the crater closer to the deformation center. In October 2004, two receivers were slung by helicopter to sites within 50 m of the actively growing new lava dome. One of these stations, Cliff-4 (CLF4) was positioned on the south side of the new dome. Its rapid southward motion literally made it a geo-speedometer, recording more than 38 m of southward motion--nearly one-half the length of a football field--in the three weeks between its installation on October 28 and its demise on November 21. On November 20, a GPS receiver was lowered onto the top of the new dome.

Portable Telemetered GPS Stations. Main image shows a Portable Telemetered GPS Station on top of the new growth (circled). USGS Photo by Dan Dzurisin, November 20, 2004. Large inset shows slinging a station to the dome. USGS Photo by Dan Dzurisin, November 20, 2004. Small inset shows closeup of a Portable Telemetered GPS Station "spider". USGS Photo by Willie Scott, November 12, 2004.

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Photogrammetry and LIDAR

Photogrammetry is the measurement of changes using precisely oriented photographs. Our topographic maps of the changing landscape within the crater come from remote-sensing and digital photogrammetric methods. The photogrammetry and LIDAR create digital elevation models that we use to calculate the volume of new dome rock extruded on the crater floor or the area and volume of uplifted crater floor and deformed glacial ice. The image above an example produced by these combined techniques.

[Click image to enlarge]

Animation of LIDAR images. LIDAR of September 2003 and October 4, 2004, showing the increasingly deformed crater floor. New dome rocks didn't penetrate the floor until October 9, 2004.

The volume of dome and uplifted area has been calculated from imagery obtained Oct 4, 14, and Nov 4, as shown in the accompanying table.

Date of imagery	Volume change (million cubic meters)	Elapsed days since previous imagery	Cubic meters per second since previous imagery
October 14, 2004	7	9	9
November 4, 2004	8	22	4

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Gas

Rising magma releases its gas as it encounters the low pressures near the Earth's surface. The chief gases are water vapor, carbon dioxide, and sulfur compounds. During periods of quiescence, sulfur gases are weak or absent at Mount St. Helens, so they provide useful measures of the vigor of the renewed eruption. Gas discharge at an erupting volcano is commonly measured by aerial traverse. Sensitive spectrometers are flown through and beneath the gas-laden steam plume that rises from the crater.

Gas Monitoring at Mount St. Helens. Left, gas-monitoring equipment being readied for aerial traverse by helicopter at Mount St. Helens. USGS Photo by M.P. Doukas, November 20, 2004. Right, ready to go. Small ultraviolet spectrometer for measuring SO2 attached to aircraft's step. Flexible tubing extends from rear window for sampling gases during flight. USGS Photo by K.A. McGee, November 20, 2004.

Sulfur is a naturally occurring component in magma. Of the sulfur species, SO₂ has been the dominant gas throughout the eruption, with emission rates ranging from 50 to 250 tons per day. Carbon dioxide (CO₂) emissions have been generally higher than SO2, but only small amounts of hydrogen sulfide (H₂S) have been released.

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Hydrology

The drainages leading away from Mount St. Helens correspond to some of the highest hazard zones around the volcano. If conditions change abruptly, lahars--volcanic mudflows--may sweep down some of these valleys. At greatest risk is the North Fork of the Toutle River, the drainage that begins at the north-facing ampitheater of the Mount St. Helens crater.

Locations of five Acoustic Flow Monitors (AFM) and one seismometer. Four AFMs (red squares) are located in the North Fork Toutle River, and one is located in the South Fork Toutle River. The seismometer (black circle) is located at the Johnston Ridge Observatory (JRO).

To increase the amount of time available for warning, special sensors have been placed in the North Fork of the Toutle River. These acoustic flow monitors (AFMs) are geophones that sense the ground vibration associated with high-volume floods. Their data is sent real-time to the Cascades Volcano Observatory, where it is monitored through an automated alarm system.

Acoustic Flow Monitoring (AFM). Left, monitoring equipment transported to site by a tried-and-true method. USGS Photo by D.R. Saunders, October 25, 2004. Loowit site, a typical setup for acoustic flow monitoring. USGS Photo by W.C. Stokes, September 9, 2002. Stream leads upslope 2 km into crater of Mount St. Helens.

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