Monitoring:  | Gas | Ground Deformation | Hydrologic | Remote Sensing | Seismicity |

Tiltmeters Help Scientists
Predict Eruptions at Mount St. Helens

Installing tiltmeters near the dome is critical

Photograph by S.R. Brantley on 9 June 1982 Tiltmeter site on crater floor of Mount St. Helens

The nearly flat crater floor adjacent to the lava dome provided an excellent "platform" on which to measure a change in inclination or "tilt" of the ground during the first few years of dome growth. Tiltmeters had to be installed within a few hundred meters of the vent (located directly beneath the dome) to record significant tilt changes. Several tiltmeters were needed around the dome because the crater floor did not tilt uniformly at all locations. At each tiltmeter site (black canister in foreground), a radio sent data to the USGS Cascades Volcano Observatory, located about 80 km away in Vancouver, Washington.

Tiltmeters detect increasing rates of deformation before eruption

This graph shows a three-month record of tilt from a tiltmeter located less than 100 m north of the dome before the March 19, 1982, eruption (vertical red line). The tiltmeter recorded a change in ground slope in two directions, radial and tangential to the dome. Note the gradual tilt that began in mid-January and accelerated sharply in mid-March. Then, within 30 minutes of the eruption, the radial ground tilt reversed direction; this reversal indicated the ground stopped tilting away from the dome and began to subside rapidly.

Eruption Warnings Based on Monitoring Data

March 12, 8:00 am:

"Seismicity beneath Mount St. Helens continues at elevated levels, but individual earthquakes are of low magnitude, but earthquakes are of low magnitude. Earthquakes have been occurring in 1- to 2-day-long episodes separated by 1- to 2-day intervals of decreased activity. The earthquakes are occurring between the surface and a depth of about 6 miles. Rates of ground deformation in the crater area have increased during the last 2 weeks, and they are similar to patterns observed before previous dome building events."

"Based on rates of deformation, an eruption is likely within the next 3 weeks. Deformation is confined to the crater areas, suggesting that renewed dome growth will occur. The current seismic patterns differ from any observed before 1980-81 eruptions, however, and raise the possibility of more hazardous various in eruptive behavior. If there were to be any pyroclastic flows, from either an explosive eruption of collapse of the steep north face of the dome, the possibility of rapid snowmelt would be of concern."

March 15, 7:00 pm:

"Accelerating rates of ground deformation in the crater of Mount St. Helens suggest that an eruption, most likely of the dome-building type, will probably begin within 1 to 5 days. Deep earthquakes have almost ceased, and shallow-earthquakes continue at a moderated rate. A further increase is shallow seismicity is likely before the eruption starts."

March 19, 9:00 am:

"Seismicity at Mount St. Helens has increased significantly during the past day. This indicates that an eruption will begin soon, probably within the next 24 hours. The character of both the seismicity and deformation in the crater area indicated that the most likely type of activity is dome growth."

Reference

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