Using Spirit Lake as a TiltmeterExcerpt from: Swanson, 1992, The Importance of Field Observations for Monitoring Volcanoes, and the Approach of "Keeping Monitoring as Simple as Practical": IN: Ewert and Swanson, (eds.), 1992, Monitoring Volcanoes: Techniques and Strategies Used by the Staff of the Cascades Volcano Observatory, 1980-1990: USGS Bulletin 1966 |
|
The first explosion from Mount St. Helens took place on March 27, 1980, and
immediately thereafter volcanologists began to wonder if a magma body were
rising beneath the volcano and, if so, what were its depth and volume. The
volcano was covered by deep snow, but it was possible to drive to its base and a
short distance up its northeast flank. Hence plans were quickly made to
initiate tilt measurements by both single-setup leveling and electronic
tiltmeters. But it would take time to acquire the necessary equipment, wait for
new bench marks and foundations to stabilize, and obtain data. Moreover, the
thick pumice blanket at the base of the volcano near Spirit Lake made it
difficult to find stable sites for instruments and bench marks.
Why not use Spirit Lake as a giant carpenter's level? This idea had several advantages. The lake was far enough from the foot of the volcano to be an effective monitor of deep-seated changes under the volcano. The lake was oriented with its long direction radial to the volcano, and quick calculations suggested that tilts on the order of 2 urad would be detectable with reasonably careful measurement procedures. The lake was covered with ice, so that wind-related waves would be damped out. Boat docks and partly submerged stumps in small areas of open water provided sites to install measurement devices. But what devices could be obtained quickly? We couldn't afford the time to obtain normal water-level sensors and install them. An inexpensive and readily available alternative was immediately apparent. A visit to a local lumber yard resulted in an outright gift of several wooden yardsticks (metersticks), which were easily nailed to piers and stumps. Within a day the level of the lake was monitored by individuals visiting each site, reading the level of the lake on the partly submerged meterstick, recording the time of their visit, and closing back on a station chosen as a reference point. Later we used several observers with synchronized watches, but the results were the same: The shore of Spirit Lake was not tilting. This told us that a large volume of magma was not intruding under the volcano, so that we could concentrate our attention on the edifice of the volcano itself (Lipman and others, 1981). |
Return to:
[Mount St. Helens May 18, 1980 Menu] ...
[Mount St. Helens Eruptive History Menu] ...
[Mount St. Helens Monitoring Menu] ...
[Mount St. Helens Volcano Menu] ...
[Spirit Lake Menu] ...
