South Sister Volcano:
A Network of Short Traverses to Determine Tilt Changes

In 1985, CVO established a network of four linear leveling traverses ranging in length from 196 m to 317 m at South Sister (Yamashita and Doukas, 1987). The purpose of the network is to provide baseline geodetic information for comparison with future surveys, especially at times of volcanic unrest. Triangular or L-shaped arrays would have been preferable, but those configurations were precluded by the steep terrain. The flanks of the cone are sparsely vegetated, so the biggest obstacle to longer traverses was the steepness of the flanks. The traverses are located at elevations of 2,200 m to 2,500 m along radial ridges on the north, east, south, and west flanks of the volcano. In spite of numerous short (10 m and less) setups required by the terrain, the network can be double-run in one or two days with helicopter support.

The South Sister network was remeasured in 1986 to verify the initial observations and determine if measurable deformation had occurred since the network was established. Measured tilt changes ranged from 0.8 to 6.5 microradians. No pattern was apparent in the changes, which are attributed to measurement error. Better precision (1-2 microradians) could be obtained from longer traverses (about 1 km), but the time required for the measurements would increase accordingly. The existing network is thought to be a good compromise, adequate to detect precursory deformation in the event of renewed volcanic activity.