USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Measurements of Slope Distances and Zenith Angles at
Newberry and South Sister
Volcanoes, Oregon, 1985-1986
--
Iwatsubo, E.Y., Topinka, L., and Swanson, D.A., 1988,
Measurements of Slope Distances and Zenith Angles at
Newberry and South Sister
Volcanoes, Oregon, 1985-1986:
U.S. Geological Survey Open-File Report 88-377, 51p.
Introduction
In 1985 the U. S. Geological Survey's David A. Johnston Cascades Volcano Observatory (CVO)
added Newberry and South Sister volcanoes, both located in the central High Cascades Range of
Oregon, to its geodetic monitoring program of Cascade volcanoes. Both volcanoes are
potentially active and warrant monitoring similar to that with CVO has already implemented at
Mount Baker, Mount Rainier, and Mount St. Helens in Washington, Mount Hood and Crater Lake in
Oregon, and Mount Shasta and Lassen Peak in California. Newberry volcano is similar to Crater
Lake in that both have summit calderas that contain lakes. Newberry, however, contains two
lakes and was last active 1400 years ago (MacLeod and others, 1981). South Sister volcano,
3157 meters high, is the tallest of the three major peaks known as the Three Sisters and was
last active during two brief episodes between 2300 and 2000 years B.P. (Scott, 1987).
The geodetic networks at Newberry and South Sister consist of Electromagnetic Distance
Measurement (EDM) and leveling lines. The EDM network at Newberry lies mainly within the
summit caldera and consists of 11 lines and zenith angles, all measured from one instrument
station. One 8.7 kilometer leveling lines also crosses the summit caldera (Yamashita and
Doukas, 1987). At South Sister are 24 EDM lines and zenith angles, measured from 5 instrument
stations, and four 200-500 meter long leveling lines. The leveling results at Newberry and
South Sister between 1985 and 1986 showed little or no change beyond the expected leveling
error. Presented here are the calculated mark-to-mark slope distances using both endpoint and
flightline temperatures. Zenith angles were measured for both surveys but were used only to
calculate benchmark elevations.
Monitoring of volcanoes has proven useful in helping to detect magma movement prior to
eruptions (Kinoshita and others, 1974; Lipman and others, 1981). Consequently, CVO hopes to
establish baseline data on all potentially hazardous volcanoes in the Cascade Range. The
networks recently established at Newberry and South Sister volcanoes are designed to provide
such baseline data. If future activity should occur, the networks could be expanded to meet
any perceived needs. The networks are planned to be measured every 3 to 5 years as an
additional precautionary measure.
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05/30/00, Lyn Topinka