Current Hazards of the Area
records have shown that the Cascades including the Three Sisters area have
experienced large eruptions of wide impact. In the past 700,000 years
there were four explosive eruptions that produced pyroclastic flows from Sisters
to south of Bend, Oregon. Deposits as thick as 13 meters containing fist-sized and
smaller white pumice clasts have been uncovered in several pumice
quarries. Related deposits have been found in northern California and in
the Pacific Ocean (USGS, 2002d). Events of this magnitude are infrequent but make the
potential for eruption in the area does exist.
This information along with the eruption of volcanoes in the area, such as Mt. St. Helens in 1980, pointed toward the need to monitor the volcanic and seismic activity of the Cascade area.
populated areas are near these volcanoes of the Cascade Range. The
diagram to the right shows the proximity of several volcanoes and
population centers. The Three Sisters area has growing communities
nearby as well as several resorts. The location of such places has
also encouraged monitoring of the area (Ewert, 1992).
Created to monitor this area was the U.S. Geological Survey's David A. Johnston Cascades Volcano Observatory (CVO). CVO established baseline data on the area between 1980 and 1984. South Sister was included in the monitoring program in 1985 (Ewert, 1992).
Several different methods and
technologies are used to monitor this volcanic area as well as others around the
Initially used was geodetic leveling. This is a technique used to measure elevation differences between successive designated areas. Geodetic leveling is a good technique for monitoring elevation changes and vertical displacements over extended periods of time. CVO established geodetic leveling on South Sister in 1985 (Dzurisin, 1992).
Real-time seismic-amplitude measurement system (RSAM) is another monitoring system. It uses an inexpensive eight-bit analog-to-digital converter controlled by a laptop computer. It calculates 10-minute amplitude averages for the Cascades. This provides a continuous time history of seismic activity. RSAM has been used in the past to predict activity at Mount St. Helens and at some volcanoes in Alaska (Endo, 1992).
Another technique used to monitor volcanoes is InSAR which stands for Interferometric Synthetic Aperture Radar. It is a form of remote sensing that uses radar satellite images. Radar satellites continuously shoot radar waves toward earth and then record them after they are reflected back. InSAR is used to detect deformation of volcanoes (USGS, 2002b).
Recently, USGS scientists have detected an uplift of the ground surface 5 kilometers west of South Sister. This uplift was detected using InSAR. The uplift occurred sometime between 1996 and 2000 and covers an area about 10 to 20 kilometers in diameter. At the center, the uplift measures about 10 centimeters (USGS, 2002e). The diagram below is a USGS image of the uplift.
As of current information, there is no supporting evidence that a catastrophic eruption could occur at Three Sisters any time soon (USGS, 2002d). The occurrence of the uplift detected in 2001 has prompted increased monitoring of South Sister, but the exact explanation of the uplift and prediction for the Three Sisters area has yet to be uncovered.
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