Current Hazards of the Area
Geologic
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.
| Many
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).
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Several different methods and
technologies are used to monitor this volcanic area as well as others around the
world.
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.
