USGS
USGS
scientists have detected a slight swelling, or uplift, of the ground surface over
a broad area centered 5 kilometers (3 miles) west of South Sister volcano in the
Three Sisters region of the central Oregon Cascade Range. The Three Sisters region
is located 35 kilometers (22 miles) west of Bend, Oregon, and 100 kilometers (60
miles) east of Eugene, Oregon. The uplift, which occurred between 1996 and 2000,
covers an area about 15 to 20 kilometers (9 to 12 miles) in diameter and the maximum
amount of uplift at its center is about 10 centimeters (4 inches). It is too broad
and low to be noticed from the ground, and several close aerial inspections of
the area have revealed no unusual surface features.
The relatively new
technique employed by the scientists, Satellite Radar Interferometry (or InSAR),
uses satellite data to make radar images of a portion of the Earth.s surface.
Images acquired at different times, but from the same location in space, can be
used to detect even minor (a few centimeters or one inch) changes in the elevation
of the ground. The images that reveal the 10-centimeter uplift near South Sister
were obtained in 1996 and 2000. The exact timing of the uplift, or whether it
is continuing at present, is unknown, but is being studied further.
The
specific cause of the uplift is uncertain. Because the Three Sisters region is
a volcanic area, the uplift may re flect intrusion of a small volume of magma
(molten rock) deep under the surface-probably at a depth of about 7 kil ometers
(4 miles). Such a process, which keeps volcanic areas "alive" and prepares them
for future eruptions, is a common occurrence under volcanoes , but until development
of techniques such as InSAR, it has been difficult to d etect. If intrusion of
magma were to continue, it could eventually lead to a volcanic eruption; however,
an eruption is unlikely without significant precursory activity. I
n addition
to continued or accelerating uplift, precursors to an eruption would include earthquakes,
typically swarms of small events generated by fracturing of rock as magma moves
upward, and large emissions of volcanic gases, such as carbon dioxide, which are
released from the magma. At present, earthquake activity appears to be at or near
background level and gas emissions are low. In order to be prepared to more accurately
detect possible precursors and to better understand this uplift phenomenon, USGS
plans to enhance the existing monitoring network. Installation of one or more
additional seismometers and a Global Positioning System (GPS) receiver, resurvey
of existing benchmarks and installation of new benchmarks, and periodic airborne
and ground-based sampling of gases are currently being considered in consultation
with managers of the Willamette and Deschutes National Forests.
A number
of public officials and agencies in the State of Oregon and Lane and Deschutes
Counties have been briefed on these findings and they and scientists will work
together to address any questions or concerns the public may have. |
