March 2004 Earthquake Swarm at Three Sisters, Oregon
U.S. Geological Survey Cascades Volcano Observatory, Vancouver,
Washington
University of Washington, Pacific Northwest Seismograph Network,
Seattle, Washington
THREE SISTERS, OREGON, INFORMATION STATEMENT
At approximately 10 a.m. yesterday (Tuesday, March 23), an ongoing swarm
of small earthquakes began in the Three Sisters volcanic center in the
central Oregon Cascade Range. This activity poses no immediate threat
to the public. As of this morning, the regional seismic network has
detected approximately 100 earthquakes ranging in magnitude up to about
1.5. The rate of earthquakes peaked late yesterday and appears to be
declining slowly. The earthquakes are occurring in the northeast part
of an area centered 5 kilometers (3 miles) west of South Sister volcano
in which the ground has been uplifted by as much as 25 cm (about 10
inches) since late 1997. On the basis of multiple lines of evidence,
scientists infer that the cause of the uplift is the continuing
intrusion of a modest volume of magma (molten rock). The magma appears
to be accumulating at a depth of about 7 kilometers (4 miles) below the
ground surface and now measures about 40 million cubic meters (about 50
million cubic yards) in volume. Until yesterday, only a few earthquakes
have accompanied this process, but scientists have expected that swarms
of small earthquakes such as the present one would eventually accompany
the uplift. The most likely cause of the earthquakes is small amounts
of slippage on faults as the Earth's crust adjusts to the slow ground
deformation of the past 7 years. Heat and gases related to the magmatic
intrusion have probably caused increases in fluid pressure deep
underground, which also helps to trigger minor faulting events.
The processes that have been causing the uplift over the past seven
years could eventually lead to shallower intrusion of magma or even to a
volcanic eruption; however, both are unlikely without significantly more
intense precursory activity. Scientists continue to monitor the
situation closely and to evaluate data from field instruments.
Today scientists are deploying another seismometer in order to locate
earthquakes more precisely. With the assistance of the Willamette and
Deschutes National Forests, additional fieldwork over the next week will
fix problems with some field instruments that resulted from the heavy
winter snow-pack and will assess sites for new instruments.
Closeup Map of 2004 Swarm Earthquake Distribution at Three Sisters.
Additional information, including maps and a volcanic-hazards
assessment, may be found on the Internet at Web at URL:
http://vulcan.wr.usgs.gov/Volcanoes/Sisters/framework.html
Updates on Seismicity can be found at:
http://www.pnsn.org/SISTERS/welcome.html
Information contacts:
U.S. Geological Survey-Cascades Volcano Observatory, Vancouver,
Washington
(360) 993-8900
Pacific Northwest Seismograph Network, University of Washington,
Seattle, Washington
(206) 685-2255