March 24, 2004

March 2004 Earthquake Swarm at Three Sisters Volcanos, 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:
Three Sisters PNSN web page

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