Source

Three Sisters, Oregon, Information Statement
March 24, 2004

Update -- March 24, 2004

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.

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 and
http://www.pnsn.org/SISTERS/welcome.html.

U.S. GEOLOGICAL SURVEY
CASCADES VOLCANO OBSERVATORY