Mount Hood, Oregon
Seismic Activity, January 1999

Information Statement, January 14, 1999

Current situation:

An earthquake swarm is occurring near Mount Hood in northern Oregon. It began early Monday morning (Jan 11, 1999) and produced more than two dozen earthquakes that day, the largest of which were magnitudes 3.2 (at 2:04 pm PST) and 3.0 (at 8:54 am PST). Following a pause in activity on Tuesday and Wednesday, another magnitude 3.2 earthquake occurred at 3:56 am PST Thursday (Jan. 14, 1999), followed by a magnitude 3.0 earthquake at 8:13 am PST. The largest earthquakes were felt at Timberline, Brightwood, Parkdale, and Mount Hood Meadows. As of noon on Jan. 14, we have detected 66 earthquakes, 33 that are large enough to be well located.

Background Information:

Since 1990, Mount Hood has produced about 15 earthquake swarms similar to the current one. These swarms have lasted from a few hours to several days, have produced from several to tens of well-located earthquakes, have been clustered between 4 and 7 km (2.5-5 miles) south of the volcano's summit, and have generally produced maximum magnitudes between 1.6 and 3.5. Seismic data of lower quality indicate that swarms also occurred in the 1970s and 1980s. The largest recorded earthquake at Mount Hood was a magnitude 4.0 in December 1974.

Interpretation of Recent Activity:

All of the earthquakes in the Mount Hood swarms have characteristics similar to tectonic earthquakes rather than volcanic earthquakes (indicative of magma movement). The recent Mount Hood earthquakes most likely result from regional tectonic stresses, although they may also be caused by deep seated changes in the volcano's plumbing system. Additional and significantly different geological and geophysical indicators would be expected before any future eruptive activity. Scientists will continue to monitor the situation closely. They may add additional instrumentation to the monitoring network to learn more about earthquakes in this region.

We continue to monitor the situation closely and will issue additional information statements should they be warranted.

Maps and and general information about Mount Hood may be found on the WORLD WIDE WEB at URL:
http://vulcan.wr.usgs.gov/Volcanoes/Hood/framework.html

Information about seismic activity near Mount Hood may be found at:
http://www.geophys.washington.edu/SEIS/PNSN/HOOD/