An earthquake swarm occurred near Mount Hood in northern Oregon beginning on 11 January. More than two dozen earthquakes were produced that day, the largest of which were M 3.2 (at 1404) and M 3.0 (at 0854). Following a pause during 12-13 January, a M 3.2 earthquake at 0356 and a M 3.0 earthquake at 0813 occurred on 14 January. The largest earthquakes were felt at Timberline, Brightwood, Parkdale, and Mount Hood Meadows. By noon on 14 January the Cascades Volcano Observatory had detected 66 earthquakes, 33 of them large enough to be well-located.

All of the earthquakes in the recent swarms had characteristics similar to tectonic earthquakes rather than volcanic earthquakes (indicative of magma movement). They most likely resulted from regional tectonic stresses, although they may have also been 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.

Since 1990 Hood has produced about fifteen earthquake swarms similar to the recent one. These swarms have lasted from a few hours to several days and have generally produced maximum magnitudes between 1.6 and 3.5. Many earthquakes have been well-located, and are generally clustered 4-7 km S of the volcano's summit. Seismic data of lower quality suggest that swarms also occurred in the 1970s and 1980s. The largest recorded earthquake at Mount Hood was a M 4.0 in December 1974.

Geological Summary. Mount Hood, Oregon's highest peak, forms a prominent backdrop to the city of Portland. The eroded summit area consists of several andesitic or dacitic lava domes. Major Pleistocene edifice collapse produced a debris avalanche and lahar that traveled north down the Hood River valley and crossed the Columbia River. The glacially eroded volcano has had at least three major eruptive periods during the past 15,000 years. The last two occurred within the past 1,800 years from the central vent high on the SW flank and produced deposits that were distributed primarily to the south and west along the Sandy and Zigzag rivers. The last major eruptive period took place beginning in 1781 CE, when growth of the Crater Rock lava dome was accompanied by pyroclastic flows and lahars down the White and Sandy rivers. The Sandy River lahar deposits extended to the west as far as the Columbia River and were observed by members of the 1804-1805 Lewis and Clark expedition shortly after their emplacement. Minor 19th-century eruptions were witnessed from Portland.

Information Contacts: Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Geophysics Program, University of Washington, Seattle, WA 98195 USA (URL: http://www.geophys.washington.edu/).