Global Volcanism Program

Volcanoes of the World

Hood

Volcanic Activity Reports

Mount Hood, Oregon's highest peak, forms a prominent backdrop to the state's largest city, Portland. The eroded summit area consists of several andesitic or dacitic lava domes; Pleistocene collapses produced avalanches and lahars that traveled across the Columbia River to the north. The glacially eroded volcano has had at least four major eruptive periods during the past 15,000 years. The last three occurred within the past 1,800 years from vents 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 eruptive period took place around 170-220 years ago, when dacitic lava domes, pyroclastic flows and mudflows were produced without major explosive eruptions. Minor 19th-century eruptions were witnessed from Portland.

Index of Volcanic Activity Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

07/80 (SEAN 05:07) Earthquake swarm

01/99 (BGVN 24:01) Mid-January swarms of tectonic earthquakes

Complete Contents of Volcanic Activity Reports

All information contained in these reports is preliminary and subject to change.

07/80 (SEAN 05:07) Earthquake swarm

[Seismic data in the following report have been extensively modified using Rite and Iyer (1981).] A series of earthquakes in the vicinity of Mt. Hood, about 100 km SSE of Mt. St. Helens, began with a M [2.8] shock on 6 July at 1817. [Within 30 minutes, 7 events of M 1.6-2.8 occurred in the vicinity of the first shock. Depths were tightly clustered at 4-7 km.]

[Activity remained anomalously high for about 48 hours after the initial shock, with 40 events recorded on 3-9 stations in the first 24 hours (figure 1). Epicenters were generally on the S flank (figure 2).] A M [2.44] earthquake at 0259 and a M [2.25] event at 1315 were centered [near the summit] at [5.9 and 5.3] km depth. [Between 6-16 July, 143 small events were recorded on a single station (VHE) about 5 km SE of the summit. About 50 were correlated with nearby stump blasting, but the other 90 appeared to be swarm events, most with coda magnitudes less than 0.5.]... The swarm's total seismic energy release [was about 8 x 10¹⁵ ergs]....

Figure 1. Histogram of seismic activity at Mt. Hood, 6-20 July 1980, after Rite and Iyer (1981).

Figure 2. Epicenters of most of the July 1980 events at Mt. Hood. Sizes of crosses are proportional to event magnitudes. After Rite and Iyer (1981).

The USGS issued a hazard watch formally notifying government officials. On 11 July, the USGS and the University of Washington installed three portable seismographs on Mt. Hood. No harmonic tremor has been recorded, and no new surface activity has been observed. Gas analyses made from aircraft on 11 July showed no increase in atmospheric SO₂ or CO₂ above normal levels.

Because no additional seismicity and no new eruptive activity have occurred at Mt. Hood, the USGS ended the hazard watch on 5 August.

References: Rite, A. and Iyer, H.M., 1981, July 1980 Mt. Hood earthquake swarm: USGS Open File Report 81-48, 22 p.

Information Contacts: R. Decker, R. Tilling, and C. Zablocki, USGS; S. Malone, R. Crosson, and E. Endo, Univ. of Washington.

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01/99 (BGVN 24:01) Mid-January swarms of tectonic earthquakes

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.

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

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