DESCRIPTION:
Mount Hood, Oregon
Seismic Activity, 1977 - 1996

Earthquakes occur sporadically at Mount Hood, typically as short-lived swarms of small events (less than or equal to magnitude 3.5) that locate chiefly on the south flank and below the summit at depths of less than 11 kilometers (information from USGS and University of Washington Geophysics Program). One to several swarms per year have been recorded since the seismic system was upgraded in 1980. A typical swarm occurred in summer 1980; a magnitude-2.9 event was followed by seven aftershocks ranging in magnitude from 1.6 to 2.8.

A swarm in February 1990 had 30 earthquakes, all smaller than Magnitude 1.3. Later that year, a magnitude 3.5 earthquake was followed by 12 aftershocks. Focal mechanism for the Magnitude 3.5 event shows dominantly dip slip with slight component of lateral slip (nodal planes N30E,43SE, minor left-lateral; and N30W,60SW, minor right-lateral).

Recently, on the morning of April 7, 1996, events of magnitude 3.0 and 2.4 occurred at a depth of 7 kilometers below the summit.

Felt earthquakes on Mount Hood occur every 2 years on the average. Seismic monitoring, in effect since 1977 (Weaver et.al., 1982), indicates a generalized concentration of earthquakes just south of the summit area and 2-7 kilometers below sea level.

A seismic swarm in July 1980, during which nearly 60 earthquakes (mostly 5-6 km deep with a maximum bodywave magnitude of 2.8) recorded in a 5-day period (Rite and Iyer, 1981), prompted development of an emergency response plan to coordinate local authorities in the event of future eruption.

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