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Earthquakes

Earthquake and other natural hazards -
Oregon seismicity in 2001

From the Spring, 2002 issue of Oregon Geology

These preliminary data have been excerpted from the Quarterly Network Reports on Seismicity of Washington and Oregon, published by the Pacific Northwest Seismograph Network, Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, WA 98195-1310 (http://www.ess.washington.edu/SEIS/PNSN/). They include earthquakes between lat 42.0° and 45.5°N., and long 117° and 125°W.


Earthquakes don’t respect geographic boundaries. In 2001, the earthquake most widely felt in Oregon was the February 28 Nisqually earthquake, centered near Olympia, Washington. The deep-seated (52.4 km), magnitude 6.8 event was felt throughout most of Oregon, and as far away as Salt Lake City.

The map below shows the earthquakes centered in Oregon in 2001 from magnitude 1 to 2.9. Only a few of these were felt, and no damage was reported. The largest events are summarized in the table insert.

For the year 2001, the earthquakes recorded in Oregon numbered 221, a total of 41 for the first quarter, 38 for the second, 96 for the third, and 46 for the fourth.

The earthquake most widely felt in Oregon was the February 28 Nisqually earthquake, centered near Olympia, Washington. The deep-seated, magnitude 6.8 event was felt throughout most of Oregon and as far away as Salt Lake City. The only Oregon earthquake reported as felt was a relatively small one (magnitude 1.9) on July 22. It was located at about 13 km depth, 21.4 km southeast of Canby in Clackamas County.

For 27 earthquakes, magnitudes were 2.0 or larger, but none of them exceeded magnitude 2.9. Two of these earthquakes were located offshore: On May 10, a magnitude 2.6 event was located at a depth of about 31 km, off the coast of southern Curry County. And on September 6, a magnitude 2.9 event was located at a depth of 35 km, off the southern Clatsop County coast.

The Mount Hood region registered several swarms of events. The first began on January 10, 2001, and continued for most of the month of January. Between January 10 and January 23, 2001, a total of 24 earthquakes were recorded here. The earthquakes were located approximately 5.0–8.0 km south-southeast of Mount Hood at depths ranging from 3.0 to 7.0 km. The magnitudes of the earthquakes ranged from –0.8 to 2.0.

Two more swarms of earthquakes occurred near Mount Hood in late summer. A total of 66 shallow (depths <10 km) earthquakes were located between lat 45° and 45.5°N. and long 121° and 122°W. The first swarm, 5 km south-southeast of Mount Hood, occurred mainly during August 20, when 9 events were located, none larger than magnitude 1.3, and mostly at depths of <5 km. The second swarm was more vigorous, longer lasting, and slightly deeper (most events 7–9 km deep). It was located in a different area, 8 km south-southwest of Mount Hood, and began on August 21 with very small magnitude (<1) events. Its most intense activity was September 11–16 and included 5 events with magnitude 2.0 or larger and 15 events with magnitude 1.0 or larger. The largest event was magnitude 2.9 on September 14.

Finally, three earthquakes on December 2 and 3, with magnitudes of 2.1, 2.4, and 2.6, were located about 5 km southwest of Mount Hood.

In the Klamath Falls area, 36 earthquakes occurred, most of them in the second half of the year. Since 1994, most earthquakes in the Klamath Falls area have been considered aftershocks or earthquake activity related to a pair of damaging earthquakes in September 1993. The 1993 earthquakes were followed by a vigorous aftershock sequence which decreased over time.

Other notable earthquake activity in Oregon included a magnitude 2.7 earthquake on May 6, located at a depth of 54 km, about 34 km southeast of Tillamook. Events of this depth are unusual in Oregon.
Elsewhere in Oregon, on August 21, a magnitude 1.9 earthquake at about 5 km depth was located near the Three Sisters volcanic area. Earthquakes are uncommon in this region.




Number Magnitude Location
1 2.9 8 km SW of Mt. Hood
2 2.7 14 km NW of Sheridan
3a 2.6 8 km SE of Condon
3b 2.6 20 km SW of LaPine
3c 2.6 5 km SW of Mt. Hood
6a 2.5 7 km SE of Condon
6b 2.5 8 km SW of Mt. Hood
8a 2.4 8 km SE of Condon
8b 2.4 5 km SW of Mt. Hood
10a 2.3 7 km SE of Hood River
10b 2.3 10 km NW of Klamath Falls
10c 2.3 13 km SW of Newberg
10d 2.3 8 km SW of Mt. Hood
10e 2.3 27 km SW of Forest Grove
¨



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