Abstract

A magnitude 5.5 (ML) earthquake occurred on 14 February 1981 near Elk Lake in the southern Washington Cascade Range. It was the largest earthquake to occur in Washington since the 1965 Seattle earthquake (Mb = 6.5). The epicenters of 1050 located aftershocks (Mcoda ≧ 0) delineate a fault zone 6 km long north-south by 3 km wide that extends from 5 to 12 km in depth. Immediately after the main shock, aftershocks occurred at generally deeper depths to the north and south, indicating that the break propagated bilaterally and downward. First-motion patterns for the main shock and 84 aftershocks (Mcoda ≧ 2.0) indicate predominantly right-lateral strike-slip faulting along a nearly vertical plane striking essentially north-south, in agreement with the alignment of epicenters.

Earthquake activity at Elk Lake for the 7-yr period to 1980 was low. Nine months before the main shock, an earthquake swarm began just southeast of what was to become the present aftershock zone. The swarm lasted 2 months and was followed by 7 months of quiescence before two immediate foreshocks occurred on 11 February 1981. The overall pattern of seismicity observed throughout the Elk Lake sequence corresponds well to that predicted by an asperity model.

Examination of the magnitude-frequency relation (b) indicates that there was no significant temporal variation in b throughout the swarm-main shock-after-shock sequence. However, b is significantly higher on the east side of the fault zone than on the west side. For the entire aftershock sequence, b = 0.77 ± 0.05.

This earthquake sequence is important for three reasons: (1) the main shock is the largest earthquake to occur in the Pacific Northwest since 1965; (2) the sequence has been long and aftershocks are numerous; and (3) its occurrence indicates that the potential seismic hazard in southwest Washington must be reevaluated.

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