Abstract

Earthquakes occurring between 1971 and 1988 are evidence for a small zone of crustal seismicity under the western North Cascades near Darrington, Washington. Better-quality hypocenters imply the activity occurs on a fault or fault zone striking N80°W ± 20°, dipping nearly south at 40° ± 15°, with a length along strike of at least 10 km and possibly 20 km or more. We term this feature the Darrington Seismic Zone (DSZ). Focal depths range between 3 and 15 km. A single-event and a composite focal mechanism show nearly pure thrust faulting with one nodal plane in agreement with the hypocenter pattern. P axes strike N20°W to N25°W, in accord with a regional stress direction due to relative motion of the Pacific and North American Plates. No mapped fault can be identified as the surface expression of the zone. The area of the DSZ is adequate to generate a magnitude 5+ earthquake should it rupture in a single event, and an ML 5.6±earthquake on 29 April 1945 in the Cascades ESE of Seattle demonstrates that crustal earthquakes having such magnitudes are possible beneath the western North Cascades. The DSZ is the first crustal seismogenic structure to be identified beneath the North Cascades.

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