A.15 Puget Sound Fault Zones, Focusing on the South Whidbey Fault Zone and Darrington-Devils Mountain Fault ZoneBrian Sherrod

Picture of a LiDAR image showing a large landslide and fault scarpIn this class, we will take a look at active faults around the Puget Lowland. These faults all have evidence of producing large earthquakes in the recent geological past. We will examine geological evidence that point to these past earthquakes at several sites. We also will examine high-resolution images produced from LiDAR surveys that help geologists doing field studies to document this evidence.

Through this careful examination of active faults, we now know that at least 13 active faults in the Puget Lowland produced 28 large earthquakes (magnitude 7 or larger) over the last 15,000 years. More importantly, most of these earthquakes occurred in the last 4000 years.  This suggests that large earthquakes strike our region on average about every 200 years. 

Note about photograph:This is an oblique LiDAR image showing a large landslide and fault scarp - both were produced during the largest historic shallow earthquake in the Pacific Northwest on December 14, 1872 near Entiat, Washington. Fault movement during the earthquake offset the ground surface and produced a fault scarp, seen in the LiDAR image as the dark shadow running diagonally from the image center to the lower left side