USING LIDAR IMAGERY TO IDENTIFY YOUNG EARTHQUAKE FAULT SCARPS ON THE TACOMA FAULT ZONE BENEATH DENSE FOREST OF THE PUGET LOWLAND, WASHINGTON, USA

Prehistoric earthquakes on the Tacoma fault zone in the southern Puget Lowland were first identified in the early 1990s through geologic mapping of shorelines uplifted suddenly during very large (magnitude 6-7) earthquakes. In the past decade, field research has focused on investigations of the stratigraphy beneath fault scarps identified and mapped using high-resolution airborne Light Detection And Ranging (LiDAR) topographic imagery.

A major problem in studying prehistoric earthquakes in the Puget Lowland is the thick forest and undergrowth that hide small, discontinuous scarps. These scarps are the most common evidence of past large earthquakes caused by upper plate faults in the region. We work with bare-earth LiDAR to produce digital elevation models (DEM) of the ground surface on which fault scarps are readily indentified. To further enhance potential scarps, we generate two hillshade layers of the same dataset using different sun angles, azimuths, and transparencies, and superimposed them. This technique commonly reveals details of the fault scarps that are not visible in single hillshade layers.

After scarps are identified, we transfer waypoints from the DEMs to handheld GPS units to help locate scarps in the field. With the location verified to within 3-4 m, even 0.5-m-high scarps can be located beneath dense vegetation. We then excavate 15- to 35-m-long trenches perpendicular to the scarps to expose, map, and date the stratigraphy beneath the scarps. Once we identify the fault and reconstruct the sequence of faulting from the stratigraphy, we collect organic material for radiocarbon dating to bracket the ages of surface-rupturing earthquakes.