Holocene Fault Reactivation and Landscape Evolution in the Eastern Cascades, WA

Significant uncertainty remains in how and where modest, distributed shortening is accommodated throughout the eastern Cascade Range in Washington State. Using lidar imagery, I identified a ~5 km long lineament in Swakane Canyon near Wenatchee, roughly coincident with a strand of the Entiat fault. Topographic profiles show the lineament is formed by a southwest-side-up break in slope with between 2 and 42 m of vertical separation of the ground surface. Trenching reveals deformed saprolite and colluvium consistent with southwest-side-up folding caused by blind reverse faulting at depth. Radiocarbon and luminescence dating combined with stratigraphic constraints suggest up to three Holocene earthquakes on the southern Entiat fault with an average vertical separation of ~0.7 m per event. A ground penetrating radar survey reveals up to 3.8 m of cumulative vertical separation of the basal trench unit, consistent with up to six earthquakes on this structure. Rupture of the 5 km fault segment in Swakane Canyon could produce an earthquake of M5.9 or M6.8 based on rupture length or average slip, respectively. Holocene earthquakes on the southern Entiat fault could suggest reactivation of the entire Entiat fault as well as other bedrock faults in the eastern Cascades. Low rates of strain accumulation, as indicated by GPS surface velocities, likely result in long earthquake recurrence intervals. Although active erosion and slow strain rates lead to a subdued geomorphic expression of recent deformation, this study shows that the Entiat fault is an active, seismogenic structure that should be considered in regional seismic hazard analyses