GEOLOGIC DEVELOPMENT AND ONGOING ACTIVITY OF THE VAN ZANDT LANDSLIDE COMPLEX, NORTHWEST WASHINGTON

Geomorphic mapping based on field reconnaissance and high-resolution lidar data indicates that the Van Zandt Landslide Complex in northwestern Washington comprises three distinct crosscutting debris lobes (up to 51.4 x 10⁶ m³) with long runouts (H/L= 0.14; 0.21) typical of catastrophic rock avalanches. AMS ¹⁴C dates from in situ logs and lake sediment cores yield overlapping ages for emplacement of Debris Lobe 2 (1330-1285 cal. yrs. B.P) and Debris Lobe 3 (1300-1285 cal. yrs. B.P.) Although Debris Lobe 3 overlies a portion of Debris Lobe 2, it is possible that emplacement of the two deposits was nearly synchronous or in rapid succession. The debris lobes are not contemporaneous with any known paleoseismic events from local shallow-crustal faults but their emplacement does overlap with a known Cascadia megaquake. Abundant transverse surface fractures, a distal “splash zone”, as well as debris exposures showing basal mixing, soft-sediment deformation, and substrate injection features provide evidence for significant rock avalanche-substrate interaction and mobility-enhancing liquefaction. In addition, the headwall region is experiencing ongoing retrogressive block sliding. We evaluated deformation in three prominent tension fractures with wire extensometers that tracked their movement over an 18-month period between 10/15 and 4/17; all three sites experienced displacement (up to 5.7 cm total), with progressive movement largely accelerating and decelerating in response to short- and long-term precipitation conditions. Although recorded strain rates were relatively slow during this period, incipient detachment scarps across the headwall involve slabs with a total volume of ~35.2 x 10⁶ m³, the potential release of which poses a serious hazard to people and property in the valley below. The potential response of these incipient detachments to seismic events represents a major uncertainty.