Geologic Development and Ongoing Activity of the Van Zandt Landslide Complex, Northwest WA, USA

Geomorphic mapping based on high-resolution lidar data indicates that the Van Zandt Landslide Complex (VZLC) has multiple 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 do overlap with a known Cascadia megaquake (event T4). 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. To evaluate ongoing retrogressive block sliding in the headwall region, we installed wire extensometers in three prominent tension gaps and tracked their movement over an 18-month period between 10/15 and 4/17; all three sites experienced measurable 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 could pose a serious hazard to people and property in the valley below.