GEOLOGIC DEVELOPMENT AND ONGOING ACTIVITY OF A LATE-HOLOCENE LONG-RUNOUT LANDSLIDE COMPLEX, NORTHWESTERN WASHINGTON

The Van Zandt Landslide Complex (VZLC) is high-mobility bedrock landslide located in the western foothills of the North Cascades of Washington, a region that is particularly susceptible to large slope failures because of its high relief, seismic activity, and abundant precipitation. Geomorphic mapping based on high-resolution lidar data indicates that the VZLC has multiple cross-cutting debris lobes with long runouts (H/L=0.16) typical of deep-seated catastrophic landslides. Despite recent tragedies like that at the nearby Oso slide (3/22/2014; 43 fatalities), our understanding of the mechanics, timing, and possible triggers of such landslides remains poor, particularly for bedrock slides. To help address this deficiency, we combine detailed geomorphic mapping and dating of the prehistoric lobes of the VZLC with data from a monitoring network of ongoing deformation in the headscarp region to assess possible triggers and future hazards at the site.

Basal AMS ¹⁴C dates from sediment cores of three surface ponds on the two youngest debris lobes indicate that both formed during or shortly before ca. 1270-1530 cal. yr B.P. (2-σ). Two additional ¹⁴C samples from distal deposits exposed in a stream bank indicate a possible precursor high-mobility slide sometime between ca. 3300-4300 cal yr B.P. Neither of these two intervals coincide with known paleoseismic events from local shallow-crustal faults (e.g., Boulder Creek Fault) or with other dated landslides in the region; both intervals do, however, overlap with known Cascadia megaquakes.

Tension fractures that disrupt soil and tear tree roots in the headscarp region indicate a potential for future large failures in the area. To test the activity of these fractures, we installed wire extensometers in three of the tension gaps; all of them have experienced significant (up to 1.7 cm) of both progressive and episodic displacement since installation in mid-October, 2015. Preliminary results indicate that the strain is dominantly rainfall-driven and that certain precipitation threshold conditions may influence the timing and magnitude of deformation. Four seismographs will test whether this activity produces shallow microseismicity. These results will provide crucial new constraints for hazard assessments of this poorly understood type of catastrophic landslide.