GEOLOGIC DEVELOPMENT AND ONGOING ACTIVITY OF A LATE-HOLOCENE HIGH-MOBILITY BEDROCK LANDSLIDE COMPLEX, NORTHWESTERN WASHINGTON

Geomorphic mapping based on high-resolution lidar data indicates that the Van Zandt Landslide Complex (VZLC) has multiple cross-cutting debris lobes with long runouts 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 suggest a possible precursor high-mobility slide sometime between ca. 3300-4300 cal. yr B.P. (2-σ). Neither of these two intervals coincide with known paleoseismic events from local shallow-crustal faults or with other dated landslides in the region; both intervals do, however, overlap with known Cascadia megaquakes. A possible basal fluidized layer preserved as a clay-rich horizon may have enhanced the mobility of the largest debris lobe (~105 x 10⁶ m³; H/L= 0.16) and contains several embedded logs from which ¹⁴C samples are currently in process to provide further emplacement age constraints.

Tension fractures 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 2.4 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 microseismicity. These results will provide crucial new constraints for hazard assessments of this poorly understood type of catastrophic landslide in the Pacific Northwest.