INVESTIGATING CONTINUED DEFORMATION IN THE HEADSCARPS OF TWO LONG-RUNOUT LANDSLIDES, NOOKSACK DRAINAGE, NORTHWEST WASHINGTON

Lidar data reveal a remarkable concentration of large deep-seated Holocene landslides in the Nooksack River basin, NW Washington. Our study focuses on two particularly large, long-runout landslides: the Van Zandt Landslide Complex (VZLC) and the Middle Fork landslide (MFL). Both slides originate in steeply dipping Chuckanut Formation, a highly deformed and friable Eocene alluvial deposit comprising thick sandstone layers interbedded with weak layers of shale and coal. Both slides failed catastrophically along such weak beds, resulting in height-to-length runout ratios of 0.14 and 0.23, respectively. Both slides exhibit hummocky debris-fields, but the slightly more muted relief of the MFL debris field suggests it may be older than the ~1400 cal yr BP age of the VZLC (Malick, 2017). The source headwalls of both slides exhibit sharply defined parallel ridges and troughs that extent behind the headscarps. These sharp ridges indicate ongoing retrogressive slip of the bedrock, potentially presaging future collapse events. To monitor displacement of these features on VZLC, we monitored extensometers installed in fresh tension cracks that are associated with several of these ridges. The instruments recorded up to 3.2 cm of extension over the last six months. Both these short-term and other longer-term records indicate that the movement in these fractures is highly sensitive to precipitation, suggesting deformation is friction-limited. To augment the extensometer measurements, we are currently testing the viability of using Structure-from-Motion (SfM) with ground-based imagery to detect 3-D movement of one of these fractures. Initial investigations of the headwall of MFL have yet to yield similar tension fractures, although recent localized rockfall and a well-defined bench on the north side of the source area suggest progressive deformation similar to that at VZLC is occurring. If fractures are located, we plan to install an additional extensometer at MFL. The latest results of our monitoring will be presented at the meeting.