DENDRO-GEOMORPHOLOGICAL ANALYSIS OF THE SLATE CREEK LANDSLIDE, ALASKA

Mass movements are costly and deadly natural hazards that will accelerate and become more widespread as high latitudes continue to warm. The Parks Highway in Alaska is a critical transportation corridor linking Fairbanks and Anchorage. Both the highway and a fiber-optic cable traverse the foothills of the Alaska Range that are underlain by discontinuous permafrost. Many mass movements are now occurring on these hillslopes. This research focuses on the Slate Creek landslide, whose advancing toe is encroaching upon the Parks Highway. Much of the slide is forested, and the dendrochronology of these trees provides a chronology of the landslide. In particular, the occurrence and thickness of reaction wood in leaning trees provide the timing and relative intensity of past slope instability. In addition, living trees are being split from the bottom up due to the expansion of transverse cracks allowing us to identify rates of movement. This data is obtained by cutting cross sections of split trees at succeeding heights, which provide tree-ring dating of ‘splitting scars’ that enable an estimate of the width of transverse cracks at different times in the past. We use a combination of real-time kinematic GPS, aerial and LiDAR imagery, historical climate data, and dendrochronology to reconstruct when this landslide moved in the past, and to identify what climatic conditions enhanced the rates of movement in order to assess the hazard this and nearby landslides pose to the road corridor. Tree-ring estimates of split spruce trees indicate that since 1970 the main body of the landslide has moved from 0.2 to 28 cm/year. Based on this data and the number of leaning trees (n=45), the mid- to late 1970s was a period of enhanced landslide movement. We tentatively correlate increased movement of the Slate Creek landslide with the abrupt shift to warmer air temperatures that accompanied the regime shift in the Pacific Decadal Oscillation occurring in 1976.