A COMPARISON OF PRECIPITATION TO LANDSLIDE FREQUENCY AND EXTENT IN THE OLYMPIC MOUNTAINS, WASHINGTON

In the Olympic Mountains of northwestern Washington, landsliding is responsible for setting the pace of erosion and for delivering the vast majority of hillslope sediment to river networks, yet it remains unknown whether volumetrically, the majority of landslides are triggered by earthquake-induced ground shaking or by high-magnitude precipitation events. To test the hypothesis that precipitation is a primary driver of landslide activity, the density and total area of landslides across a 1250 km²swath of the Olympic Mountains were compared to gridded precipitation data acquired from the PRSIM climate group for the period 1981-2010. Using all available satellite imagery in Google Earth, 800+ landslides were mapped and exported to ArcGIS for geospatial analysis. Landslide density and area were compared to annual precipitation totals across the mapped area, which, as a result of the rain shadow of the Olympic Mountains, has a sharp precipitation gradient of 600 cm/yr near the range crest to less than 200 cm/yr on the eastern side of the range. Linear regression of landslide, precipitation, and topographic datasets shows no correlation between landsliding and mean elevation or slope, but reveals a significant moderate correlation (r > 0.5) between mean annual precipitation and landslide density as well as mean annual precipitation and total landslide area.

Overall, the lack of large historic earthquakes in the vicinity of the Olympic Mountains makes it difficult to assess the relative contributions of tectonics and climate to total erosion, but our results suggest that climate is an important driver of landscape evolution during periods of seismic quiescence. Over longer time scales, it is likely that both tectonic and climatic factors play considerable roles in balancing uplift of the Olympic Mountains via landsliding, but the remaining uncertainty in triggering mechanisms necessitates a closer look at the bigger picture of landslide cause and effect in the state of Washington and beyond.