HR: 15:25h
AN: H43G-08    [Abstracts]
TI: Quantifying topographic, structural, and lithologic controls on large landslides
AU: * Roering, J J
EM: jroering@uoregon.edu
AF: University of Oregon, Dept. of Geological Sciences, Eugene, OR 97403-1272 United States
AU: Kirchner, J W
EM: kirchner@seismo.berkeley.edu
AF: University of California, Berkeley, Dept. of Earth and Planetary Sci., Berkeley, CA 94703-4767 United States
AU: Dietrich, W E
EM: bill@seismo.berkeley.edu
AF: University of California, Berkeley, Dept. of Earth and Planetary Sci., Berkeley, CA 94703-4767 United States
AB: The occurrence of large landslides often reflects material properties that favor slope instability. Uncertainty associated with analyses of individual bedrock landslides is often significant and we lack the ability to test and apply mechanistic slope stability criteria at the regional scale. In the Oregon Coast Range (OCR), deep-seated landslides within the gently folded Tyee Formation (Eocene deltaic-submarine ramp sediments) have been recognized, but their relationship to bedrock properties is poorly constrained. We developed an automated algorithm that uses the topographic signature (specifically the relationship between curvature and gradient) of deep-seated landslides to map their distribution. In contrast to steep and highly dissected terrain frequently identified as characteristic of the OCR (which exhibits steep, planar sideslopes and highly curved, low-gradient ridgetops and valleys), terrain prone to large landslides tends to have low values of both drainage density and curvature, and gradients that cluster between 0.16 and 0.44. Our analysis indicates that the distribution of failure-dominated terrain in our 10,000-km2 study area is influenced by systematic variations in sedimentary facies and bedrock structure. The fraction of terrain altered by large landslides (>100,000 m3) varies from 5% in the sand-rich (delta-slope and proximal ramp facies) southern section of our study area to ~25% in the north (distal ramp facies), coincident with an increase in the thickness of siltstone beds and a decrease in the sandstone:siltstone ratio. Structural controls are superimposed on facies-related variations as deep-seated landslides are frequently found on slopes whose downslope aspect corresponds to the bedrock dip direction. For 1516 strike and dip measurements in our study area, we calculated the fraction of proximal terrain (< 2.5 km) altered by deep-seated landsliding. In the sand-rich southern region, the proportion of proximal slide-dominated terrain increases modestly with bedrock dip; an increase in dip angle from 0ø to 16ø corresponds to a change from 1% to 9% in the fraction of local terrain shaped by landslides. In the silt-rich northern region, terrain altered by deep-seated landsliding is pervasive and an increase in dip from 0ø to 16ø corresponds to a change in the fraction of slide-prone terrain from 10% to 28%. This pattern likely reflects a bedrock-driven change in the mechanical requirement for instability; the increased frequency of weak, siltstone innerbeds in the northern region favors instability even where bedrock dips gently. Given structural and lithologic data, our calibrated model enables us to predict the occurrence of large landslides and their potential impacts.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting