Geologic zoning of slope movements in western Oregon, U.S.A.

The Japanese approach to geologic zoning of slope movement types has useful application in the geologically diverse landscape of western Oregon, U.S.A. The slope movement regime of an area can be characterized quantitatively in terms of total area affected, size distribution, and velocity and frequency of movement for slope movements of different types. Slope movements in Tertiary sedimentary rocks of the Oregon Coast Range include a high frequency (number per square kilometer per year) and low average volume of debris avalanches, high frequency of debris torrents down first- to fourth-order channels, and large, deep-seated slope movement features that may experience some of their movement in brief pulses lasting hours to days. Tertiary volcanic rocks in the Cascade Range have a lower frequency of debris avalanches and torrents and the large, deep-seated slope movements are mainly of the earthflow type that move slowly for prolonged periods (months) during the wet season. Further quantification of slope movement regimes will aid in planning land use and erosion control programs and in studying sediment production and landscape evolution.