Photographs taken last week at the "Frazier" landslide in the Willamette National Forest outside of Oakridge, OR - that has shut down the Union Pacific railroad line (and Amtrak) through the Cascades since 19 January - suggest a 1992 clearcut and logging road may have played a role in the slide.
According to geologists, the slide area was the site of an "ancient landslide that reactivated."
On the ground photos document that a portion of the massive slide broke off directly beneath a logging road (known to disrupt soils and drainage) while nearly half of the 1992 Forest Service clearcut / 1995 replant sloughed off in the slide.
According to the Forest Service's "preliminary geologic investigation" on 30 January, hired geologist Doug Shank claims there is "no evidence" to link the clearcut or logging road with the landslide.
Science suggests otherwise:
Exhibit A: "Timber harvest in sensitive areas has also been associated with increased incidence of mass movement. Clearcut harvest and/or slash burning on steep slopes may increase failure rates from two to forty times over rates on undisturbed sites."
-Cumulative Effects of Forest Practices in Oregon, Oregon Department of Forestry, 1995.
Exhibit B: 1975. F.J. Swanson and C.T. Dyrness concluded that: "slide erosion from clear-cut areas in the unstable zone has totaled 6,030 M3/KM2, or 2.8 times the level of activity in forested areas of the unstable zone... Along road rights-of-way, slide erosion has been 30 times greater than on forested sites in the unstable zone."
Exhibit C: 1981. M.S. thesis (OSU, geology) Daniel A. Marion said: "Clearcutting and road construction appears to strongly affect landslide frequency and location. Landslides occur 24 and 253 times more frequently (relative to forest rate) in clearcuts and road areas, respectively."
Exhibit D: 1986. M.S. thesis (OSU, geology) Margaret McHugh said: "...harvested areas showed an increase in failure rate of 7 times and roaded areas an increase of 48 times that of forested terrain."