Ground Reconnaissance of Landslide Activity in Seattle, Washington

U.S. Geological Survey geologists Alan Chleborad and Rex Baum conducted reconnaissance on January 8, 9, and 10, 1997, of landslides in the Seattle area. The reconnaissance was conducted to determine the extent and type of landslide activity that resulted from storms the previous week, on or about New Years Eve and New Years Day. News reports suggested that slides occurred mainly in and north of Seattle. Accounts from local residents indicated that most of the slides occurred following heavy rain on top of a foot or more of wet snow.

Landslides were most abundant along the bluffs of Puget Sound and bluffs overlooking Lake Washington. The highest concentrations of landslides occurred in the West Seattle, and Magnolia areas. Slides were also abundant on bluffs overlooking Lake Union and Portage Bay. A few slides occurred along the I-5 corridor and at scattered locations elsewhere in the city.

Shallow slides were by far the most common type; many of the shallow slides had mobilized into debris flows. A number of the debris flows had partially blocked roads or impacted structures. Structural damage from the flows ranged from slight to severe. At least two homes in the Magnolia bluffs area were damaged by debris flows.

Deep-seated slides and slumps, though relatively uncommon, caused severe damage to land and structures. A deep slide in the Magnolia area damaged the back of one house and endangered several others. Part of this same slide mobilized into flows that have damaged supports for the Magnolia bridge. Another deep slide in the Magnolia area destroyed part of a residential street and four homes sitting on the slide have been heavily damaged. A deep slide in the extreme northwest part of Seattle has damaged several homes. Debris flows from the front of this slide have deposited debris on the railroad, which had been cleared by the time of the USGS visit.

Area map of Seattle, Washington, showing approximate locations of landslides depicted in photographs.

Figure 1 A landslide blocking a street in West Seattle. (190K)

Figure 2 Head of landslide in Magnolia area that damaged a house and endangered several others. (146K)

Figure 3 Debris flows from deep-seated slide (previous photo) have damaged supports of the Magnolia bridge. (193K)

Figure 4 A home in the Magnolia area that was struck by a debris flow. Note damage where debris collapsed the front wall and flowed into the house. (124K)

Photographs by Alan F. chleborad, USGS.

Rex L. Baum U.S. Geological Survey Geologic Hazards team Landslide Hazards Group email: baum@usgs.gov (baum@gldvxa.cr.usgs.gov) P.O. Box 25046, MS 966 Lakewood, CO 80225

Alan F. Chleborad U.S. Geological Survey Geologic Hazards team Landslide Hazards Group email: chleb@usgs.gov (chleb@gldvxa.cr.usgs.gov) P.O. Box 25046, MS 966 Lakewood, CO 80225