Nisqually Earthquake, Washington February 28, 2001

The Geologic Hazards Team from Golden, CO
Post-Nisqually-Eathquake Investigations—Alan Nelson

Evidence of liquefaction, such as sand blows and lateral spreads, commonly occurs in young, sandy alluvial sediments in river valleys. Nelson spent the late afternoon of 1 March in the northernmost Kent Valley just south of Tukwila looking for signs of liquefaction. No evidence was found, with the possible exception of a broken water main that was being repaired near the Green River. Nelson and David Lidke (Earth Surface Processes Team) plan to canoe a 20-km-long reach of the Green River bewteen Tukwila and Auburn on 4 March to search for liquefaction features.

Although liquefaction features were reported on the day of the earthquake in Seattle, more than 45 km to the northeast of the epicenter, little surface evidence of strong ground motion was reported in the area northwest of the epicenter. For this reason, Nelson assisted Michael Polenz, a geologist with the Washington Department of Natural Resources, in a marine reconnaissance of landslides and shoreline features northwest of Olympia. Features along shorelines and bluffs from Boston Harbour (10 km north of Olympia) through Squaxin Passage and Hammersley Inlet to the head of Oakland Bay (25 km northwest of Olympia) were carefully examined for signs of landsliding and liquefaction. The gravelly sand spit at Cooper Point (2 km northwest of Boston Harbour), which failed during the 1949 earthquake, showed no signs of lateral spreading at low tide. Evidence of earthquake-induced movement of active landslides south of Hunter Point (4 km northwest of Boston Harbour) was inconclusive. The Carlyon slide west of Hunter Point, which has received much media attention in the past year, did move at least a few tens of centimeters, as evidenced by a freshly fractured bulkhead at the toe of the slide.

But elsewhere along the many tens of kilometers of shoreline on both sides of Hammersley Inlet and Oakland Bay almost no evidence of strong ground motion was observed. Exceptions include a 2-m-long section of 1-m-high bulkhead composed of boulders in the western part of Hammersley Inlet that apparently failed during the earthquake and a few <1-m-diameter blocks of till that spalled from vertical cliffs in the outer part of Hammersley Inlet. Many much larger recently fallen blocks predate the earthquake. Similarly, many large and small landslides that appear to have moved in recent years showed no sign of movement in the past week. Tidal marshes at the head of Oakland Bay (mouth of Deer Creek) and adjacent to the Bayshore Golf Club (halfway up the bay) showed no evidence of liquefaction despite the presence of sand within 1 m of the surface. The only damage suffered by the Golf Club was a small crack in the patio.