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Some Observations of Geotechnical Aspects of the February 28, 2001, Nisqually Earthquake in Olympia, South Seattle, and Tacoma, Washington

Jonathan D. Bray, Rodolfo B. Sancio, Ann Marie Kammerer, Scott Merry, Adrian Rodriguez-Marek, Bijan Khazai, Susan Chang, Ali Bastani, Brian Collins, Elizabeth Hausler, Douglas Dreger, William J. Perkins, and Monique Nykamp

A report sponsored by the National Science Foundation, Pacific Earthquake Engineering Research Center, University of California at Berkeley, University of Arizona, Washington State University, Shannon & Wilson, Inc., and Leighton and Associates March 8, 2001

A significant earthquake (M_w = 6.8) occurred in the Puget Sound area of western Washington at 10:54 a.m. on February 28, 2001. This intraslab subduction zone event, named the Nisqually earthquake, is a result of high-angle normal faulting due to downdip tension in the subducting Juan de Fuca Plate, and it is similar in mechanism to events that occurred on April 13, 1949 (Puget Sound event of Mw = 7.1) and on April 29, 1965 (Seattle event of M_w = 6.7). The hypocenter for the earthquake was preliminarily located at N47.1525° and W122.7197° at a depth of 52.4 km, and the causative fault plane has a strike of 357° and dips to the east at 69°.

The Nisqually earthquake produced strong ground shaking over a wide area and caused damage in the Olympia, Seattle, and Tacoma areas of Washington. No fatalities are directly attributable to the earthquake, but damage has been preliminarily estimated at $2 billion. Although the intensity of ground motions was not especially severe, dozens of buildings in the area have been red-tagged, and hundreds more have been damaged. Observations of liquefaction were widespread in parts of Olympia and South Seattle, and several significant lateral spreads, embankment slides, and landslides occurred. The relatively long duration of the event and the relatively low cyclic resistances of some of the fills in the area are likely causes for the significant liquefaction and ground failure observed for this event.

This preliminary report is based on a field reconnaissance of the geotechnical effects of the Nisqually earthquake in the Olympia, South Seattle, and Tacoma areas by a group of researchers and consultants during the period of February 28, 2001 to March 7, 2001. This effort did not intend to document all of the geotechnical effects of the Nisqually earthquake. Instead, our reconnaissance effort focused on the selected areas identified previously with the goal of developing well-documented case histories of liquefaction and ground failure and their effect on engineered systems.

Outline

NOTE: All photographs and maps presented at this web site, with the exception of those credited to outside sources, were developed by the authors of this report. Please provide this citation: "from Bray et al., 2001 - a NSF-PEER sponsored reconnaissance effort" when using materials downloaded from this web report.

Photos: By the Authors