GEOLOGIC CONTROLS ON SITE RESPONSE AND GROUND FAILURES IN SEATTLE DURING THE 2001 NISQUALLY EARTHQUAKE

Ground failures during the February 28, 2001 Nisqually earthquake were similar to those reported after the previous Benioff-zone earthquakes in 1965 and 1949, and they included landslides, settlement, and lateral spreads. Existing subsurface geologic data were reviewed to evaluate if the measured site response variations could be explained by corresponding variations in the underlying Quaternary deposits. Despite limited data, clear spatial patterns have emerged. Although the observed ground-failure patterns do not always correlate with the distribution of surficial geologic materials, some of the site response variations are consistent with subsurface geologic variability within the upper 100 m. Evaluation of existing borehole data provides a means to study non-linear site responses in select areas, such as where there are marked differences in the thickness of certain extensive subsurface units like the overconsolidated Esperance sand.

Ground failures and high amplification were noted in weak surficial geologic materials. Liquefaction-related settlement and lateral spreading, by far the most common forms of ground failure in these soils, were concentrated in the northern Duwamish valley, and in the northern half of Boeing Field. Smaller and more widely scattered features were recognized elsewhere in the Duwamish valley, on the now-exposed pre-1911 shorelines of Lake Washington and Lake Union, and in a few filled wetlands in north Seattle. Virtually all observed liquefaction features occurred within deposits of loose Holocene alluvium or artificial fill; perhaps more remarkable, however, are the many areas of similar deposits within the city that did not liquefy.