SEISMIC TRENCH EXPOSURES OF PRE-HOLOCENE DEFORMATION FEATURES ADJACENT TO THE SOUTHERN WHIDBEY ISLAND FAULT ZONE, SOUTHERN SNOHOMISH COUNTY, WASHINGTON

Seismic hazard studies by the U.S. Geological Survey (USGS) in the Pacific Northwest using bare-ground LiDAR topography in forested terrain have revealed linear, northwest-striking, post-glacial scarps, spatially associated with aeromagnetic lineaments. Excavations across the scarps have exposed deformed and faulted glacial and post-glacial sediments. In 2004, the Southern Whidbey Island fault (SWIF) was identified by the USGS near the Brightwater Wastewater Treatment Plant site (BWTP), located in southern Snohomish County near its boundary with King County. Seismic trenches were excavated across portions of the site in between lineament 4 and the postulated lineament X in order to prevent the placement of certain chemical facilities over unknown seismic faults and to minimize the risk of a chemical spill or release during a seismic event.

Two sets of seismic trenches were constructed at the proposed site of the South Chemical Building and a seismic trench was excavated near the proposed site of the North Chemical Building (spanning beyond the planned buildings) to evaluate the fault-rupture hazard. The trench exposures, mapped by the authors, are part of geologic/seismic studies undertaken at the site since 2004. Based on detailed examination of the seismic trenches excavated at the BWTP site in June, July, and August 2006, we found no evidence of active faults that meet the 2003 IBC definition passing through, or within 50 feet of, the foundations of the two Chemical Buildings. The primary geologic evidence to support this conclusion consists of:

1. Unfaulted and undeformed stratigraphy deposited in a subglacial or recessional outwash environment no younger than 16,570 cal yr B.P. (Trenches South 1, South 1a, South 1b, South 1c, South 2a, and South 2b) 2. Uniform dense conditions in diamict adjacent to fractures and fracture-filling materials (North Trench) 3. Continuous glacially scoured ridges depicted in LiDAR images of the landscape adjacent to the site 4. Absence of LiDAR scarps in the glacial landscape adjacent to the site.

Geologic evidence of minor deformation, cracking, liquefaction, and small glacial faults were observed at several locations. These pre-Holocene deformation features were overlain by unfaulted and undeformed stratigraphy in the trenches excavated for the South Chemical Buildings.