SHN conducted an extensive fault rupture hazard investigation for a proposed hospital, a senior care facility, and a large residential subdivision on adjacent parcels in Willits. The study area is located on a Holocene age alluvial plain in very close proximity to the active Maacama fault.

Surface rupture hazard was addressed through a comprehensive subsurface investigation. The investigation was complicated by the depth of age-appropriate materials, which are some 25 feet deep beneath the site. To reach that depth, very large pits were excavated on the hospital and Senior Care sites to depths of 20 to 25 feet, and standard fault trenches were excavated on the floors of the deep pits, reaching depths up to about 32 feet Despite these challenges, the fault rupture investigations resulted in clearance of large areas sufficient to locate the hospital and senior care facilities.

The trenching for the residential subdivision used more traditional shallow trenches, but they had to span the entire width of the Alquist-Priolo Earthquake Fault Zone. Over 1,000 feet of trench were required to clear this area. Interpretation of the fault hazard on the residential subdivision site was complicated by a step-over in the fault zone. Reduced width exclusion zones were approved at this site because the fault was well-delineated and we were able to demonstrate an absence of secondary faulting.

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