GEOLOGIC MAPPING AND PALEOSEISMIC INVESTIGATIONS OF THE WASHINGTON FAULT ZONE, WASHINGTON COUNTY, UTAH, AND MOHAVE COUNTY, ARIZONA (MP 15-6)

The Paleoseismology of Utah series makes the results of paleoseismic investigations in Utah available to geoscientists, engineers, planners, public officials, and the general public. These studies provide critical information regarding paleoearthquake
parameters such as earthquake timing, recurrence, displacement, slip rate, fault geometry, and segmentation, which can be
used to characterize potential seismic sources and evaluate the long-term seismic hazard of Utah’s Quaternary faults.
This Miscellaneous Publication presents the results of four individual investigations undertaken to acquire new geologic and
paleoseismic data on the Washington fault zone in Utah and northernmost Arizona. (1) New 1:24,000-scale geologic mapping provides improved information on the location and length of young surface ruptures and the relative ages of displaced
surficial deposits along the Washington fault zone. A principal result of this new mapping is that the former Northern section
of the Washington fault zone has been subdivided into the newly defined Fort Pearce and Washington Hollow sections. (2)
A detailed paleoseismic trenching investigation of a scarp formed on a latest Quaternary alluvial fan near Dutchman Draw
in Arizona provides new information on paleoearthquake timing, displacement, and recurrence necessary for evaluating the
seismic hazard presented by the Fort Pearce section to the St. George metropolitan area. (3) Trace element and major oxide
geochemical correlation and radiometric dating (40Ar/39Ar) of mafic volcanic flows displaced across the fault zone in Arizona
provide long-term (early to middle Quaternary) vertical slip-rate estimates for the Fort Pearce and Sullivan Draw sections of
the fault. (4) A geotechnical consultant’s surface-fault-rupture-hazard investigation conducted for the Utah Department of
Transportation provides new information on fault locations, number of earthquakes, and displacement per earthquake on the
central part of the Fort Pearce section. Based on the results of this investigation, the Utah Department of Transportation relocated two proposed elevated interchange structures and several hundred meters of roadway to avoid risk from future surface
rupture.
Determining the paleoseismic parameters of the Washington fault zone is critical to understanding the segmentation of the
northern, urbanized part of the fault zone; for refining probabilistic earthquake-hazard assessments; and improving earthquake-hazard evaluations for the St. George metropolitan area, all of which help reduce earthquake-related risks to the region’s residents.