PRELIMINARY PALEOSEISMOLOGY OF THE NORTHERN NEPHI SEGMENT, WASATCH FAULT ZONE, UTAH

The Utah Geological Survey excavated two paleoseismic trenches across the northern part of the Nephi segment of the Wasatch fault zone in the eastern Basin and Range Province of central Utah. The 42-kilometer-long Nephi segment marks a major west step in the southern trace of the Wasatch fault zone, and is divided into two strands separated by a 5-kilometer-wide right step: the 17-kilometer-long northern (Santaquin) strand and the 25-kilometer-long southern (Nephi) strand. Previous paleoseismic studies along the southern strand identified three post-mid-Holocene fault ruptures, but poorly constrained the timing of each event. In the absence of paleoseismic data for the northern strand, geologic mapping and morphologic fault-scarp analyses confirmed multiple Holocene earthquakes, and raised the possibility of rupture spill-over to the northern Nephi segment from the Provo segment to the north. The new trenches on the northern strand, in conjunction with trenches recently excavated on the southern strand by the U.S. Geological Survey, will allow the fault-rupture parameters for the individual fault strands and the overall earthquake behavior of the Nephi segment to be resolved.

Our trenches were excavated about 2 kilometers east of the town of Santaquin below the elevation of the Bonneville shoreline, and exposed evidence for faulted Holocene alluvial-fan deposits and a paleosol buried by distinctive fault-scarp colluvium. Units correlated across the fault zone indicate that 2.9 +/- 0.2 meters of vertical displacement accompanied the youngest surface-faulting earthquake at the site. Fault displacement is distributed across a 3-5-m-wide deformation zone, which includes a main normal fault dipping 65-80 degrees northwest and several steeply dipping synthetic and antithetic faults. We did not expose evidence for an earlier earthquake in the trenches, possibly due to a rapid alluvial-fan sedimentation rate or long earthquake return period. Our new paleoseismic data being developed have important implications for refining the timing, displacement, and extent of surface-faulting earthquakes on the Nephi segment, and for understanding fault segmentation and earthquake hazards along the southern Wasatch fault zone.