The West Valley fault zone, Salt Lake Valley, Utah - Paleoseismic summary and preliminary results from recent trenching

摘要

The West Valley fault zone (WVFZ) comprises a distributed system of intrabasin normal faults in northern Salt Lake Valley that are antithetic to the west-dipping Salt Lake City segment (SLCS) of the Wasatch fault zone. The WVFZ and SLCS both show evidence of recurrent Holocene surface faulting; however, the structural relation between the two fault zones is poorly understood. Consequently, the extent to which the WVFZ is seismogenically independent of or moves coseismically with the SLCS is uncertain. Extensive urban development and a typically shallow water table have presented challenges to trench studies of the WVFZ. Previous detailed paleoseismic investigations produced abundant cumulative displacement data but very limited data pertaining to the timing and displacement associated with individual earthquakes. A few radiocarbon ages from consultant trenches on the WVFZ suggest a similarity in the timing of two WVFZ earthquakes with the two most recent earthquakes on the SLCS; however, large uncertainty exists in the relation between the radiocarbon ages and earthquake timing. To obtain new timing and displacement data for individual earthquakes on the WVFZ, we excavated three trenches across two strands of the western part of the WVFZ (the Granger fault) in September 2010; this paper summarizes our preliminary results. The trenches exposed evidence of four or five surface-faulting earthquakes occurring after the 18-ka highstand of Lake Bonneville and having a per-event average vertical displacement of 0.5 m (1.6 ft). We are using ostracode biostratigraphy to evaluate the timing of earthquakes relative to various phases of the Bonneville lake cycle, and radiocarbon and luminescence analyses (results pending) will provide numerical constraints on earthquake timing. The WVFZ surface-faulting chronology will be compared with that of the SLCS to provide insight into the seismogenic relation of the two faults.