Synchronous Ruptures along a Major Graben‐Forming Fault System: Wasatch and West Valley Fault Zones, Utah

Abstract

Abstract The Salt Lake City segment (SLCS) of the Wasatch fault zone and the antithetic West Valley fault zone (WVFZ) form a large, Holocene‐active, intrabasin graben in northern Salt Lake Valley, Utah. We integrate previous paleoseismic data with new data from recent trench investigations and compare earthquake timing and displacement for both the master and antithetic faults of this major graben‐forming system to address whether the WVFZ ruptures simultaneously with the SLCS or is a separate, independent source of earthquakes. Nine SLCS surface‐faulting earthquakes postdate the Lake Bonneville highstand (∼18 ka); however, the record is most complete since ∼14 ka, yielding latest Pleistocene and Holocene mean recurrence estimates of ∼1.5 ky and ∼1.3–1.6 ky, respectively. Six post‐Bonneville‐highstand WVFZ earthquakes yield a mean recurrence of ∼2.0–3.6 ky; however, we consider the WVFZ earthquake record incomplete because of distributed faulting and limited paleoseismic data. Five of six WVFZ earthquakes have mean and 2 σ times that are very similar to those of SLCS earthquakes. WVFZ earthquake W5 lacks an apparent temporal correlation with an SLCS earthquake but occurred during a period for which the SLCS chronology may be incomplete. Mean WVFZ per‐event vertical displacement (∼0.5 m) is 26%–42% of that for the SLCS (∼1.2–1.9 m), consistent with that predicted by previous mechanical modeling of antithetic faulting triggered by slip on a listric master fault. We conclude that large WVFZ earthquakes are likely synchronous with, or triggered shortly after, SLCS surface‐faulting earthquakes. Although earthquake‐timing uncertainties preclude determining an unequivocal coseismic link between the WVFZ and SLCS, structural models suggest a high likelihood for synchronous rupture. These results have important implications for forecasting earthquake probabilities in complex normal‐faulting environments.