OVERVIEW OF RECENT PALEOSEISMIC STUDIES ON THE BRIGHAM CITY, WEBER, AND SALT LAKE CITY SEGMENTS OF THE WASATCH FAULT ZONE, UTAH

Three decades of paleoseismic study on the central segments of the Wasatch fault zone (WFZ) in central Utah have revealed abundant evidence of Holocene surface-faulting earthquakes. However, major questions remain regarding the timing and rupture extent of the most recent earthquake (MRE) on the Brigham City segment (BCS), the correlation of earthquakes between sites on the Weber segment (WS), and the rupture history of the prominent East Bench fault (EBF) of the Salt Lake City segment (SLCS). To address these questions, we excavated trenches at six sites on the BCS, WS, and SLCS between 2007 and 2010. The elapsed time since the MRE on the BCS is 2.1 ky, which is more than 50 percent longer than the mean Holocene earthquake recurrence interval for the BCS of about 1.3 ky. We excavated trenches at three sites on the BCS to confirm the previously documented 2.1 ka time for the MRE. Two sites on the northern BCS indicate that the MRE is slightly older than 2 ka, but on the southern BCS near Pearsons Canyon, the youngest earthquake occurred at 1.1–1.3 ka, which we interpret to be coseismic rupture from a large, 1.2-ka earthquake on the adjacent WS. On the WS, we integrated earthquake-timing data from our Rice Creek site with the results of previous paleoseismic studies and defined five earthquakes on the WS that occurred between 5.9 and 0.6 ka. This yields a mean middle Holocene recurrence interval of 1.3 ky. Our results refine the earthquake history of the WS, including the correlation of events between trench sites and answer important questions about the timing and rupture extent of the MRE. On the EBF of the SLCS, two trenches at the Penrose Drive site exposed evidence of five and possibly six earthquakes that are younger than the Provo phase of Lake Bonneville (17–14 ka). Pending radiocarbon and luminescence ages will help define the earthquake chronology. Collectively, these new paleoseismic data reduce uncertainties in the timing and extent of paleoearthquakes on the WS, BCS, and SLCS, and reveal a complex history of Holocene surface faulting on the WFZ that includes at least one rupture across a segment boundary. Our results have important implications for segmentation of the WFZ and earthquake-probability analyses, such as the time-dependent forecast for the central WFZ being developed by the Working Group on Utah Earthquake Probabilities.