Christopher B. DuRoss

Integration of Paleoseismic Data from Multiple Sites to Develop an Objective Earthquake Chronology: Application to the Weber Segment of the Wasatch Fault Zone, Utah

We present a method to evaluate and integrate paleoseismic data from multiple sites into a single, objective measure of earthquake timing and recurrence on discrete segments of active faults. We apply this method to the Weber segment (WS) of the Wasatch fault zone using data from four fault-trench studies completed between 1981 and 2009. After systematically reevaluating the stratigraphic and chro- nologic data from each trench site, we constructed time-stratigraphic OxCal models that yield site probability density functions (PDFs) of the times of individual earth- quakes. We next qualitatively correlated the site PDFs into a segment-wide earthquake chronology, which is supported by overlapping site PDFs, large per-event displace- ments, and prominent segment boundaries. For each segment-wide earthquake, we computed the product of the site PDF probabilities in common time bins, which emphasizes the overlap in the site earthquake times, and gives more weight to the narrowest, best-defined PDFs. The product method yields smaller earthquake-timing uncertainties compared to taking the mean of the site PDFs, but is best suited to earth- quakes constrained by broad, overlapping site PDFs. We calculated segment-wide earthquake recurrence intervals and uncertainties using a Monte Carlo model. Five surface-faulting earthquakes occurred on the WS at about 5.9, 4.5, 3.1, 1.1, and 0.6 ka. With the exception of the 1.1-ka event, we used the product method to define the earthquake times. The revised WS chronology yields a mean recurrence interval of 1.3 kyr (0.7-1.9-kyr estimated two-sigma (2σ) range based on interevent recurrence). These data help clarify the paleoearthquake history of the WS, including the important question of the timing and rupture extent of the most recent earthquake, and are essential to the improvement of earthquake-probability assessments for the Wasatch Front region.

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

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.

High-resolution seismic profiling reveals faulting associated with the 1934 Ms 6.6 Hansel Valley earthquake (Utah, USA)

The 1934 Ms 6.6 Hansel Valley, Utah, earthquake produced an 8-km-long by 3-km-wide zone of north-south−trending surface deformation in an extensional basin within the easternmost Basin and Range Province. Less than 0.5 m of purely vertical displacement was measured at the surface, although seismologic data suggest mostly strike-slip faulting at depth. Characterization of the origin and kinematics of faulting in the Hansel Valley earthquake is important to understand how complex fault ruptures accommodate regions of continental extension and transtension. Here, we address three questions: (1) How does the 1934 surface rupture compare with faults in the subsurface? (2) Are the 1934 fault scarps tectonic or secondary features? (3) Did the 1934 earthquake have components of both strike-slip and dip-slip motion? To address these questions, we acquired a 6.6-km-long, high-resolution seismic profile across Hansel Valley, including the 1934 ruptures. We observed numerous east- and west-dipping normal faults that dip 40°−70° and offset late Quaternary strata from within a few tens of meters of the surface down to a depth of ∼1 km. Spatial correspondence between the 1934 surface ruptures and subsurface faults suggests that ruptures associated with the earthquake are of tectonic origin. Our data clearly show complex basin faulting that is most consistent with transtensional tectonics. Although the kinematics of the 1934 earthquake remain underconstrained, we interpret the disagreement between surface (normal) and subsurface (strike-slip) kinematics as due to slip partitioning during fault propagation and to the effect of preexisting structural complexities. We infer that the 1934 earthquake occurred along an ∼3-km wide, off-fault damage zone characterized by distributed deformation along small-displacement faults that may be alternatively activated during different earthquake episodes.