Michael D. Hylland

Fault segmentation: New concepts from the Wasatch Fault Zone, Utah, USA

The question of whether structural segment boundaries along multisegment normal faults such as the Wasatch fault zone (WFZ) act as persistent barriers to rupture is critical to seismic hazard analyses. We synthesized late Holocene paleoseismic data from 20 trench sites along the central WFZ to evaluate earthquake rupture length and fault segmentation. For the youngest (<3 ka) and best-constrained earthquakes, differences in earthquake timing across prominent primary segment boundaries, especially for the most recent earthquakes on the north-central WFZ, are consistent with segment-controlled ruptures. However, broadly constrained earthquake times, dissimilar event times along the segments, the presence of smaller-scale (subsegment) boundaries, and areas of complex faulting permit partial-segment and multisegment (e.g., spillover) ruptures that are shorter (~20–40 km) or longer (~60–100 km) than the primary segment lengths (35–59 km). We report a segmented WFZ model that includes 24 earthquakes since ~7 ka and yields mean estimates of recurrence (1.1–1.3 kyr) and vertical slip rate (1.3–2.0 mm/yr) for the segments. However, additional rupture scenarios that include segment boundary spatial uncertainties, floating earthquakes, and multisegment ruptures are necessary to fully address epistemic uncertainties in rupture length. We compare the central WFZ to paleoseismic and historical surface ruptures in the Basin and Range Province and central Italian Apennines and conclude that displacement profiles have limited value for assessing the persistence of segment boundaries but can aid in interpreting prehistoric spillover ruptures. Our comparison also suggests that the probabilities of shorter and longer ruptures on the WFZ need to be investigated.

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.