Late Holocene Paleoseismic History and Segmentation of the Central Wasatch Fault Zone, Utah, USA

Abstract:

Late Holocene earthquake timing and displacement data from 20 trench sites along the 260-km-long central Wasatch fault zone (WFZ) clarify whether structural complexities that define the five central segments act as persistent barriers to rupture. From these sites we compiled earthquake-timing probability density functions for 69 earthquakes younger than 7 ka. However, we rely mostly on earthquakes that are less than 3 ka because they have the smallest timing uncertainties and thus are the most suitable for discerning differences in earthquake timing along the fault and assessing fault segmentation. Along the northern segments, clear differences in earthquake timing across prominent structural and topographic boundaries (e.g., bedrock salients and areas of complex faulting) are consistent with models of a segmented fault. In contrast, along the southern segments, overlapping earthquake times and the greater structural complexity of the fault make it difficult to determine rupture extent. On the central WFZ as a whole, older, less-well constrained earthquake times, per-event vertical displacements, and the presence of smaller-scale (intra-segment) boundaries permit partial-segment, spillover, and multi-segment ruptures that are shorter (~20­–40 km) or longer (~100 km) than the defined lengths of the segments (35–59 km). We present a segmented model for the central WFZ that includes 24 ruptures since ~7 ka and yields mean estimates of vertical displacement (1.7–2.6 m), recurrence (1.0–1.3 kyr), and vertical slip rates (1.3–2.0 mm/yr) for the segments. However, for hazard analysis, additional rupture scenarios that include segment-boundary spatial uncertainties, earthquakes of varying rupture length allowed to float along the fault, and multi-segment ruptures are necessary to fully address epistemic uncertainties in segmentation and rupture length.