Relaxing Segmentation on the Wasatch Fault Zone: Impact on Seismic Hazard

Abstract

The multisegment Wasatch fault zone is a well-studied normal fault in the western United States that has paleoseismic evidence of recurrent Holocene surface-faulting earthquakes. Along the 270 km long central part of the fault, four primary structural complexities provide possible along-strike limits to these ruptures and form the basis for models of fault segmentation. Here, we assess the impact that the Wasatch fault segmentation model has on seismic hazard by evaluating the time-independent long-term rate of ruptures on the fault that satisfy fault-slip rates and paleoseismic event rates, adapting standard inverse theory used in the Uniform California Earthquake Rupture Forecast, Version 3, and implementing a segmentation constraint in which ruptures across primary structural complexities are penalized. We define three models with varying degrees of rupture penalization: (1) segmented (ruptures confined to individual segments), (2) penalized (multisegment ruptures allowed, but penalized), and (3) unsegmented (all ruptures allowed). Seismic-hazard results show that, on average, hazard is highest for the segmented model, in which seismic moment is accommodated by frequent moderate (moment magnitude Mw 6.2–6.8) earthquakes. The unsegmented model yields the lowest average seismic hazard because part of the seismic moment is accommodated by large (Mw 6.9–7.9) but infrequent ruptures. We compare these results to model differences derived from other inputs such as slip rate and magnitude scaling relations and conclude that segmentation exerts a primary control on seismic hazard. This study demonstrates the need for additional geologic constraints on rupture extent and methods by which these observations can be included in hazard-modeling efforts.

Structural inheritance and border fault reactivation during active early-stage rifting along the Thyolo fault, Malawi

“…This presents a challenge when segmentation criteria based on shallow structures is used for assessing earthquake magnitudes for seismic hazard analyses (e.g. Field et al, 2009;Petersen et al, 2015;Valentini et al, 2019): where depth-dependent segmentation is not correctly identified, multi-segment and multi-fault ruptures such as those observed in the 2016 earthquakes in central Italy (Mw6.2,6.1 & 6.6) and…”

Strain and Velocity Across the Great Basin Derived From 15‐ka Fault Slip Rates: Implications for Continuous Deformation and Seismic Hazard in the Walker Lane, California‐Nevada, USA

“…Since there are few long strike‐slip faults in the Walker Lane (Wesnousky et al., 2012), strike slip may occur mostly on shorter faults that are not capable of producing large (≥M7) earthquakes, and therefore slip in more frequent moderate earthquakes than in rarer larger ones (Anderson, 1979; Molnar, 1979). More frequent moderate earthquakes may pose greater hazard than fewer large earthquakes (Minson et al., 2020; Valentini et al., 2020). If missing strike slip is accommodated by moderate earthquakes that do not always rupture the surface (e.g., the 2020 Monte Cristo earthquake and dePolo, 1994), seismic hazard in the Walker Lane may be higher than estimates based solely on the paleoseismic record.…”

A Large Paleoearthquake in the Central Apennines, Italy, Recorded by the Collapse of a Cave Speleothem

“…Further developments of the approach could integrate fault segmentation relaxation and the possible fault interactions into the seismic hazard model (e.g., Valentini et al, 2020;Verdecchia et al, 2018;Visini et al, 2020) and a better treatment of the uncertainties involved in the definition of the mechanical properties of broken speleothems and in the age record of speleoseismic events. F. Visini who helped us during the seismic noise measurements acquisition and discussed with us the seismic hazard results; D. Berardi who led us in the Cola Cave; F. Rizzo who helped us during the initial part of FEM modeling; S. Agostini for useful discussions on speleoseismology and for providing his papers collection.…”

“…Further developments of the approach could integrate fault segmentation relaxation and the possible fault interactions into the seismic hazard model (e.g., Valentini et al, 2020; Verdecchia et al, 2018; Visini et al, 2020) and a better treatment of the uncertainties involved in the definition of the mechanical properties of broken speleothems and in the age record of speleoseismic events.…”