Understanding the Impact of the North American Crustal Stress Field on Probabilistic Seismic Hazard Assessment (PSHA)

Probabilistic seismic hazard assessment (PSHA) provides the frequency of exceeding various levels of ground shaking at a location. At the most basic level, a PSHA considers all possible earthquake scenarios, estimates of the resulting ground motion (GM), and associated rates of occurrence. The PSHA integrates across uncertainties to yield estimated frequencies of exceedance. Current practices in PSHA systematically address both aleatory variabilities and epistemic uncertainties (uncertainties associated with limited knowledge and often reflected by reasonable alternate analysis assumptions). In locations without detailed characterization of the seismic hazard associated with individual faults, significant epistemic uncertainties relate to source characterizations, including identification of potential rupture characteristics and their likelihoods.

Knowing the state of stress in the Earth's crust can reduce epistemic uncertainties because it provides insights regarding potential earthquake rupture surface directions and mechanisms as well as the seismotectonic characteristics that dominate the regional hazard. The orientation of the maximum horizontal principal stress (S_Hmax) and the style of faulting (relative principal stress magnitudes) determine which orientations of pre-existing faults are more and less likely to fail seismically. The probability that faults of any orientation may be potentially active may be estimated by accounting for the uncertainties on stress, fault, and rock property parameters.

Recently, stress maps have become available for North America that provide both S_Hmax orientations and quantify the style of faulting, together with uncertainties [1]. Use of crustal stress data during development of fault source parameters for a PSHA may improve GM estimates, particularly in locations sensitive to GM models or where fault orientations are well characterized but paleoseismic data or other fault hazard indicators are not. This study explores how knowledge of the state of stress may inform a PSHA through augmentation and potential reduction in epistemic uncertainties associated with source characterization.

[1] J.-E. Lund Snee & M. D. Zoback, "Multiscale variations of the crustal stress field throughout North America," Nature Communications, 11(1), Apr. 2020.