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Orientation Dependence of Probabilistic Seismic Hazard Results From Physics-Based Simulations

Description: 

Probabilistic seismic hazard analyses (PSHA) currently consider ground motion intensity measures that reduce the variability of horizontal motion into a single value. For example, the U.S. Geological Survey National Seismic Hazard Model uses the median pseudo-spectral acceleration (PSA) of all horizontal orientations, commonly referred to as RotD50. However, ground motion intensities from earthquake records often exhibit significant variation with horizontal orientation, primarily due to physical mechanisms that polarize seismic waves. If these effects are consistent across the earthquakes considered in the analysis, they can introduce a dependence of seismic hazard results on horizontal orientation. This study investigates the dependence of PSHA results on horizontal orientation using outputs from CyberShake Study 22.12, a physics-based PSHA model of Southern California with simulated ground motions from over 600 thousand earthquake scenarios. PSAs were computed at periods from 1 to 10 s and at all possible horizontal orientations directly from the simulated ground motions, bypassing the use of ground motion models typically employed in conventional PSHA. The results reveal that PSAs with a 2% probability of exceedance in 50 years, a standard hazard level used for designing earthquake-resistant structures, can vary significantly with horizontal orientation at several sites, particularly those near active strike-slip faults. For instance, at a period of 5 s, the variation of PSA with horizontal orientation averages around 35% (maximum relative to minimum PSA), with some sites experiencing variations exceeding 100%. Since most structures have mechanical properties that also depend on horizontal orientation, at these sites, the same structure would be subjected to different seismic hazard levels depending on its orientation. These findings suggest that orientation-dependent PSHA could provide more accurate seismic hazard estimates for designing structures with known orientations, especially at sites where the seismic hazard varies considerably with horizontal orientation.

Session: Recent Advances in Modeling Near-source Ground Motions for Seismic Hazard Applications - I

Type: Oral

Date: 4/16/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Alan

Student Presenter: No

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