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Evaluation and Integration of Seismic Directivity Models for the USGS National Seismic Hazard Model

Description: 

Several directivity models (DMs) have been developed in recent years to describe the near-source spatial variations in ground motion amplitudes related to propagation of rupture along the fault. We recently organized an effort towards incorporating these directivity effects into probabilistic seismic hazard analysis (PSHA), by first evaluating the community’s work and selecting an approach that can be readily implemented into the USGS National Seismic Hazard Model (NSHM). Guided by this evaluation and comparison among the considered DMs, we chose to incorporate a computationally simple approach, which provides an azimuthally varying adjustment to the median ground motion and its aleatory variability. This method allows us to assess the impact on hazard levels and provides a platform to test the DM amplification predictions using a generalized coordinate system, necessary for consistent calculation of source-to-site distance terms for complex ruptures.

We give examples of the directivity-related impact on hazard, progressing from a simple, hypothetical rupture, to more complex fault systems, composed of multiple rupture segments and sources. The directivity adjustments were constrained to strike-slip faulting, where DMs have good agreement. We find that seismic directivity adjustments using a simple median and aleatory adjustment approach can impact hazard both from a site perspective and on a regional scale, increasing shaking off the end of the fault trace and potentially reducing it for sites along strike. Statewide hazard maps of California show that the change in shaking along major faults can be a significant factor to consider for assessing long-period near-source effects within the USGS NSHM going forward. Finally, we suggest consideration of minimum parameter ranges and baseline requirements as future DMs are developed to minimize single approach adaptations and enable more consistent application within both ground motion and hazard studies.

Session: USGS National Seismic Hazard Models: 2023 and Beyond [Poster]

Type: Poster

Date: 4/18/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Kyle B. Withers

Student Presenter: No

Invited Presentation: 

Authors