18-11. Towards non-ergodic ground motion models in the U.S. National Seismic Hazard Model

The National Seismic Hazard Model (NSHM) is an important product of the U.S. Geological Survey and forms the basis for the seismic provisions of U.S. building codes, as well as other engineering-design requirements, risk assessments, and planning decisions (e.g., Petersen et al., 2014; 2019). The probabilistic seismic hazard analyses in the NSHM currently use ergodic ground-motion models (GMMs) that only account for the broadest variations in ground-motion excitation and attenuation between seismotectonic regions—such as for crustal earthquakes in the western U.S. and central and eastern U.S. and from subduction zones. Increasingly, the seismological and engineering communities recognize that seismic hazard assessments will be improved by incorporation of regional variations in source characteristics and the effects of local geologic structures on seismic wave propagation. For example, one topic of current interest to the NSHM is the effect of sedimentary basins on earthquake ground motions.

In previous updates of the NSHM, progress in incorporating local and regional effects into ground-motion models has largely been limited by the observations from the moderate- to large-magnitude earthquakes that are used to develop GMMs. However, recent results from 3-D ground-motion simulations (e.g., Frankel et al., 2007; Graves et al., 2011; Frankel et al., 2018; Moschetti et al., 2018; Wirth et al., 2018) provide valuable insights into how ground-motions behave under conditions that are not sufficiently sampled by recordings to be constrained in empirical models. Additionally, researchers have developed new methods for developing non-ergodic ground-motion models (e.g., Landwehr et al., 2016; Abrahamson et al., 2019) from ground-motion recordings or simulation results.

The Research Opportunity seeks to leverage recently developed methods for non-ergodic ground-motions, existing ground-motion simulations in multiple urban areas, ground-motion simulation methodologies, and enhanced ground-motion processing capabilities within the U.S. Geological Survey to develop new frameworks for the seismic hazard calculations of the National Seismic Hazard Model.

We seek a candidate to conduct research on earthquake ground-motions and probabilistic seismic hazard analyses that can be used to improve the NSHM. Priority regions for study include Los Angeles, CA; San Francisco Bay Area, CA; Seattle, WA; and Salt Lake City, UT. Multiple approaches to advancing probabilistic seismic hazard analysis through non-ergodic ground-motion models may be employed, including through 3-D ground-motion simulations, modifying empirical ground-motion models with observations or simulated ground-motion data, and by other novel means. Example research directions that may be addressed by the Fellow are provided below, but other research efforts to improve ground motion characterizations in seismic hazard products are also encouraged.

1. 3-D ground-motion simulations: How can 3-D simulations be used in probabilistic seismic hazard assessments? What features of ground-motion simulations are sufficiently characterized for application in probabilistic seismic hazard assessments? How can simulated ground-motions and their variability be validated?

Earthquake ground-motion simulations can constrain the ground-shaking from scenario earthquakes and are inherently non-ergodic; however, simulated values are highly dependent on the details of the earthquake rupture model and require various validation approaches. Research topics may also include the development of simulation-based probabilistic seismic hazard models (i.e., urban seismic hazard models). The Salt Lake City, UT region is of particular interest for simulation-based seismic hazard modeling.

2. Development and implementation of non-ergodic ground-motion models in PSHA: How can ground-motion recordings or ground-motion measurements from simulations be used to develop non-ergodic ground-motion models? How can non-ergodic ground-motion models be implemented in the National Seismic Hazard model and what are the implications for hazard assessments?

Recordings from small- to moderate-magnitude earthquakes and from ground-motion simulations may further constrain components of empirically based ground-motion models. Potential research questions may address methods to regionalize ground-motion models to account for important geologic structures, such as the sedimentary basins that underlie Los Angeles, CA; San Francisco Bay Area, CA; Seattle, WA; and Salt Lake City, UT, and to implement modified these GMMs into the probabilistic seismic hazard assessments of the NSHM.

Interested applicants are strongly encouraged to contact the Research Advisor(s) early in the application process to discuss project ideas.

References: 

Abrahamson, N.A. et al. (2019) Probabilistic Seismic Hazard Analysis in California Using Nonergodic Ground‐Motion Models. Bulletin of the Seismological Society of America, 109(4), 1235-1249

Frankel, A.D., Stephenson, W.J., Carver, D.L., Williams, R.A., Odum, J.K, and Rhea, S. (2007). Seismic hazard maps for Seattle, Washington, incorporating 3D sedimentary basin effects, nonlinear site response, and rupture directivity: U.S. Geological Survey Open-File Report 2007-1175, 77 p.

Frankel, A., Wirth, E., Marafi, N., Vidale, J., and Stephenson, W. (2018) Broadband synthetic seismograms for magnitude 9 earthquakes on the Cascadia megathrust based on 3D simulations and stochastic synthetics (Part 1)—Methodology and overall results. Bulletin of the Seismological Society of America, 108(5A), 2347-2369.

Graves, R., Jordan, T. H., et al. (2011). CyberShake: A physics-based seismic hazard model for southern California. Pure and Applied Geophysics, 168(3-4), 367-381.

Landwehr, N., Kuehn, N. M., Scheffer, T., and Abrahamson, N. (2016). A nonergodic ground‐motion model for California with spatially varying coefficients. Bulletin of the Seismological Society of America, 106(6), 2574-2583.

Moschetti, M. P., Hartzell, S., Ramírez‐Guzmán, L., Frankel, A. D., Angster, S. J., & Stephenson, W. J. (2017). 3D ground‐motion simulations of Mw 7 earthquakes on the Salt Lake City segment of the Wasatch fault zone: Variability of long‐period (T≥ 1 s) ground-motions and sensitivity to kinematic rupture parameters. Bulletin of the Seismological Society of America, 107(4), 1704-1723.

Petersen, M.D., et al. (2014) Documentation for the 2014 update of the United States national seismic hazard maps: U.S. Geological Survey Open-File Report 2014–1091, 243 p.

Petersen, M.D., et al. (2019) 2018 Update of the U.S. National Seismic Hazard Model: Overview of Model and Implications. Earthquake Spectra (accepted)

Wirth, E.A., Frankel, A.D., Marafi, N., Vidale, J.E., and Stephenson, W.J. (2018) Broadband synthetic seismograms for magnitude 9 earthquakes on the Cascadia megathrust based on 3-D simulations and stochastic synthetics (Part 2)—Rupture parameters and variability. Bulletin of the Seismological Society of America, 108(5A), 2370-2388.

Proposed Duty Station: Golden, CO

Areas of PhD: Geophysics, earthquake engineering or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet the qualifications for: Research Geophysicist, Research Physical Scientist, Research Civil Engineer. 

(This type of research is performed by those who have backgrounds for the occupations stated above.  However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Human Resources Office Contact: Audrey Tsujita, 916-278-9395, atsujita@usgs.gov