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USGS Mendenhall Postdoctoral Research Fellowship Program

17-2. The relation of earthquake stress drop to source physics and ground motion

The goals of our research under this Opportunity are to understand differences in methods for estimating stress drop in order to distinguish trends, more fully understand the energetics of earthquake rupture, and reduce uncertainty in measurements. Earthquake stress drop is an essential parameter for understanding the energetics of the earthquake source, as well as modeling high-frequency ground motion. Given the relationship between earthquake stress drop and strong ground motion, stress drop is one of the largest sources of uncertainty in ground-motion prediction equations (GMPEs). The uncertainty in GMPEs propagates into increased uncertainty in key USGS products such as ShakeMap and early warning alerts. Additionally, uncertainty in GMPEs dominates the uncertainty (over the seismic source characterization) in probabilistic seismic hazard analysis (PSHA), the backbone of USGS hazard maps.

In theory, stress drop is the difference between the initial and final stress of an earthquake rupture, but in practice is often modeled from a single parameter (i.e., the corner frequency) on a spectral recording, or from macro-observations such as the area over which an earthquake faulted. In addition, stress drop is a fundamental control on the excitation of high-frequency ground motion. The relationship between these different representations of stress drop is fraught with uncertainty, and so the exact physics governing these details is not well resolved. Additionally, many analytical methods are currently employed to estimate seismological stress drop from ground motion records, and often result in inconsistent estimates.

With current levels of uncertainty and variability in earthquake stress drop, it is difficult to truly discern source properties or determine the relationship of stress drop to predictive parameters, such as magnitude, location, depth, rupture area or focal mechanism. Additionally, this level of uncertainty renders any absolute stress drop values impractical for use as a predictive measure in ground-motion modeling, either with empirically based GMPEs or dynamic rupture models. The work undertaken by the Fellow is expected to reduce uncertainties in stress drop for earthquakes and improve our understanding of stress drop as a control on high-frequency ground motion, as well as determining physical variations (spatially, temporally or due to other source properties) of stress drop.

We seek a postdoctoral researcher to address one or more of the following topics:

  1. Relation of stress drop to source properties (i.e., geometry, area, depth, mechanism, pore pressure, etc.); consider spatial or temporal relations or changes in stress drop; differences in mainshocks vs. aftershocks; address whether stress drop can be used to distinguish different types of events, such as tectonic (natural) versus induced earthquakes.
  2. Exploration of stress drop as a control on ground motion parameters that are important to hazard. Consider interplay between stress drop and other controlling factors (see above) in ground-motion estimation or modeling.
  3. Comparison of stress drops derived using a range of methodologies and rigorously assess uncertainties in the measured source parameters. This could be through participation in or development of a stress drop validation experiment to test many of the above ideas using a vetted data set of earthquake records, potentially in collaboration with other researchers and using a variety of methods.

Interested applicants are strongly encouraged to contact the advisors below early in the application process to discuss project ideas.

Proposed Duty Station: Pasadena, CA; Menlo Park, CA

Areas of Ph.D.: Geophysics, seismology, 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 one of the following qualifications: Research Geophysicist (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.)

Research Advisor(s): ): Elizabeth Cochran, (626) 583-7238,; Annemarie Baltay, (650) 329-4759,; Jeff McGuire, (650)-329-5153,; Oliver Boyd, (303) 273-8617,; Morgan Moschetti (303) 273-8464,; and Dan McNamara (303) 273-8550,

Human Resources Office Contact: Yumi Sakakibara,, 916-278-9384

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U.S. Department of the Interior, U.S. Geological Survey
URL: Cochran.htm
Direct inquiries to Cara A. Campbell at
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Last modified: 11:07:39 Fri 08 Feb 2019
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