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USGS Mendenhall Postdoctoral  Research Fellowship Program

14-17. Development and assessment of new methodologies for better estimating fault slip rates from space geodetic data.

We seek a postdoctoral fellow to develop original and innovative methods that better utilize the growing space geodetic database for improved seismic hazard assessment.  Seismic hazard maps are among the most visible and societally important USGS products. They are used to set and revise building codes, allocate resources for earthquake preparedness and response, and have economic impacts measurable in billions of dollars.  Other factors being equal, fault slip rate is directly proportional to seismic hazard, and using best science in assessing hazard is one of the most important goals of the U. S. Geological Survey.  Traditionally, seismic hazard assessment has employed slip rates estimated via the classical methods of tectonic geology.  However, for the first time slip rate estimates obtained through newer, more experimental analysis methods that rely on space geodetic measurements of surface ground displacement (Global Positioning System, GPS; and Interferometric Synthetic Aperture Radar, InSAR, imaging) are being applied in both California and nationwide seismic hazard estimation.

Specifically, the 3rd Unified California Earthquake Rupture Forecast (UCERF3) and the next generation of the USGS’s National Seismic Hazard Map (NSHM) now employ both traditional geologic and novel geodetic “Deformation Models” as input to earthquake occurrence models.  Each method has its own strengths and limitations; use of both provides an independent check on each and a useful range on the true unknown fault slip rate.  However, since the direct use of space geodetic results in hazard assessment is new, many correspondingly new scientific issues have arisen. While the current assessments are nearing completion, it is essential that these questions be addressed well ahead of the future issuance of improved, updated hazard maps for California, the U.S., and other seismically active regions important to the Nation’s global economic, political, and social interests.

The current UCERF3 and NSHM development has brought to light several outstanding scientific questions centered on earthquake-related crustal deformation and its relation to fault slip rate estimation and seismic hazard.  As such, targets for new research are numerous.  For example, why do geodetic models that use the same input data and assumed fault geometry produce (apparently) significantly different slip rate estimates?  Likewise, why are geodetic and geologic slip rate estimates significantly different in some locales while agreeing well in many others?  What role might time-dependent changes in deformation processes between geodetic (past ~20 years) and geologic (last ~10 ka) time scales play?  Do long-lived post-earthquake transients contaminate the assumed steady state geodetic velocity field and lead to systematic biases in slip rate estimates (e.g. Chuang and Johnson, 2011; Hearn et al., 2013)?  How much crustal strain is accommodated not on major faults but in the regions between them?  What is the nature of strain release in these regions, and what is the implication for regional seismic potential?  Can combined analysis of geodetically determined strain rates, geologic slip rates on smaller faults, and principal stress orientations determined from earthquake fault plane solutions help quantify this?

Research aimed at critically evaluating questionable assumptions in existing geologic and geodetic methods, as well as developing new methodologies for obtaining fault slip rate information, will make a large impact on hazard assessment and goes hand-in-hand with work that focuses on the geophysical sources of possible discrepancies.  For example, what contribution do off-fault deformation (e.g. Oskin et al., 2008) or assumptions about the initiation of fault offset on geomorphic features (e.g. Cowgill, 2007) introduce into geologic slip rate estimates?  How do assumptions regarding fault geometry, the “block-like” behavior of the Earth’s crust, and the parameterization of geodetically-observable elastic strain due to continuous slip on faults below 10-15 km influence geodetic estimates of slip rate and off-fault strain?  How can approaches such as Bayesian inversion (Segall, 2002; Fukuda and Johnson 2010; Minson et al., 2013) be employed to go beyond obtaining a single slip rate estimate with formal uncertainties to fully characterize the range of rates that are consistent with data and prior information?  Can new methods such as cluster analysis (Simpson et al., 2012) be extended to provide an objective method for defining block boundaries by delineating subtle gradients in the GPS velocity field?

While the Mendenhall Fellow is encouraged to frame innovative new approaches to the target problems, s/he will be able to draw upon a wealth of resources in developing and carrying out the research plan.  A variety of datasets are readily available including survey-mode and continuous GPS and InSAR data spanning two decades, a complete inventory of consensus fault slip rates used in the UCERF3 and NSHM assessments, and the USGS online Quaternary Fault and Fold database.  The Research Advisor team is experienced in block modeling, cluster analysis, Bayesian inversion, and cutting edge methods for estimating fault slip rates from geologic data.  The USGS Menlo Park computer cluster is available for running computationally intensive models.

REFERENCES.

Cowgill, E., Impact of riser reconstructions on estimation of secular variation in rates of strike slip faulting: Revisiting the Cherchen River site along the Altyn Tagh Fault, NW China: Earth Planet. Sci. Lett., v. 254, no. 3, 239-255, 2007.

Chuang, R. and K. Johnson (2011), Reconciling geologic and geodetic model fault slip-rate discrepancies in Southern California: Consideration of nonsteady mantle flow and lower crustal fault creep, Geology, 39, 627-630.

Fukuda, J., and K. M. Johnson, Mixed linear-non-linear inversion of crustal deformation data: Bayesian inference of model, weighting and regularization parameters, Geophys. J. Int., 181, 1441-1458, 2010.

Hearn, E.H., Pollitz, F.F., Thatcher, W.R.,  andOnishi, C.T., How do ‘ghost transients from past earthquakes affect GPS slip rate estimates on southern California faults? Geol. Geochem. Geophys., doi: 10.1002/ggge20080, 2013.

Minson, S. E., M. Simons, and J. L. Beck, A Bayesian approach to finite fault earthquake modeling I: Theory and synthetics, Geophys. J. Int., doi: 10.1093/gji/ggt188, 2013.

Oskin, M., Perg, L., Shelef, E., Strane, M., Gurney, E., Singer, B., and Zhang, X.,  Elevated shear zone loading rate during an earthquake cluster in eastern California: Geology, v. 36, no. 6, doi:10.1130/G24814A.24811., 2008.

Segall, P., Integrating geologic and geodetic estimates of slip rate on the San Andreas fault system: Int. Geol. Rev., v. 44,  62-82, 2002.

Simpson, R. W., Thatcher, W., and Savage, J. C., Using cluster analysis to organize and explore regional GPS velocities: Geophys. Res. Lett.,. v. 39, doi:10.1029/2012GL052755, 2012.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D. Geophysics, Geology, Geodesy, 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, Research Geologist, Research Physicist.

(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): Wayne Thatcher, (650) 329-4810, thatcher@usgs.gov.; Jessica Murray, (650) 329-4864, jrmurray@usgs.gov.; Fred Pollitz, (650)329-4821, fpollitz.usgs.gov; Carol Prentice, (650) 329-5690, cprentice@usgs.gov.; and Kate Scharer, (626) 583-7240, kscharer@usgs.gov.

Human Resources Office Contact: Lisa James, (916) 278-9405, ljames@usgs.gov.


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U.S. Department of the Interior, U.S. Geological Survey
URL: http://geology.usgs.gov/postdoc/opps/2014/14-17 Thatcher.htm
Direct inquiries to Rama K. Kotra at rkotra@usgs.gov
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Last modified: 15:16:04 Tue 23 Jul 2013
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