20-25. Imaging northern California faults and their connectivity in 3D

CLOSED

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Background: The USGS relies on continual improvement of earthquake physics and fault representations to research the earthquake source process and map the nation’s seismic hazard. Fundamental to this work are the mapped traces of Quaternary active faults, but these only capture part of the picture; fault geometry in the subsurface is important to understand faulting and seismic hazard, but remains less well understood and challenging to document. Indeed, many of the recent major earthquake ruptures in California have occurred along faults that were previously incompletely mapped and only partially observable based on surface geomorphology alone. Identification and structural understanding of these features requires methods like relocating microseismicity to map faults in the subsurface (e.g., Plesch et al., 2020). Uncertainties in three-dimensional fault geometry have ramifications across earthquake science and seismic hazard analysis. Uncertainties in fault length, connectivity, existence, and 3D geometry increase uncertainty regarding possible earthquake magnitudes in rupture models and thus seismic hazard assessments. Efforts to establish the suite of multi-fault earthquake ruptures that may occur are highly sensitive to fault geometry and continuity.

While inherently challenging to map, fault geometry in three dimensions forms the basis upon which to answer a host of scientific questions and underpins probabilistic regional and site-specific seismic hazard models. In southern California, ample geophysics and borehole data inform these models as a result of decades of natural resource exploration, but northern California lacks the density of similar subsurface data. Using alternative approaches to improve 3D fault models may reduce uncertainty in hazard models, and provide new opportunities to understand how tectonic motion is partitioned across a complex plate boundary.

Recent research using relocated earthquake hypocenters has identified new faults, refined the geometry of known faults, found connections between faults, evaluated the frictional properties of faults, and enhanced our understanding of the physical phenomena through which faults slip. Nonetheless, gaps exist in our understanding of the way the plate boundary fault system in northern California behaves. These knowledge gaps include unmapped seismogenic structures, uncharacterized connectivity between the region’s major faults, and an evolving understanding of how aseismic slip is distributed along several of the major faults. We invite research proposals that can address these science questions through innovative, integrated techniques that improve our 3D representation of faults and thereby provide better physical models of the earthquake process.

We seek a postdoctoral researcher who will use innovative techniques to analyze 3D fault geometry in northern California with the aim of constructing accurate, structurally and kinematically coherent models of faults within the regional fault system. Improvements in our ability to represent the fault system may be achieved through a variety of methods largely centered around refining hypocenters and focal mechanisms of seismicity and/or developing tools to construct structurally coherent and justifiable fault geometries through integration of these data with geological, geophysical, and geomorphological information. Motivating proposal questions may involve a diverse array of subjects, including rupture propagation, structural geology & tectonic evolution, aseismic slip, fault growth and mechanics, seismic cycle processes, or others.

The results and approaches developed by this project may be used in seismic source models, velocity models, earthquake rupture forecasts, models of seismic versus aseismic moment release, and numerical simulations of rupture along frictionally and geometrically realistic faults. We encourage proposals that expand upon and integrate existing USGS research products, including the Bay Area velocity model, comprehensive catalogs of seismicity from regional seismic instrumentation, locally deployed seismic network arrays, and recently updated fault representations for the National Seismic Hazard Model as well as the definitive Quaternary Active Faults of the U.S. database. Mentors from multiple USGS projects are available to advise this postdoctoral research, ranging from earthquake geologists to statistical seismologists and numerical modelers, across multiple USGS Science Centers.

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

References:

Plesch, A., Shaw, J. H., Benson, C., Bryant, W. A., Carena, S., Cooke, M., ... & Yeats, R. (2007). Community fault model (CFM) for southern California. Bulletin of the Seismological Society of America, 97(6), 1793-1802.

Shaddox, H. R., Schwartz, S. Y., & Bartlow, N. M. Afterslip and Spontaneous Aseismic Slip on the Anza Segment of the San Jacinto Fault Zone, Southern California. Journal of Geophysical Research: Solid Earth, e2020JB020460.

Skoumal, R. J., Kaven, J. O., & Walter, J. I. (2019). Characterizing seismogenic fault structures in Oklahoma using a relocated template‐matched catalog. Seismological Research Letters, 90(4), 1535-1543.

Thomas, A. M., Bürgmann, R., & Dreger, D. S. (2013). Incipient faulting near Lake Pillsbury, California, and the role of accessory faults in plate boundary evolution. Geology, 41(10), 1119-1122.

Thurber, C. H., Brocher, T. M., Zhang, H., & Langenheim, V. E. (2007). Three‐dimensional P wave velocity model for the San Francisco Bay region, California. Journal of Geophysical Research: Solid Earth, 112(B7).

Proposed Duty Station: Moffett Field, California

Areas of PhD: Earth sciences, geology, seismology, planetary sciences, 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 one of the following: Research Geologist, Research Geophysicist, Research Geodesist.

(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:  Paj Shua Cha, 650-439-2455, pcha@usgs.gov