20-27. Fault processes and mechanics near Earth’s surface during coseismic and aseismic slip

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The processes and mechanics of faulting at Earth’s surface and in the shallow crust (<~1 km depth) are poorly known. In part, this knowledge gap stems from the fact that the near-surface environment is not adequately represented by an elastic dislocation model of a fault, a paradigm that dominates fault studies at greater lithospheric depths. Near Earth’s surface, where confining pressure decreases to zero, competent bedrock often transitions to unconsolidated material that is more prone to plastic, granular deformation. In terrestrial settings, faults nearly always traverse the water table, so that the shallowest faulting occurs in unsaturated material where the frictional properties may differ from those at depth. Fault tips often are buried beneath Earth’s surface due to fault healing or sedimentation between slip events. Thus, rupturing Earth’s surface during an earthquake or creep requires fracture propagation and interaction between neighboring fractures and with Earth’s surface. Over time, faults may become enveloped in a damage zone characterized by a high fracture density and a reduced shear modulus, which may influence how subsequent deformation is distributed and how seismic energy is radiated. Though these factors are thought to affect surface rupture and shallow fault behavior throughout the earthquake cycle, most have remained relatively unstudied.

Understanding shallow fault processes is important not only for advancing basic science, but also for characterizing seismic hazard. Overarching, unanswered questions include how fault-driven deformation (i.e., discrete fault slip and off-fault distributed deformation) varies temporally and spatially along strike and with depth, and how the factors in the preceding paragraph control this variation. Co- and aseismic near-surface faulting directly impact the built environment, including above-ground construction and buried lifeline infrastructure. Current efforts to forecast the distribution of fault slip (e.g., Probabilistic Fault Displacement Hazard Analysis) would benefit from a process-based framework based on knowledge of the local site conditions. Furthermore, our most important seismic hazard products, including the National Seismic Hazard Model (NSHM), rely on measurements of fault slip made at Earth’s surface. These measurements, however, may not accurately reflect fault activity at depth, even at tens-of-meters depth, leading to unquantified uncertainty in the modeled earthquake magnitudes and recurrence intervals. Developing a mechanical understanding of how slip at Earth’s surface relates to slip at depth is thus critically important to accurately characterizing seismic hazard.

We seek a Fellow to conduct independent, original research that advances our knowledge of fault processes in Earth’s shallowest crust (0-1 km depth) during coseismic and/or aseismic slip and how these processes relate to deeper fault loading. We encourage an interdisciplinary approach that integrates field, geodetic, seismic and/or lab data with mechanical modeling. Potential research topics include, but are not limited to:

• How do fluids affect shallow fault behavior? The steady-state frictional strength of many minerals is greatly reduced under wet versus dry conditions. The increased strength expected in fault zones in the unsaturated region above the water table is expected to hinder rupture propagation and fault slip. Other poroelastic effects and changes in dynamic weakening behavior (e.g., due to fluid pressurization) may also accompany the transition across the water table. Avenues for investigating these effects include analysis of well/borehole fluid pressure data, geodetic data, seismic tomography, laboratory experiments, and mechanical modeling.
• What is the relationship between near-surface structure, the distribution of slip and plastic deformation, and ground motions? Surface rupture along strike-slip faults often requires fracture propagation through an initially unbroken layer. This usually produces a discontinuous array of fault segments with dimensions and orientations that depend on the local stress state and dictate the overall roughness of the surface rupture. In turn, this roughness may influence the proportion of slip reaching Earth’s surface from depth, in addition to the distribution of off-fault plastic deformation. How fine-scale slip distributions, plastic deformation, and surface rupture geometry relate to near-field ground motions remains an open question. These topics can be investigated through detailed analysis of surface rupture morphology from field examples and analog (e.g., claybox) experiments, near-field geodetic data, seismic data, and mechanical modeling.
• What are the mechanisms of coseismic and aseismic deformation near Earth’s surface? Constraining the appropriate constitutive equations for coseismic and aseismic faulting requires knowledge of the deformation mechanisms, ascertained through field observations, laboratory analysis of core or outcrop samples, and/or experiments. Knowledge of the constitutive behavior may shed light on how interseismic creep does or does not store elastic strain energy available to drive or limit, respectively, coseismic rupture. Testing constitutive equations will be possible through comparison of forward mechanical models constrained by site-specific properties (e.g., from active source seismic tomography) and near-field geodetic data.

This Research Opportunity is a broad call, spanning topics in crustal deformation, field geology, seismology, and experimental rock mechanics and includes research advisors with expertise and interests that span these and other fields of earthquake science. Interested applicants are strongly encouraged to contact the Research Advisors early in the application process to discuss project ideas.

Proposed Duty Station: Moffett Field, California

Areas of PhD: Geophysics, geology, geodesy, 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 the qualifications for one of the following: Research Geophysicist or Research Geologist.

(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: Victor Mendoza, 650-439-2454, vjmendoza@usgs.gov