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19-1. Broadband earthquake rupture physics


Closing Date: January 4, 2021

This Research Opportunity will be filled depending on the availability of funds. All application materials must be submitted through USAJobs by 11:59 pm, US Eastern Standard Time, on the closing date.

How to Apply

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Stress drop is an elastic measure of deformation within the source region and is a key earthquake source property that can be estimated over a broad range of frequencies. Static stress drop is the product of the shear modulus and strain in the source, while dynamic stress drop is the change in shear stress driving earthquake faulting released as radiated seismic energy, which controls the amplitude and frequency content of ground shaking. Both static and dynamic stress drops are derived source properties dependent upon directly measurable quantities: rise time, duration, corner frequency (for dynamic stress drop); earthquake area or slip (for static stress drop). All of these are difficult to directly measure or model, so while dynamic and static stress drops should be equivalent under idealized theoretical models, uncertainty in the observable parameters hampers attempts to truly characterize the source; hence, the underlying broadband physical and temporal aspects of this connection are still poorly understood.  

The USGS National Seismic Hazard Model (NSHM) is an example of a sophisticated probabilistic seismic hazard analysis (PSHA), which has two main components: seismic source characterization (SSC) and ground motion prediction equations (GMPEs). Earthquake source properties, parameterized by the stress drop, figure prominently in both components. In the SSC, static stress drop links fault slip, length and magnitude, as used in empirical magnitude-area relations. In a GMPE, dynamic stress drop is inferred from propagating elastic waves rather than from static slip. In theory static and dynamic stress drop are equal, yet robust relationships between the two are not well established and uncertainty in their calculations impede the development of better GMPE. 

Improved constraints on earthquake source properties and reduced uncertainty in source contributions to hazard assessment require observations over a range of frequencies and should include insights from disciplines in addition to seismology. The Fellow will seek to understand the connections amongst earthquake source properties from low-frequency, static measurements to high-frequency dynamic observations. Work could focus on field, seismic or geodetic observations, laboratory experiments, simulations of rupture propagation or ground motion. Some ideas include, but are not limited to:  

  • What are the connections between event duration, dynamic fault slip, dynamic stress drop, and high-frequency radiated energy? How does fault slip heterogeneity correlate with dynamic stress drop and the spectra of near-field ground accelerations during an extended rupture? Can source parameter variability be characterized by statistically robust spatial or temporal patterns, dependence on depth/stress, or fault slip history? Do aftershocks show reduced stress drop compared to mainshocks, as proposed by some investigators, and if so, why? What can observed variations in focal mechanisms, or other stress field indicators (e.g. borehole breakouts) at a local scale tell us about heterogeneity in pre-stress and radiated energy in relation to stress drop? 

  • How can observables from lab experiments and theoretical models derived from these experiments be scaled up to model in situ earthquakes? What insight can be gained through controlled laboratory experiments where, for example, normal stress can be prescribed and signals related to the earthquake source can be measured? This could involve work on the 2m long x 0.4m deep simulated strike-slip fault in Menlo Park that is outfitted with relatively broadband sensors, or smaller-scale lab experiments in which the energy released in an earthquake can be partitioned into radiated and dissipated (e.g., through shear heating) energies by direct near-field measurements. 

  • Static stress drop is inherent in magnitude-area relationships, yet these static stress drops do not always correlate with high-frequency, dynamic ones. How can this be reconciled? At the core of this relationship is the proportionality of the rupture dimension, r to event duration and by extension, corner frequency fc, i.e., fc~Vs (with Vs the shear wave velocity). The implicit mapping of a static value (r) to a dynamic one (fc) assumes that any large, complex rupture can be parameterized with just a single, averaged stress drop.  

  • Fault slip may decrease toward the Earth’s surface and be accompanied by distributed deformation off the principal fault surface. What is the impact of shallow slip reduction (e.g., by interseismic creep) and/or off-fault deformation on ground motions? Is there a systematic relationship between earthquakes with significant off-fault deformation and static/dynamic stress drop estimates? 

  • While it is well-known that surface earthquake accelerations are frequency-independent white noise above the corner frequency and that the mean (RMS) acceleration amplitude reflects a dynamic stress drop, is the phase of the high-frequency, white noise truly random? What is the physical cause for source accelerations to be of random, uncorrelated phase (white noise), and how is that reflected in near-field and borehole-recorded seismograms?  

  • How can the seismic source in physical models be represented as more complex than a planar fault in elastic surroundings, for example as a source volume or as non-planar? How does off-fault deformation or slip relate to the dynamic energy released?  

This broad Research Opportunity spans topics in seismology, crustal deformation, field geology and rock physics 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 Advisor(s) early in the application process to discuss project ideas. 

Proposed Duty Station: Moffett Field, CA or Menlo Park, CA 

Areas of PhD: Geophysics, seismology, geology, engineering 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 GeophysicistResearch GeologistResearch EngineerResearch GeodesistResearch PhysicistResearch Computer ScientistResearch StatisticianResearch Civil EngineerResearch MathematicianResearch Physical ScientistResearch Computer 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: Beverly Ledbetter, 916-278-9396, 

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