18-7. Static vs. dynamic stress drops to characterize broadband earthquake rupture physics

“What is earthquake stress drop, and what does it represent physically?” is a long-standing issue in earthquake rupture physics. Seismologists and ground-motion modelers often mean dynamic stress drop, the change in shear stress driving earthquake faulting that goes into radiated seismic energy, which controls the amplitude and frequency content of ground shaking during earthquakes and is thus of great interest to structural engineers. Geologists often mean static stress drop, the change in average stress resolved onto the fault before and after an earthquake rupture, which controls the mechanics of crustal deformation and should be related to slip on a fault, which can feed into earthquake occurrence statistics. In idealized, theoretical earthquake models, static and dynamic stress drops are equivalent. To a first order, this equivalency has been observed, suggesting that earthquakes rupture in approximately the same way in a variety of geologic settings and over a wide range of magnitudes, allowing us to extrapolate current models and knowledge to predict ground-motion, slip, recurrence rates and other parameters to poorly recorded large-magnitude events, close distances, or new regions of interest; however, a closer look reveals discrepancies between static and dynamic stress drops and thus a need for a deeper understanding of the earthquake rupture process, apart from the idealized physics.

This Research Opportunity seeks to answer the basic questions of: What is the connection between static and dynamic stress drop? Can any observed differences between the two be indicative of rupture process, or simply attributed to uncertainty in the measurements? What is the best way to incorporate complexity of dynamic and static stress drop in updated hazard models, for both ground-motion modeling and crustal deformation? Hampering many attempts to correlate these parameters are uncertainties in spectral corner frequency, magnitude-area relations, and rupture rise time, the parameters from which stress drops are often estimated. Work could focus on seismic or geodetic observations, laboratory experiments or ground motion simulations. Some possible research avenues include, but are not limited to:

• How is stress drop a predictor of far-field ground motion? How does it pertain to high-frequency, stochastic or long-period, deterministic ground-motion simulations? How can we relate stress drop in ground-motion models across a wide range of frequencies, to either better describe the observations or better predict ensuing ground-motion?

• Are there statistically robust patterns in spatial or temporal variability in stress drop or dependence on source parameters such as depth? Do aftershocks show reduced stress drop compared to mainshocks, as several studies have proposed, and if so, why?

• 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, which is outfitted with broadband sensors, or smaller-scale lab experiments in which the released, radiated and dissipated energies are measured directly in the near-field.

• Static stress drop is inherent in magnitude-area relationships, and some studies invoke an increasing stress drop for larger events in order to increase the slip to be physically consistent with observations. Yet, we do not observe an increase of dynamic stress drops with magnitude. How can this be reconciled, and how are magnitude-area stress drops related to dynamic stress drops?

• Generally, how can observables from lab experiments and theoretical models derived from these experiments be scaled up to model in situ earthquakes?

• What is the connection between long-period displacement or slip and stress drop or high-frequency radiated energy? Long-period observations, such as GPS displacements or strain measurements may be used to probe these relationships. How can variations in the long-period displacement or slip be modeled through stress drop estimates and thus extended to predicted variations in high-frequency ground motion, or visa-versa?

• At the core of the relationship between static and dynamic stress drops is the proportionality of the rupture dimension, r to the inverse of the corner frequency, fc, i.e., fc~Vs/r (with Vs the shear wave velocity). This is implicitly mapping a static value (r) to a dynamic one (fc) and assumes that any large, complex rupture can be parameterized with just a single, averaged stress drop. Is this actually observed?

• How does slip heterogeneity or surface displacement/rupture along a fault correlate with stress drop or high-frequency ground motion, during an extended rupture or for smaller events (or aftershocks) along a fault? What can observed variations in focal mechanisms/orientations or other stress field indicators on a highly localized scale tell us about heterogeneity in pre-stress and radiated energy in relation to stress drop?

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

Areas of PhD: Geophysics, geology, civil engineering 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: Research Geophysicist, Research Geologist, Research 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: Audrey Tsujita, 916-278-9395, atsujita@usgs.gov