34. Constraining crustal structure, faulting, and earthquake effects through the application of new imaging and modeling methods in the Cascadia region.

Research under this Mendenhall opportunity will provide the scientific inputs needed for the next generation of USGS earthquake hazard assessments in the U.S. Pacific Northwest (PNW) and other subduction zone regions (for example, that of the 2004 Sumatra earthquake). Tremendous progress in our understanding of faulting and earthquake effects, particularly in the Puget Sound region of the PNW, has come largely by combining paleoseismologic and geologic observations with geophysical imaging (using LIDAR, aeromagnetic and gravity, and seismic data). Despite this progress, near-surface observations permit a range of different geometric models of crustal faults, their long-term slip patterns and the resulting earthquake effects. These observations provide few constraints on earthquakes on the Cascadia subduction interface, or on the smaller but more frequent deep interplate events. Additionally, an enigmatic relationship remains in the PNW between inferred long-term deformation and contemporary processes that are reflected mostly in modern, small to moderate earthquakes. For example, shallow earthquakes are sparse where faulting is clear in the geologic record, and are more abundant in the lower crust than in the shallow crust. At all depths, earthquake characteristics do not delineate faults of significant dimensions or that have been identified geologically.

Research conducted as part of this Mendenhall opportunity will focus on developing models of faulting and earthquake effects in the PNW that integrate observations of long-term crustal processes recorded in the geologic structure with those reflecting contemporary processes of ruptures and radiated wavefields of modern earthquakes. This may involve better defining geologic structures in the crust and subducted plate using new and improved imaging techniques, or it may involve modeling of earthquakes or deformation. New data sets and instrumentation that can be employed come from Earthscope’s Transportable Array of seismic stations and Plate Boundary Observatory GPS network, the Seismic Hazard Investigations of Puget Sound (SHIPS), and other seismic experiments. While the PNW is the test bed, the research proposed may bring in observations from other subduction or compressional environments, and from theoretical modeling.

Proposed research can fall into one of several categories:

Characterizing geologic structures. Crustal structures in the PNW need to be discerned to constrain structural models and ground shaking. The location of the subduction interface, and thus the rupture plane of future M8–9 Cascadia earthquakes, is poorly known. In particular, its down-dip limit may lie beneath the most populated regions of the PNW. For crustal faults and earthquake effects, improved images of crustal structures and the subducted plate could result from using newly developed methods of noise or microtremor analysis (cross-correlation; inversion of Rayleigh-wave ellipticity) and using earthquake and blast data for receiver function or attenuation studies. Knowing the S-wave velocity and attenuation structure of the crust and basins, and imaging the subducted plate and (serpentized?) mantle will improve models of rheology and wave propagation.

Evaluating faulting mechanics and interactions. Current geologic/geophysics-based models of upper-plate faulting in the Puget Sound region involve complex systems of thrust (often listric) faults. Geologic evidence suggests that sometimes multiple faults slipped concurrently and other times independently. In addition, the paleoseismic record suggests that a major subduction earthquake and multiple crustal fault earthquakes occurred nearly simultaneously ~1100 years ago. In contrast, the 1700 M9 earthquake on the Cascadia subduction zone occurred without apparent coseismic deformation in the overlying plate. Current crustal fault models and fault interaction scenarios may be tested by addressing the questions: “Can ruptures effectively propagate on listric thrust faults? How much does failure of one fault load another?” Dislocation, boundary element, or other types of numerical modeling may provide answers.

Connecting contemporary and long-term fault slip. The instrumental earthquake record in the PNW includes mostly small and few moderate events. As noted above, the relationship between these earthquakes and regional tectonics or specific faults remains enigmatic. Studies may focus on understanding the physical significance of the seismicity patterns we observe today and their apparent connection (or lack of) to tectonic processes and features in the PNW. One such study might employ new higher resolution earthquake location algorithms, modified to require allowable locations to be within some distance from a posited causative fault. If the data cannot be fit under such constraints several definitive conclusions can be drawn about the posited fault; that is, it is locked and fails only in large earthquakes or it doesn’t exist as inferred, and also must consider additional source faults that give rise to the observed earthquakes.

Proposed Duty Station: Seattle, WA

Areas of Ph.D.: Seismology, geophysics, engineering, or associated fields

Qualifications: Applicants must meet one of the following qualifications: Research Geophysicist, Research Civil Engineer/Civil Engineer, Research Geologist (Seismologist)

(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): Thomas Pratt, (206) 543-7358, tpratt@usgs.gov; Brian Sherrod, (206) 553-0153, bsherrod@usgs.gov; Joan Gomberg, (206) 616-5581, gomberg@usgs.gov; Art Frankel, (303) 273-8556, afrankel@usgs.gov; and John Vidale, (206) 543-6790, john.vidale@gmail.com or vidale@u.washington.edu

Human Resources Office contact: Erica Settlemyer, (916) 278-9383, esettlemyer@usgs.gov

Summary of Opportunities