29. Quantitative Structural Analysis of the Seattle Fault

Recent paleoseismic evidence indicates that the Seattle fault lying beneath downtown Seattle, Bremerton, and Bellevue, Washington, is one of the most hazardous crustal faults in the Pacific Northwest. Three or more surface rupturing earthquakes have occurred on the fault in Holocene time, at least one of them greater than M 7 about 1100 years ago. Surprisingly, most of the surface ruptures occurred on north-dipping thrusts, whereas the Seattle fault zone overall is thought to be a major south-dipping structure forming the northern boundary of the Seattle uplift. Although shorelines were uplifted over 7 m 1100 years ago, the lack of a major scarp along the leading edge of the uplift suggests that much of the slip occurred on a blind thrust. The hazard from blind-thrust faulting is well known but poorly characterized along the plate boundary in the western U.S. Defining the seismic hazard posed by blind thrusts requires knowledge of the subsurface geometry and Holocene earthquake history of these active faults. Several models for the subsurface geometry of the Seattle fault have been proposed based on seismic-reflection profiles, gravity and magnetic data, high-resolution seismic tomography models, paleoseismic excavations, sparse borehole control, and surface geologic mapping. The most recent suggests that the Seattle fault consists of a south-dipping blind thrust that rises toward a shallow, north-dipping roof thrust. None of the proposed models, however, completely relate surface rupture, fault displacement, subsurface geometry, and fault history.

Quantitative structural models for the Seattle fault and for the Tacoma fault to the south could answer several questions needed to more accurately characterize the seismic hazards posed by these faults: (1) Are existing models consistent with respect to fault structure and history (i.e., are they retrodeformable and balanced)? (2) How does the Seattle fault geometry change along strike, especially east of Lake Washington? (3) Based on surface deformation and subsurface geometry, how was slip distributed at depth in the last large earthquake? In addition to the data used by previous workers, Boise State University and the USGS acquired high-resolution seismic reflection data in June 2004 across the Seattle fault east of Lake Sammamish to compare and contrast with the better mapped region west of Lake Washington. We also anticipate that in the next year additional coseismic uplift data will become available from the last major rupture of the Seattle fault about 1100 years ago. These data can be used to help develop models for the three-dimensional distribution of slip on the Seattle fault system.

A structural geologist/geophysicist is sought to develop quantitative two- and three-dimensional structural and slip models for the Seattle and Tacoma faults. The models must be compatible with existing seismic-reflection and potential-field data, paleoseismic excavations, borehole control, and surface geologic mapping. Balanced cross sections must be developed that integrate these data and quantitatively test against other proposed structural models. Models could be developed for the eastward extension of the Seattle fault to the western foothills of the Cascade Mountains and the faults possible association with southeast-trending structures of the Olympic-Wallowa lineament.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D.: Geology, geophysics, structural geology

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Geophysicist

(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 Personnel specialist.)

Research Advisor(s): Tom Brocher, (650) 329-4737, brocher@usgs.gov; Rich Blakely, (650) 329-5316, blakely@usgs.gov; Ralph Haugerud, (206) 553-5542, rhaugerud@usgs.gov; Tom Pratt, (206) 543-7358, tpratt@usgs.gov; Ray Wells, (650) 329-4933, rwells@usgs.gov

Personnel Office contact: Marie Guillory, (650) 329-4112, guillory@usgs.gov

Summary of Opportunities