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Quantitative Structural Analysis of the Seattle Fault : Three-Dimensional Constraints on Thrust Fault Structure, Kinematics, and Seismic Hazard

Project Title: Quantitative Structural Analysis of the Seattle Fault: Three-Dimensional Constraints on Thrust Fault Structure, Kinematics, and Seismic Hazard
Mendenhall Fellow: Megan Anderson, (650) 329-5204,
Duty Station: Menlo Park
Start Date: October 24, 2005
Education: Ph.D. (Geoscience), University of Arizona, Tucson, 2005
Research Advisors: Rick Blakely, (650) 329-5316,; Tom Brocher, (650) 329-4737,; Ray Wells, (650) 329-4933,; Tom Pratt, (206) 543-7358,; Ralph Haugerud, (206) 553-5542,
  Megan Anderson

Project Description: The Puget Lowland is located in the Cascadia forearc of Washington, situated between the subduction zone accretionary wedge complex ( Olympic Mountains) to the west and the associated volcanic arc to the east (fig. 1). The Puget Lowland is seismically active, due to the interaction of two tectonic forces: oblique subduction of the Juan de Fuca plate under North America and clockwise rotation of the Washington/Oregon forearc. Forearc rotation may be caused by the oblique subduction of the Juan de Fuca plate, northward movement of the Sierra Nevada block, and/or Basin and Range extension. Recent GPS and seismicity studies show that the forearc continues to rotate and translate northward at 3 to 7 mm/yr. The stationary Canadian coastal mountains resist this northward migration, thus creating north-south compression in the northern Washington forearc.

Structural elements of
      the Puget Lowland   Figure 1. Structural elements of the Puget Lowland [Johnson and others, 1999] . S = Seattle; SF = Seattle Fault; SWF = southern Whidbey Island fault; T = Tacoma; SB = Seattle Basin; HC = Hood Canal; CBF = Coast Range Boundary Fault. The dotted line near Tacoma is the Tacoma fault. Triangles represent arc volcanoes.

Some of this compression is accommodated by the Seattle fault, a seismically active thrust fault in the Puget Sound, which lies beneath the cities of Seattle, Bellevue, and Bremerton. It is part of a group of east-west–oriented thrust structures with documented Holocene slip that exist throughout the Puget Lowland (here termed the Puget Lowland fault system). Because of their recent activity, these faults pose great hazard for Seattle and other communities in the Puget Lowland. Trenching across postglacial fault scarps revealed by laser terrain mapping indicate that multiple earthquakes have occurred along the Seattle fault in the last 3000 years. Moreover, many of the Puget Lowland communities lie on sediments deposited in the structural lows created by the Puget Lowland fault system and tectonic forearc subsidence, and are likely to experience amplified shaking in the event of a large magnitude earthquake. A solid understanding of the structure of the Puget Lowland fault system, its interaction with basin sediment, and its place in the greater tectonic setting is necessary for accurate characterization of seismic hazards in the Puget Lowland.

Published models of thrust structures in the Puget Lowland are strongly grounded in data, including geologic mapping, trench excavations, geomorphology (lidar), seismicity, tomography, seismic reflection, gravity, and aeromagnetic anomalies. These data, considered as a whole, give quite good constraints on many structures in 2- and even 3-D, especially in the near surface. There is, however, some discordance between patterns of recent uplift from geomorphic data and fault geometry at depth described by seismic and potential field data. Several 2-D, north-south–oriented models have been developed to try and resolve these discrepancies, but many areas of disagreement still exist between models and several have some shortcomings.

The main goal of this project is to rectify these inconsistencies through investigation of the Seattle fault zone in 3-D. I will work towards creating a new, structurally consistent model for the Puget Lowland fault system that integrates all available data sets (geomorphic, paleoseismic, geologic, reflection seismic, tomographic, potential field, and seismicity). A few questions I would like to target with this approach are:

  1. Paleoseismologic and geomorphic data indicate recent southward-directed slip on northward-dipping thrust faults in the Seattle fault zone. Yet seismic imaging of the upper- to mid-crust indicates the Seattle fault is a southward-dipping fault with northward-directed slip. How can we merge the surficial and shallow crustal structures within one structural model?

  2. Are existing 2-D models of the Puget Lowland fault system structurally balanced?

  3. Some structural models of the lowland are tied to mid- to lower-crustal structures, but not the newer models that try to integrate the disagreeing kinematics in the shallow crust. Can these newer models be placed within a regional fault system?

  4. There are many indications that structures in the Puget Lowland are not inherently 2-D (fig. 2). Does the accommodation of shortening change its structure along strike of the Seattle fault? If so, can we quantify this change and attribute it to a particular source, such as change in shortening along strike of the Seattle fault, or movement of thrust slices oblique to the strike of the Seattle fault?

Tomographic and potential-field
         data from the Seattle uplift   Figure 2. Tomographic and potential-field data from the Seattle uplift [Brocher and others., 2004]. Strands of the Seattle fault zone bound the uplift to the north and are indicated by the dotted lines. Note the changes in seismic velocity structure (A) and potential field signal (B, gravity; C, aeromagnetic) along strike of the Seattle fault zone within the Seattle uplift, especially the aeromagnetic anomalies (C). Changes in these geophysical anomalies could indicate changes in structural style and/or shortening along strike of the Seattle fault.

Understanding the origin and geometry of the Puget Lowland fault system in 3-D will improve our understanding of strain partitioning or changes in crustal properties along the forearc. The process of creating this model will also target data collection in areas where discrepancies are still unresolved. Moreover, a 3-D model will serve as a tool for characterizing several aspects of seismic hazard for communities in the Puget Lowland, including likelihood and magnitude of potential fault slip in a major earthquake and geometry of structures at depth and their influence on shaking.

References Cited

Brocher, T.M., Blakely, R.J., and Wells, R.E., 2004, Interpretation of the Seattle uplift, Washington, as a passive-roof duplex: Bulletin of the Seismological Society of America, vol. 94, no.4, p. 1379–1401.

Johnson, S.Y., Dadisman, S.V., Childs, J.R., and Stanley, W.D., 1999, Active tectonics of the Seattle fault and central Puget Sound,
Washington–Implications for earthquake hazards: Geological Society of America Bulletin, vol. 111, no. 7, p. 1042–1053.

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