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Linking the Yakima Fold Belt to Its Forearc Analogs Through the Cascade Arc: Constraints on Active Fold Belt Deformation between the Puget Lowland and Portland, Oregon: Sean P. Bemis
Project Title:Linking the Yakima Fold Belt to Its Forearc Analogs Through the Cascade Arc: Constraints on Active Fold Belt Deformation between the Puget Lowland and Portland, Oregon
Mendenhall Fellow: Sean P. Bemis, (650)329-5308, spbemis@usgs.gov  
Duty Station: Menlo Park, CA
Start Date: February 1, 2010
Education:Ph.D., University of Oregon, 2010
Research Advisors: Ray Wells, (650)329-4933, rwells@usgs.gov; Brian Sherrod, (206)553-0153, bsherrod@usgs.gov; Rick Blakely, (650)329-5316, blakely@usgs.gov
  Sean P. Bemis

Project Description: My project seeks to address a gap in our understanding of seismic hazards and the neotectonic framework of the Puget–Willamette Lowland, where structural analogs to the Yakima Fold Belt (YFB) cross the urban corridor (fig. 1). I am examining a transect from the northern Willamette Valley to Olympia, Wash., to constrain Quaternary activity of several potentially active faults. To the north in the Puget Lowland, studies have documented recent movement on more than 11 faults (for example, Sherrod and others, 2008). Near Portland, only three faults with known Quaternary displacement have been identified. No active faults are known between those faults and Olympia, Wash., despite topographic, stratigraphic, geophysical, and geodetic indications of young tectonic activity (for example, Blakely and others, 2000; Liberty and others, 2003; Wilson, 2003; McCaffrey and others, 2007). Furthermore, these faults and folds are similar in form to the YFB in central Washington. Together, the Yakima Fold Belt and the structures in the Cascadia forearc may be a continuous system that accommodates significant north-south shortening. Transverse fault zones near Chehalis, Wash., and Portland, Oreg., may be part of this YFB system.

I am undertaking detailed geomorphic, neotectonic, and paleoseismic investigations to characterize the late Quaternary activity on the Gales Creek–Canby/Molalla fault zone near Portland, Oreg., and the Doty fault in southwest Washington, which are both recognized for their potential activity (Liberty and others, 1999; Blakely and others, 2000; Wilson, 2000; Wongand others, 2000). Constraining the structural geometry, Quaternary slip rates, and earthquake recurrence on these faults is fundamentally important for our understanding of (1) the earthquake hazards from crustal faults along the Northwest urban corridor, (2) how margin-parallel strain is being accommodated in the Cascadia forearc, and (3) how active structures in the YFB may connect to active structures in the forearc. These results will contribute to the goals of the U.S. Geological Survey by specifically addressing the recognized need for characterizing the location, size, and occurrence of geologic hazards.

References:
Blakely, R. J., Wells, R.E., Tolan, T.L., Beeson, M.H., Trehu, A.M., and Liberty, L.M. 2000, New aeromagnetic data reveal large strike-slip (?) faults in the northern Willamette Valley, Oregon: Geological Society of America Bulletin, vol. 112, no. 8), p. 1225–1233, doi:10.1130/0016-7606(2000)112<1225:NADRLS>2.0.CO;2.

Liberty, L.M., Hemphill-Haley, M.A., and Madin, I.P., 2003, The Portland Hills Fault: Uncovering a hidden fault in Portland, Oregon using high-resolution geophysical methods: Tectonophysics, vol. 368, nos. 1–4, p. 89–103.

Liberty, L. M., Trehu, A.M., Blakely, R. J., and Dougherty, M.E., 1999, Integration of high-resolution seismic and aeromagnetic data for earthquake hazards evaluations: An example from the Willamette Valley, Oregon: Bulletin of the Seismological Society of America, vol. 89, no. 6, p. 1473–1483.

McCaffrey, R., Qamar, A.I., King, R.W., Wells, R., Khazaradze, G., Williams, C.A., Stevens, C.W., Vollick, J.J., and Zwick, P.C., 2007, Fault locking, block rotation and crustal deformation in the Pacific Northwest: Geophysical Journal International, vol. 169, no. 3, p. 1315–1340, doi:10.1111/j.1365-246X.2007.03371.x.

Sherrod, B.L., Mazzotti, S., and Haugerud, R., 2008, Comparison of geodetic and paleoseismic rates of deformation in the Puget Sound-Georgia Basin, Pacific Northwest, in Eos Transactions of the American Geophysical Union, Fall Meeting Supplement, vol. 89, no. 53, p. T21B–1953.

Wilson, D.C., 2003, Hydrogeology and water resource potential of Neogene sediments in the Tualatin Basin and comparison with the neighboring Portland Basin, Northwest Oregon: Environmental and Engineering Geoscience, vol. 9, no. 4, p. 327–338, doi:10.2113/9.4.327.

Wong, I., Silva, W., Bott, J., Wright, D., Thomas, P.,Gregor, N., Li, S., Mabey, M., Sojourner, A., and Wang, Y., 2000, Earthquake scenario and probabilistic ground shaking maps for the Portland, Oregon, metropolitan area: Oregon Department of Geology and Mineral Industries Interpretive Map Series IMS-16, 11 map sheets, 20 p.


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