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Willis, J.B., 2010

Subduction zone geometry and stress transfer during megathrust earthquakes, upper Cook Inlet basin, Alaska

Bibliographic Reference

Willis, J.B., 2010, Subduction zone geometry and stress transfer during megathrust earthquakes, upper Cook Inlet basin, Alaska: University of Utah, Salt Lake City, Ph.D. dissertation, 136 p.


Field observations and elastic and viscoelastic dislocation models of the 1964 Great Alaska Earthquake, Mw 9.2, demonstrate how complex subduction zone geometry affects static and time-dependent stress transfer from megathrust ruptures to upper plate faults. A three-dimensional elastic dislocation model of the 1964 earthquake uses triangular subfaults to capture sharp changes in trend and spacing of Benioff contours that define the top of the subducting slab as it transitions from steep subduction of the Pacific Plate (PP) to shallow subduction of the Yakutat terrane (YT), an allochthonous continental fragment. The model demonstrates that slab geometry strongly influences coseismic stress transfer into the upper plate, ascertaining that the anomalous upper Cook Inlet stress field (maximum horizontal stress oriented ~45 degrees counterclockwise from subducting plate motion), is partially due to slip on the complex subduction interface. The model further ascertains that slip on the complex interface causes marked variations in coseismic stresses transferred to the Castle Mountain fault (CMF), which shows marked along-strike variability in seismogenic behavior and straddles the underlying edge of the YT. The western CMF is the only upper plate fault with unequivocal Holocene surface rupture in the greater Anchorage area. Herein, a previously unrecognized offset margin of a postglacial outwash channel is identified and used to establish a right lateral slip rate of 3.0 +/- 0.6 mm/yr -1 and a potential rupture magnitude of 6.9-7.3 for that fault segment. A time-dependent viscoelastic Maxwell rheological model of the 1964 earthquake suggests a time-delayed rotation of principal stresses from a reverse to a strike-slip faulting regime along the CMF. The delay may indicate a temporal link between megathrust earthquakes and CMF rupture. The time-dependent model further suggest that megathrust-induced changes in shear and normal stresses, and thus in rupture potential, on the Castle Mountain fault and on transpressional faults of upper Cook Inlet basin will peak ~60 years post-1964. The transpressional faults core hydrocarbon-producing anticlines of the Cook Inlet petroleum province. These faults and the western CMF pose a significant seismic hazard to the greater Anchorage area and to petroleum infrastructure of Cook Inlet basin.

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