HR: 17:30h
AN: G44A-07    [Abstracts]
TI: Evidence of Power-Law Flow in the Upper Mantle Beneath Southern Alaska
AU: * Freed, A M
EM: freed@purdue.edu
AF: Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AU: Brgmann, R
EM: burgmann@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California Berkeley, 389 McCone Hall, Berkeley, CA 94720 United States
AU: Calais, E
EM: ecalais@purdue.edu
AF: Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AU: Freymueller, J T
EM: jfreymue@gi.alaska.edu
AF: Geophysical Institute, University of Alaska, Fairbanks, PO Box 757320, Fairbanks, AK 99775 United States
AB: During the first 2.5 years following the 2002 M = 7.9 Denali, Alaska strike-slip earthquake, a large array of Global Positioning System (GPS) receivers recorded rapid postseismic surface motions extending at least 300 km from the rupture and at rates of more than 100 mm/yr in the near-field. In an initial study, we used 3-D viscoelastic finite element models to infer that observed cumulative far-field (> 100 km) surface displacements were best explained by rapid viscoelastic flow in the mantle below a depth of 60 km, with a component of lower crustal relaxation between 30 and 50 km depth [Freed et al., JGR, 2005]. Here we extend the study by refining the model to match time-series data (daily measurements from continuous GPS receivers, semi-annual data for campaign sites) that show very rapid early postseismic transients that slow dramatically with time. Our model considered power-law rheologic structure where strain rate equals C(d) times stress raised to the power n, where C(d) is a depth-dependent constant incorporating the influence of all rheologic parameters normally found in power-law expressions (A, Q, T, etc.). For each assumption of the power-law exponent n, we iteratively solved for the C(d) that provided the minimum misfit to the time-series data. We focus our initial modeling efforts on far-field sites where the contribution to the displacements of afterslip is probably minimal, based on reasonable afterslip models, so that the time-dependence of afterslip is not aliased into our estimation of viscosity structure. We found that far-field time-series observations cannot be explained by a Newtonian rheology (n = 1), as such rheologies cannot match the rate of decay of the displacements. Across both the lower crust and upper mantle, an average power-law exponent of n = 2 provides a reasonable solution, while higher exponents under-estimate longer term strain rates. Consideration of models in which the power-law varies between the crust and mantle find that time-series data is best fit by a composite rheology of a mantle with a power-law exponent of n = 3.5 and a lower crust with a power-law of n = 1. Because of trade-offs between parameters such as activation energy, water fugacity, and temperature, a unique rheologic environment cannot be defined. However, this solution can be shown to be consistent with diffusion creep of a feldspar dominated crust overlying dislocation creep of wet olivine, making this composite rheologic model our most appealing solution. Our results illustrate the importance of considering the absolute stress field in power-law solutions, and provide insights into the 3-D distribution of stress, strain rate, and effective viscosity within the Alaskan lithosphere as a function of time after the Denali earthquake. This evolution shows that coseismic stress changes induce a temporary weak lithospheric rheology leading to observed rapid postseismic surface deformations, eventually reverting to a much stronger rheology capable of supporting long-term topographic loads. Once the contributions of a deep viscoelastic rheology is accounted for to explain observed far-field displacements following the Denali earthquake, residual near-field displacements enable inference of contributions from time-dependent afterslip and poroelastic rebound in the shallow crust.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Geodesy [G]
MN: Fall Meeting 2005