HR: 10:50h
AN: G12A-03 INVITED     [Abstracts]
TI: Deep Lithospheric Mantle and Heterogeneous Crustal Flow Following the 2002 Denali, Alaska Earthquake
AU: * Freed, A M
EM: freed@purdue.edu
AF: Dept. of Earth & Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907 United States
AU: Burgmann, R
EM: burgmann@seismo.berkeley.edu
AF: Dept. of Earth and Planetary Science, Univ. of California, Berkeley, 389 McCone Hall, Berkeley, CA 94720 United States
AU: Calais, E
EM: ecalais@purdue.edu
AF: Dept. of Earth & 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, Univ. of Alaska, Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AU: Hreinsdottir, S
EM: sigrun@giseis.alaska.edu
AF: Geophysical Institute, Univ. of Alaska, Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AB: A large earthquake can be utilized as a rock deformation experiment in which sudden stress changes trigger an observable postseismic response that can be used to infer rheological properties of the lithosphere. Such experiments seek to understand the relative contributions of postseismic viscous flow, poroelastic rebound, and afterslip, the depth at which such mechanisms are most prominently active, and the nature of the strain rate-to-stress relationship (linear versus non-linear). Here we study the response of the Alaskan lithosphere to the 2002 M7.9 Denali, Alaska earthquake. We utilize 2 years of campaign and continuous GPS observations of surface deformation to constrain finite element models of the various candidate postseismic mechanisms. We find that observed far-field deformation (100 km or more from the rupture surface) can only be explained by processes extending deep into the lithosphere. This broadly distributed deformation can be explained by viscous flow at a depth of 50 to 100 km or by deep afterslip on a downdip extension of the Denali fault, dominantly at a depth of 45 to 60 km. The problem with such deep afterslip, other than the fact that it results from a kinematic optimization with no constraining physics, is that it implies a large separation between the depth of coseismic slip (0 to 20 km depth) and the depth of afterslip, which may be difficult to physically explain. Deep flow that satisfies long wavelength surface deformation cannot, however, fully explain observed surface deformation within 50 km of the fault. Near-field displacements require additional flow near the base of the crust (24 to 30 km deep) or an afterslip distribution that includes significant (more than 1 m) shallow slip. Poroelastic rebound appears to play a minor role in the postseismic deformations. Viscous models that only vary flow parameters with depth cannot explain significant deformation observed to occur to the south of the Denali fault near its junction with the Totschunda fault. We find that this deformation can, however, be explained by localized mid-crustal flow in a region which coincides with anomalously high seismic velocities. This region could correspond to a mafic (high velocity) pluton that may be viscously weak because of partial melts or a high water content, though such an interpretation is only conjecture at this stage in our analysis. Time series data from continuous GPS stations indicate that the relationship between postseismic strain rate and stress is not linear (i.e. not Newtonian). We find that the decay of observed surface deformations with time can be explained by viscous relaxation involving a power-law in which strain rate is proportional to stress raised to a power between 2 and 3. This is below the experimentally derived powerlaw exponent of about 3.5 for dislocation creep of olivine, the mineral most likely controlling deformation within the mantle. This may indicate that viscous flow in the mantle beneath the Denali region may be comprised of a combination of dislocation and diffusion creep or contributions from other shallow mechanisms.
DE: 8160 Rheology--general
DE: 7205 Continental crust (1242)
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 1242 Seismic deformations (7205)
DE: 1243 Space geodetic surveys
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting