HR: 10:35h
AN: G12A-02 INVITED     [Abstracts]
TI: Transient Rheology of the Upper Mantle: Evidence From two Case Studies
AU: * Pollitz, F F
EM: fpollitz@usgs.gov
AF: USGS, 345 Middlefield Rd., MS977, Menlo Park, CA 94025 United States
AB: Geodetic observations indicate that both the M7.1 1999 Hector Mine and M7.9 Denali earthquakes were followed by a brief period ($\sim 0.1$ year) of rapid postseismic movements and a longer period ($\geq \sim 2$ years) of slower postseismic movements still elevated well above pre-earthquake velocities. The involvement of GPS sites more than 50 km and 100 km from the Hector Mine and Denali ruptures, respectively, suggests a deep source for the observed postseismic deformation. Viscoelastic relaxation of the regional lower crust and/or mantle governed by either a nonlinear or transient rheology (represented by a Burghers body) would be consistent with the observed time dependence. Detailed examination of 3-component GPS time series covering the first $\sim 2$ years after each earthquake indicates that relaxation of the regional upper mantle governed by a Burghers body rheology is consistent with the temporal and spatial patterns of relaxation observed after both earthquakes. A relaxation model involving a linear Maxwell rheology in the upper mantle fails to explain the same patterns. These results suggest that a combination of anelastic and viscous deformation mechanisms in the uppermost mantle is generally important in shaping the Earth's response to large earthquakes.
DE: 1208 Crustal movements--intraplate (8110)
DE: 1236 Rheology of the lithosphere and mantle (8160)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting