HR: 1340h
AN: G13A-0921    [Abstracts]
TI: Coupled afterslip and viscous flow following the 2002 Denali, Alaska earthquake
AU: * Johnson, K M
EM: kajjohns@indiana.edu
AF: Indiana University, 1001 E. 10th St., Bloomington, IN 47405, United States
AU: Burgmann, R
EM: burgmann@seismo.berkeley.edu
AF: University California, Berkeley, 307 McCone Hall, Berkeley, CA 94720, United States
AU: Freymueller, J
EM: jeff@giseis.alaska.edu
AF: University of Alaska, Fairbanks, Geophysical Institute, Fairbanks, AK 99775-7320, United States
AB: We investigate the processes of postseismic deformation following the 2002 Denali Fault, Alaska earthquake using 4.5 years of continuous and campaign GPS data. Afterslip is modeled on a fault in an elastic lithosphere overlying a Maxwell (linear) viscoelastic asthenosphere. We assume afterslip is governed by a nonlinear velocity- strengthening friction law. Postseismic GPS time-series are best explained by a combination of two mechanisms: viscous flow in the lower crust and upper mantle with viscosity of about 10¹⁹ Pa s, and afterslip on the fault above 30-40 km depth. Models with afterslip only (no distributed viscous flow) underestimate displacements at sites more than 100 km from the fault. The rate-state frictional parameter a-b, is estimated to be in the range 10^-3-10^-2, consistent experimental values for granite at conditions near the transition from velocity weakening to velocity strengthening. It has been suggested previously that nonlinear rheology of the upper mantle is necessary to explain the observed evolution of surface displacement rates with time. However, the displacement rates at continuous GPS sites are reproduced remarkably well by our model with afterslip in a fault zone with nonlinear rheology and a linear viscous upper mantle. The Denali earthquake may have caused increased locking at the interface of the subducting Pacific plate south of the Denali Fault. Northeast directed horizontal surface velocities at GPS sites over 100 km south of the Denali fault increased following the earthquake. The magnitude of the acceleration at these sites in southern Alaska cannot be explained with our simple models of postseismic deformation associated with afterslip and viscous flow directly below the Denali fault. The Denali earthquake reduced the reverse-sense of shear stress on the subduction interface, promoting increased coupling on the interface. Simple spring-slider models with rate-state friction confirm the possibility of increased coupling of the interface following the earthquake. The spring-slider model predicts that a sudden decrease in stress on a fault sliding with friction parameters near the transition from velocity strengthening to velocity weakening can cause the slider to stop sliding for a period of 5-10 years.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
SC: Geodesy [G]
MN: 2007 Fall Meeting