HR: 08:00h
AN: G21D-01 INVITED    [Abstracts]
TI: Multi-year Postseismic Deformation in South Iceland Dominated by Viscoelastic Relaxation, but not Afterslip
AU: * Jónsson, S
EM: sj@erdw.ethz.ch
AF: Institute of Geophysics, ETH Zurich, Schafmattstr. 30, Zurich, 8093, Switzerland
AB: Several time-dependent processes follow large earthquakes, such as pore-fluid flow, postseismic slip (afterslip), and viscous relaxation of the lower crust and upper mantle. These processes cause additional stress changes in the surrounding crust and therefore influence the occurrence of aftershocks and other neighboring earthquakes. However, it has proven difficult to distinguish between these processes based on field observations, as models of the different processes can predict similar ground displacements and more than one process may be acting at the same time. The postseismic deformation observed following two South Iceland earthquakes (magnitude 6.5) in June 2000 has provided a unique opportunity to work on this problem, as different postseismic processes appear to have operated at different timescales. The early phase (a few months) of the postseismic deformation was dominated by localized displacements (within 5-10 km from the faults), showing quadrants of uplift and subsidence around the two faults. This pattern of deformation was caused by postseismic groundwater pressure changes during the 2-3 months when coseismic pore-pressure changes were dissipating. Annual GPS measurements revealed another pattern of post-seismic deformation with a much slower decay than the poroelastic relaxation. However, both afterslip and viscoelastic relaxation models are able to predict the GPS observations equally well, making it impossible to reject one of these two potential explanations based on these data alone. Additional information about the multi-year deformation was derived from times-series analysis of multiple satellite radar interferograms (InSAR) using radar data from 2000-2005. The InSAR images were multi-looked and spatially unwrapped, and then the line-of-sight displacement for each multi-looked pixel was estimated at each acquisition date. The deformation pattern revealed by the time-series analysis is consistent with models of viscoelastic relaxation, but clearly inconsistent with afterslip model predictions. The results also show decaying deformation rate with a rather poorly constrained exponential decay time-constant of 500-1000 days, suggesting a low upper-mantle viscosity of circa 1×10¹⁸ Pa s. The results provide a stark contrast to results focusing on some other moderate-size earthquakes, e.g.\ the 2004 Parkfield earthquake, where afterslip was found to be the dominating postseismic mechanism.
DE: 1209 Tectonic deformation (6924)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 7250 Transform faults
SC: Geodesy [G]
MN: 2007 Fall Meeting