The postseismic response to the 2002 M 7.9 Denali Fault earthquake: constraints from InSAR 2003–2005

Abstract

InSAR is particularly sensitive to vertical displacements, which can be important in distinguishing between mechanisms responsible for the postseismic response to large earthquakes (afterslip, viscoelastic relaxation). We produce maps of the surface displacements resulting from the postseismic response to the 2002 Denali Fault earthquake, using data from the Cana-dian Radarsat-1 satellite from the periods summer 2003, summer 2004 and summer 2005. A peak-to-trough signal of amplitude 4 cm in the satellite line of sight was observed between summer 2003 and summer 2004. By the period between summer 2004 and summer 2005, the displacement rate had dropped below the threshold required for observation with InSAR over a single year. The InSAR observations show that the principal postseismic relaxation process acted at a depth of ∼50 km, equivalent to the top of the mantle. However, the observations are still incapable of distinguishing between distributed (viscoelastic relaxation) and localized (afterslip) deformation. The imposed coseismic stresses are highest in the lower crust and, assuming a Maxwell rheology, a viscosity ratio of at least 5 between lower crust and upper mantle is required to explain the contrast in behaviour. The lowest misfits are produced by mixed models of viscoelastic relaxation in the mantle and shallow afterslip in the upper crust. Profiles perpendicular to the fault show significant asymmetry, which is consistent with differences in rheological structure across the fault. Postseismic deformation is the transient response of the lithosphere to the sudden change of stresses caused by an earthquake. Observations show that displacements are typically an order of magnitude smaller than coseismic displacements and decay with longer spatial wavelengths and that the rate of displacement decreases with time over a period of years following the earthquake. A number of mechanisms have been proposed to explain this response, including afterslip on a discrete fault plane (e.g. Bürgmann et al. 2002), creep in a viscous or viscoelastic shear zone viscoelastic relaxation in the lower crust/upper mantle (e.g. Pollitz et al. 2000) and poroelastic rebound (e.g. Jonsson et al. 2003). To investigate the causal mechanism behind the postseismic transient , it is necessary to observe surface displacements with good spatial and temporal resolution over time periods ranging from days to decades following the earthquake. Triangulation and levelling surveys offer measurements of long-term postseismic response from historic earthquakes such as the 1906 San Francisco earthquake (e.g. Kenner & Segall 2000). For such historic earthquakes, space geodetic techniques such as InSAR …

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