Space geodetic observations and modeling of postseismic deformation due to the 2005 M7.6 Kashmir (Pakistan) earthquake

We analyze the L-band ALOS and C-band ENVISAT Interferometric Synthetic Apetrure data from the epicentral area of the 2005 magnitude 7.6 Kashmir (Pakistan) earthquake that occurred on the south-western edge of Himalaya. ENVISAT data are available from both the descending and ascending orbits, and span a time period of 3 years immediately following the earthquake (2005-2008), with monthly acquisitions. However, the ENVISAT data are highly decorrelated due to high topography and snow cover. ALOS data are available from the ascending orbit, and span a time period of ~2.5 years between 2007-2009, over which they remain reasonably well-correlated. We derive the mean line-of-sight (LOS) postseismic velocity maps in the epicentral area of the Kashmir earthquake using persistent scatterer method for ENVISAT data, and selective stacking for ALOS data. LOS velocities from all data sets indicate an uplift in the hanging wall of the earthquake rupture over the entire period of SAR observations (2005-2009). Models of poroelastic relaxation predict uplift of both the footwall and the hanging wall, while models of viscoelastic relaxation below the brittle-ductile transition predict subsidence of both the footwall and the hanging wall. Therefore the observed pattern of surface velocities suggests that the early several years of postseismic deformation were dominated by afterslip on the fault plane. Kinematic inversions of InSAR data, as well as forward models of stress-driven creep confirm that the observed deformation is consistent with afterslip, primarily downdip of the seismic asperity. We use the InSAR data and modeling results to place contraints on the effective viscosity of the ductile substrate in the study area. We show that in order to prevent surface subsidence, the effective viscosity has to be greater than 10^19 Pa s. The data also appear to require lateral heterogeneities in the rate-state frictional properties of the fault at the bottom of the seismogenic zone and/or and below the brittle-ductile transition.