Tandem afterslip on connected fault planes following the 2008 Nima-Gaize (Tibet) earthquake

On 9 January 2008 a M 6.4 normal-faulting earthquake occurred in central Tibet, near the border of Nima and Gaize counties and just north of the Bangong-Nujiang suture zone. A week later, a M 5.9 aftershock occurred a few kilometers to the northwest of the main shock. Here we consider interferometric synthetic aperture radar (InSAR) data from the Japanese ALOS and European Envisat satellites, covering both the coseismic phase and 9 months of postseismic deformation. The coseismic interferograms clearly show surface deformation resulting from both main shock and aftershock ruptures, and data inversions using elastic dislocation models suggest that two northwest dipping faults form a synthetic system, with the more steeply dipping aftershock plane meeting the main shock plane at depth. Postseismic interferograms show first-order similarities with their coseismic counterparts, indicating that afterslip occurred on both main shock and aftershock rupture surfaces during the months following the earthquakes. The afterslip occurred at comparable depths to the coseismic slip, but the amount of slip was about an order of magnitude smaller. A slip template method is used to obtain moment release estimates at different postseismic time intervals and hence document the time dependence of the postseismic transient. The exponential decay time of the afterslip is 34 days, and the moment release due to the afterslip was about 10% of the coseismic moment. Models of viscoelastic stress relaxation in a Maxwell half-space place a strong lower bound on midcrust to lower crustal viscosity of 3 x 10(17) Pa s. Postseismic data covering a longer time span have the potential to improve this constraint.