The lowdown on postseismic deformation: Integrated studies from Tibet

Abstract:

Postseismic surface deformation is used to investigate both fault behaviour and lithospheric rheology, with the two being closely linked. Studies of postseismic deformation are therefore important in enabling us to characterize the progression of an earthquake cycle, as well as potentially feeding into long timescale geodynamic models. We have studied a number of earthquakes on the seismically active Tibetan Plateau. Collectively, the postseismic phases of these events reveal the processes that occur in the months to decades following major earthquakes. Our aim in this presentation is to highlight what we consider are the important facets of these processes, in order to aid future postseismic studies. In some cases there is a clear signature of afterslip only (e.g. the Nima-Gaize, Yutian and Damxung earthquakes in 2008), with no detectable contribution from viscoelastic relaxation (VER). Such cases enable straightforward characterization of afterslip (e.g. duration, spatial distribution), which can be used to inform other studies. There is also one case of viscoelastic relaxation only (the 1951/1952 Beng Co/Gulu earthquakes), with no “contamination” from afterslip within the late postseismic observation period. In light of this latter case, we re-examine previously-published InSAR datasets for the 1997 Manyi and 2001 Kokoxili earthquakes. Since these were strike-slip events, interpretation of their postseismic signature has in the past caused some uncertainty. We use synthetic examples to demonstrate how to distinguish between afterslip and VER, and apply the insights gained to individual interferograms for these large earthquakes. How earthquake magnitude, focal mechanism and observation time affect postseismic deformation is also explored. Finally, we present an example of postseismic release faulting imaged by InSAR (the Zhongba earthquake, 2008), which appears to take the place of afterslip in the conventional sense. Where relevant, comparisons are made with findings from postseismic studies elsewhere in the world. As a community, we are now at the stage where enormous value can be gained from pooling results and insights from the many postseismic investigations that have been published, which will enable us to model earthquakes, faults and their associated seismic hazard more effectively.