TESTING MODELS OF LITHOSPHERIC RHEOLOGY IN NEW ZEALAND: POSTSEISMIC COULOMB STRESS CHANGES CAUSED BY THE 1848 MARLBOROUGH EARTHQUAKE

The rheology of the lithosphere underlying New Zealand is poorly known, but it certainly has significant lateral variations due to the complex tectonic setting of the region. In order to verify how significant the choice of a specific model might be in earthquake studies, we tested two different rheological models in postseismic Coulomb stress change (ΔCFS) calculations.

As our test case, we used the Awatere fault as a source fault, by applying the parameters of the M_w7.6 1848 Marlborough earthquake. We calculated the postseismic ΔCFS at various depths on the geometry and kinematics of the Hope Fault, which produced the M_w7.2 Canterbury earthquake 40 years later in 1888. Our results show that the Hope Fault lies within a positive ΔCFS lobe at the time before rupture regardless of the rheological model used, and the pattern of ΔCFS distribution changes only slightly between models. However, the values of postseismic ΔCFS calculated at a sampling point show differences up to 100% between the two rheological models for the same time step. This would influence any attempts at calculating how much sooner or later an earthquake has occurred due to the effects of a previous one. We conclude that, in a tectonic setting like New Zealand, it may still be possible to use a uniform rheological model for local studies of spatial distribution of earthquakes. Studies at regional level would however require the implementation of a laterally-variable lithospheric rheology.