Influence of lithospheric and mantle stratification on co- and post-seismic deformation due to finite faults

Abstract

Coseismic and post-seismic deformation in the fully relaxed state following larger earthquakes is modelled for finite faults by means of multilayered, viscoelastic, self-gravitating, hydrostatically pre-stressed, spherical earth models based on the preliminary reference earth model (PREM) with a Maxwell rheology. For both finite dip-slip and strike-slip sources we find that the elastic layering of the lithosphere has a major impact on the modelling of co- and post-seismic deformation. For large and deep earthquakes embedded at the base of the lithosphere in the proximity of the lithosphere-upper mantle interface in order to study the effects of mantle relaxation, coseismic and post-seismic deformation can result in several tens of centimetres of displacement, with a deformation pattern strongly dependent on the distance from the source and the azimuth from the strike of the fault. Strike-slip faulting is found to be more sensitive to mantle rheology than dip-slip faulting. Deviations between earth models with a homogeneous crust and lithosphere and those based on PREM are of the same order as the coseismic and post-seismic signals, ranging from a few centimetres to several tens of centimetres for large earthquakes with seismic moments of 1022 N m and epicentral depths of 75-100 km at the base of the lithosphere. This finding lends further support to the conclusion of a preliminary study based on dip-slip point sources that observations based on VLBI, GPS and SAR techniques, which can easily resolve crustal deformation of the order of the deviations among the various lithospheric models, can be badly misinterpreted if the results of surveying in seismogenic regions are compared with earth models that have a limited number of layers in general and a homogeneous crust in particular.