EFFECTS OF RIGIDITY LAYERING, GRAVITY AND STRESS-RELAXATION ON 3-D SUBSURFACE FAULT DISPLACEMENT-FIELDS

Elastic dislocation theory has been modified to determine 3-D subsurfa ce displacements for faults in a three-layer elastic-gravitational med ium. A new set of kernel functions for Fourier-Bessel integrals descri bing subsurface displacements have been derived, using the Thomson-Has kell propagator matrix technique, and has been used to investigate the effect of layering and gravity on subsurface displacement fields. Wit hin our three-layer model, layer 1 may be used to represent the seismo genic upper crust, layer 2 the ductile lower crust and layer 3 the duc tile mantle. For a point source within the upper layer, lower layer ri gidity moduli control the amplitude and wavelength of displacements wi thin the upper layer and the relative distribution of uplift and subsi dence within foot and hanging wall. Displacement variations, due to lo wer layer rigidity moduli changes, increase with depth and are profoun d at the base of the upper layer and within the lower layers. High-rig idity-moduli lower layers attenuate the upper layer displacement field , while a decrease gives amplification. The effect of gravity on the s ubsurface displacement field is more pronounced when the rigidity of t he lower layers is small. The elastic-gravitational dislocation model has been used to examine co-seismic and post-seismic components of sur face and subsurface displacement during extension of continental litho sphere. The model predicts surface co-seismic footwall uplift and hang ing-wall subsidence; the co-seismic subsidence being greater than the uplift. Post-seismic relaxation of stress within the lower crust and m antle by post-seismic ductile deformation, gives an increase in footwa ll uplift and a decrease in maximum hanging-wall subsidence within the upper layer. A decrease in upper layer rigidity due to post-seismic b rittle or plastic deformation within the upper crust leads to a decrea se in the wavelength of surface footwall uplift and hanging-wall subsi dence. The elastic-gravitational dislocation model has also been used to investigate the development of Moho topography during continental e xtension. Co-seismically Moho under footwall is predicted to uplift, w hile that under hanging wall subsides but by a smaller magnitude. Duri ng post-seismic relaxation Moho topography is predicted at first to in crease in magnitude and then to decay. The existence of preserved Moho topography uplift associated with old continental rifts implies a fin ite long-term ductile strength within the lower crust and mantle.