COSEISMIC AND POSTSEISMIC SUBSURFACE DISPLACEMENTS AND STRAINS FOR A DIP-SLIP NORMAL-FAULT IN A 3-LAYER ELASTIC-GRAVITATIONAL MEDIUM

A three layer elastic-gravitational fault displacement model has been developed using elastic dislocation and employed to examine the effect s of rigidity layering, gravity, and stress relaxation on surface and subsurface displacements fields for a dip-slip normal fault. Within ou r three layer model, layer 1 represents the upper crust, layer 2 the l ower crust, and layer 3 the upper mantle. The fault is embedded in the upper crust. Horizontal as well as vertical displacement components h ave been determined. Displacement field changes due to postseismic str ess relaxation have been calculated using both the correspondence prin ciple and relaxed rigidity moduli, and results are in close agreement. The relaxed rigidity method provides an accurate and computationally efficient method of examining postseismic relaxation. Postseismic rela xation within the lower two layers only, gives surface uplift, increas ing footwall uplift and decreasing hangingwall subsidence, and also in creases the wavelength of surface vertical displacement. Postseismic s tress relaxation within all three layers (i.e., uniform half-space) pr oduces a shorter wavelength of surface vertical deformation with respe ct to the coseismic response. Moho topography created during coseismic deformation is initially amplified during postseismic relaxation. The relaxed Moho topography is dependent on the strength of the upper cru st as well as the strength of the lower crust and mantle. Gravity has a significant influence on displacements only,at the postseismic stage when the effective rigidity of the lower layers is small. Coseismic a nd postseismic normal strains associated with dip-slip normal faulting have been examined. For a large normal basement fault intersecting th e free surface, the coseismic horizontal surface strain, perpendicular to fault strike, is compressive adjacent to the fault and in the foot wall, and tensile in the hangingwall away from the fault. Coseismic st ress redistribution may generate significant tensile brittle failure o f the upper crust adjacent to large basement faults.