COSEISMIC AND POSTSEISMIC SUBSURFACE DISPLACEMENTS AND STRAINS FOR A VERTICAL STRIKE-SLIP-FAULT IN A 3-LAYER ELASTIC MEDIUM

A three-layer elastic-gravitational fault displacement model using dis location theory has been developed and used to examine the effect of l ayering of earth elastic moduli on surface and subsurface displacement fields for a vertical strike-slip fault. The model has been used to e xamine the effect of depth variation of elastic properties at coseismi c and postseismic time scales. For pure strike-slip motion the effect of gravity on coseismic and postseismic horizontal deformation is negl igible. For coseismic deformation the model predicts that (for constan t Poisson's ratio) an increase in elastic moduli with depth attenuates the displacements within the upper layers with respect to displacemen t distribution for a uniform half-space, while an inclusion of a soft layer between the top layer and lower half-space amplifies upper layer displacements. The effect of variation in Poisson's ratio on surface and subsurface displacements has also been examined. The effect of pos tseismic stress relaxation on surface and subsurface displacements for a three-layer model has been calculated and compared with that of a u niformly relaxed half-space model. Layer 1 is assumed to correspond to the upper crust, layer 2 the lower crust and layer 3 the upper mantle . The effect of postseismic stress relaxation within a uniform half-sp ace and within just the lower crust and upper mantle has been examined . Stress relaxation within the whole half-space decreases the amplitud e and shortens the wavelength of displacements, while stress relaxatio n within the lower two layers increases the amplitude and broadens the wavelength of displacements. The difference between uniform and layer ed postseismic relaxation is particularly pronounced at the base of th e crust. Coseismic and postseismic normal and volumetric strains for a vertical strike-slip fault have also been examined. For a uniformly r elaxed half-space model, an increase in normal strains is shown with r espect to the coseismic elastic solution, whereas the postseismic volu metric strain is effectively zero. For a three-layer model with stress relaxation in the lower layers only, the nor-mal and volumetric strai ns within the top elastic layer resemble coseismic strains, while in t he lower layers which suffer a rigidity decrease, the postseismic volu metric strain is effectively zero.