Effects induced by an earthquake on its fault plane:a boundary element study

Abstract

SUMMARY Mechanical eiects left by a model earthquake on its fault plane, in the post-seismic phase, are investigated employing the 'displacement discontinuity method'. Simple crack models, characterized by the release of a constant, unidirectional shear traction are investigated ¢rst. Both slip componentsparallel and normal to the traction directionare found to be non-vanishing and to depend on fault depth, dip, aspect ratio and fault plane geometry. The rake of the slip vector is similarly found to depend on depth and dip.The fault plane is found to suier some small rotation and bending, which may be responsible for the indentation of a transform tectonic margin, particularly if cumulative eiects are considered. Very signi¢cant normal stress components are left over the shallow portion of the fault surface after an earthquake: these are tensile for thrust faults, compressive for normal faults and are typically comparable in size to the stress drop. These normal stresses can easily be computed for more realistic seismic source models, in which a variable slip is assigned; normal stresses are induced in these cases too, and positive shear stresses may even be induced on the fault plane in regions of high slip gradient. Several observations can be explained from the present model: low-dip thrust faults and high-dip normal faults are found to be facilitated, according to the Coulomb failure criterion, in repetitive earthquake cycles; the shape of dip-slip faults near the surface is predicted to be upward-concave; and the shallower aftershock activity generally found in the hanging block of a thrust event can be explained by 'unclamping' mechanisms.