Space-time evolution of the stress field following earthquakes and episodes of magma upwelling

Mechanical effects left by an earthquake on its fault plane, in the post-seismic phase, are investigated employing the "displacement discontinuity method" and imposing the release of a constant, uni-directional shear traction. Due to unsymmetric interaction between the fault plane and the free surface, significant normal stress components are left over the shallow portion of the fault surface after the earthquake (Figure 24) these are compressive for normal faults, tensile for thrust faults, and are typically comparable to the stress drop.

  

Figure 24: Normal stress induced by uniform stress drop over a high-dip normal fault and a low-dip thrust fault.

In Figure 24 the s-axis is along the strike of the fault, the d-axis is along the dip (positive upwards). Several observations can be explained from the present model: low-dip thrust faults and high-dip normal faults are found to be favoured, 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; the shallow aftershock activity commonly observed in the hanging block of a thrust event is easily explained. A paper has been submitted for publication.

Effects of structural inhomogeneities on the stress and displacement fields induced by strike-slip faults in layered media is presently under study. An elastic medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space characterized by different elastic parameters. The case of a strike-slip fault crossing the interface between two elastic media is particularly interesting. The dislocation density distribution is found to be affected by a jump discontinuity at the interface, which is responsible for inducing high concentrations of deviatoric stress, not only in proximity of fault edges, but also along the interface where it may be even higher than the stress drop on the fault plane. The displacement field observable over the ground surface (Figure 25) is found to be strongly affected by the presence of a soft sedimentary layer (with rigidity $\mu_2\ll \mu_1$).

  

Figure 25: Displacement at the free surface after a strike-slip event with uniform stress drop: layered half-space solutions are shown as solid lines, homogeneous half-space solutions as dashed lines.

In Figure 25 the solid line shows the surface displacement induced by strike-slip faulting at depth greater than d=2 km, computed in the heterogeneous medium, the dashed line show computation in a homogeneous half-space. It appears from panel (b) that estimates of d and $\Delta\sigma$ derived from geodetic observations can be severely biased if structural heterogeneities are not taken into account. A paper is in preparation.