Task 2: Global post-seismic rebound following strike-slip and normal faulting earthquakes

Start: March 1996 (month 1)
End: February 1997 (month 12)
Responsible partner: UBLG.DF

Maurizio Bonafede, Antonio Piersanti and Giorgio Spada

In order to study the post-seismic rebound following large lithospheric earthquakes we have built a spherical, self-gravitating earth model with viscoelastic rheology []. This model, which allows to compute coseismic and postseismic displacements associated to lithosperic earthquakes, has been originally employed to predict horizontal and vertical rates of deformations in Alaska [1]. The results were compared with geodetic data in order to better constrain the rheological structure of the upper mantle beneath Alaska. Although motions associated with rift dynamics and postglacial adjustment are expected to contribute in a dominant way to present-day velocities in this area, new insights are expected from the application of our postseismic rebound model to Iceland. Accordingly the model has been further developed to account for strike-slip earthquakes characteristic of the SISZ and for rifting activity along the middle Atlantic Ridge, described in terms of a distribution of tensile dislocations.

  

Figure: See text for discussion.

  

Figure: See text for discussion.

A simple earth model was preliminarly employed, which includes a 100 km thick elastic lithosphere, a uniform mantle with Maxwell rheology, and a fluid inviscid core. The source of deformation consists of a 200 km long tensile fault buried at a depth of 50 km. Figure 30 portrays the coseismic surface displacement u (in centimeters) observed at a given distance from the fault along different azimuths $\alpha$ (namely, , and from top to bottom). The surface displacement is decomposed along the spherical unit vectors r, $\theta$, and $\phi$ (dash-dotted, solid, and dotted curves, respectively). Figure 31 shows the long term relaxation of the displacement field. The time-scales governing the transition from coseismic to postseismic displacements depends essentially on the viscosity stratification of the mantle. For an upper mantle characterized by a relatively low viscosity (such as the mantle beneath Iceland) these time-scales amount to a few years. Large amounts of relaxation may affect all of the components of the displacement field. In particular, we observe amplifications by a factor of 2 for the $\theta$ and r components of displacements. Another interesting feature of Figure 31 is the large spatial scale of the region experiencing horizontal motions in the postseismic regime. The solutions and algorithms developed under Task 2 were employed as working tools for addressing Tasks 3 and 5. Results have been discussed at a PRENLAB workshop [], a paper is in preparation.