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.
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
(namely,
,
and
from top to bottom). The surface displacement is decomposed along the
spherical unit vectors r,
,
and
(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
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.