Task 4: Modelling of a spreading ridge

Start: October 1996 (month 8)
End: March 1997 (month 13)
Responsible partner: UBLG.DF

Andrea Antonioli, Maurizio Bonafede, Antonio Piersanti and Giorgio Spada

The dynamics of a spreading ridge has been studied by means of an approach based on a spherical earth model. The main goal of this study is to enlighten the impact of episodic uprises of magma along the Iceland rift on intermediate and large-scale ground deformations. The method employed, based on a spherical model with radially varying rheology, is particularly suitable for this purpose, since a realistic time-evolution for the opening of the rift can be easily introduced. In addition, it possible to appreciate the time-dependent effects associated with the delayed response of the ductile properties of the soft asthenosphere beneath Iceland. This method employed is more appropriate than previous ones, based on flat half-spaces with purely elastic properties, in that it also allows for a self-consistent description of the effects associated with the gravity field. The interesting possibility that the episodic uprise of magma along the Iceland rift may induce time-dependent stress accumulation in the surrounding regions, with triggering effects on the seismic activity along transform faults of Iceland is under study. A manuscript is in preparation on this topic: Time-dependent deformations driven by spreading ridges in a spherical earth [].

  

Figure: See text for discussion.

In Figure 33, we show the time-dependent displacement field associated with the opening of a ridge. The time-history employed, shown in the bottom frame, is characterized by a transient of length of 5 years during which the ridge is subject to a constant rate of spreading. The displacement is computed at the earth´s surface perpendicularly to the axis of the fault. The opening ridge is modelled by a 200 km long planar distributions of elementary tensile faults located in the middle of the elastic crust. Due to the delayed response of the asthenosphere to the imposed forces, the displacement u attains an asymptotic regime only after a considerable amount of time is elapsed since the end of the transient opening. In the vicinity of the ridge, up to a distance of $X\sim30$ km, the displacement is negative, to indicate that motions are towards the source. Maximum positive displacements, associated with motions away from the source, are found at a distance of about one half the source length ($X\sim100$ km). The surface horizontal stress field, proportional to the derivative of u with respect to X, is characterized by a quite complex spatial and temporal evolution. A fully 3-dimensional descriptions of the deformation and stresses in the regions surrounding the fault is under way.

Preliminary results on this topic have been discussed in meetings and workshops [,,]. A paper is in preparation.