WP 6.2 Model stress in the solid matrix and pressures in fluids permeating the crust

Start date or starting event:

M0

Lead contractor:

DF.UNIBO

Participants:

DF.UNIBO, CNRS-UMR 5562

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Objectives:

1) To model the lithosphere-asthenosphere interaction under the SISZ, taking into account viscoelastic constitutive relationships and intrusion events across rheological discontinuities.

2) To model crust instability in the SISZ taking into account poro-elastic constitutive relationships.

3) To model triggered seismicity and the interaction between the two large earthquakes of year 2000.

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Methodology / work description:

In the framework of previous research within the Environment Program of the EU, rigorous mathematical solutions of dislocation problems in elastic layered media have been obtained which have shown that rigidity contrasts  can be responsible for significant stress build-up localized along the interface between different media. Due to the different geometrical configurations, strike-slip, dip-slip and pull-apart motions across heterogeneous media have very different effects on the stress field and on failure conditions.  Further studies of such effects have shown that fault complexities must inevitably be generated in nascent transform zones, since it is not generally possible to prescribe the stress drop on one planar fault without violating the welded B.C. Models produced up to now were restricted to elastic heterogeneous media, so that applications were mainly oriented to describe co-seismic and early post-seismic effects of faulting. In the preparatory stage of an earthquake, different constitutive relationships should be considered. First, viscoelastic constitutive relationships should be employed to model a subcrustal asthenosphere under the SISZ. If the asthenosphere is modelled in terms of  the SLS rheology (Standard Linear Solid), its long-term rigidity is much less than the instantaneous rigidity inferred from seismic waves: accordingly we expect that very high stress concentration may arise along the brittle side of the interface following spreading motions in the asthenosphere. Second, poroelastic constitutive relationships should be employed to model the joint action of stress in the solid matrix and pore-pressure in the fluids permeating the crust. According to the modified Coulomb-Navier criterion, failure conditions are sensitive to both, stress and pore-pressure, but most studies up to now have not considered the possibility that sources of pore pressure may be present in the crust, related to exsolution of volatiles from ascending magma in the asthenosphere. Third, fault interactions studies provide a contribution to the understanding of the physical processes at the base of earthquakes sequences. In particular the explanation of short-term and long-range interactions can rely upon the study of the dynamic stress redistribution during the sequence studied. After the first magnitude 6.6 earthquake of 2000, a series of earthquakes followed immediately to a distance of 100 km along the SISZ and it prolongation along the Reykjanes peninsula (RP). The local seismic data and aftershock data collected in the SISZ can be used as a basis for dynamic detailed modelling of fault interactions. The results of such modelling and the use of fault constitutive properties, such as the rate- and state-dependent friction laws, can enable us to understand the triggered seismicity. The seismic events which took place in year 2000 within the SISZ offer a unique opportunity to test these models, increase our understanding of preparatory phases of earthquakes and of triggered seismicity. This will in turn provide a quantitative conceptual scheme in the framework of fault mechanics, capable of explaining causal links among precursory phenomena.

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Deliverables including cost of deliverable as percentage of total cost of the proposed project:

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Milestones: Delivery of the above items at the date indicated.