ABSTRACT: The localization of plastic deformation in the lithosphere is studied in numerical simulations, using a two-dimensional finite element code with non-linear elasto-plastic rheological laws. The propagation of the seismic fracture is taken to be a consequence of this localization, emphasis being made in the effects of fault dip and rheological behaviour. We study normal faults the dip angles of which range between IS and 90°.
RESUME: La localisation de la deformation plastique dans la lithosphère est etudiee à I'aide de simulations numeriques par un code d'elements finis bidimensionnel,qui utilise des lois rheologiques elasto-plastiques non-lineaires. La propagation de la rupture sismique est consideree comme une consequence de cette localisation, et I'on etudie en particulier les effets du pendage de la faille et des lois de comportement. Le phenomène est etudiè dans Ie cas de failles normales dont Ie pendage varie entre 15 et 90°.
ZUSAMMENFASSUNG: Die Lokalisierung der plastischen Verformung in der Lithosphare wird anhand numerischer Simulierungen durch einen zweidimensionalen Code finiten Elemente untersucht, derauf nicht Iinearen plast-elastischen Fliessregeln beruht. Die Fortptlanzung des sismischen Bruches wird a1sFolge dieser Lokalisierungbetrachtet, wobel insbesondere die Auswirkungen des Einfallens der Verwerfung und der Verhaltensregeln untersucht werden. Das Phanomen wird fuer Abschiebungen untersucht, deren Einfallen zwischen IS und 90° variert.
1 INTRODUCTION Seismic risk evaluation for critical facilities is usually done by using either a deterministic or a probabilistic method. Whilst the former approach is based on the assumption that earthquakes similar to the most severe historically experienced one in a seismotectonic unit are likely to occur in the future at ally point of that seismotectonic domain (RFS [1981]), the latter method calculates the likelihood of earthquake occurrence in terms of existing data in neighbouring regions (Araya and Der Kiureghian [1988]). In any case, the discovery of a fault of recent activity in the region considered (e.g. Carbon et al. [1993], Combes et al. [1993]) can raise the question of the degree of confidence one can have in the seismic risk assessment. A magnitude estimation of the earthquake that activated (or. created) the fault is then necessary. One way of tackling the problem is by modelling the fault movement and the propagation of the seismic fracture in such a way that the observable effects (fault geometry near the surface and topography) are similar in the model and in the field. We use forward modelling with a numerical method in an attempt to relate the subsurface rheology and fault geometry to the markers of recent tectonic evolution.
2 THE NUMERICAL MODEL 2.1 Modelling Parameters The behaviour of a fault is studied by considering the differences between the geological structures created when the main parameters of the mechanical system are changed systematically: the system's geometry, the fault and medium behaviour laws, and the mechanical boundary conditions of stress and displacement. In our models, the main parameters are the geometry of the system (i.e, the position and dip angle of the preexisting fault) and the rheological laws of the materials. In our models, we shall make the following assumptions:The initial geometry of the system (figure 1) is a faulted layer of bedrock with a width of 15 km (the fault having a dip, its base being at a depth of 4 km and its tip at a depth of 2 km) overlayed by one horizontal layer of another material. There is no preexistent topography;
The initial stress field is calculated assuming hydrostatic equilibrium(pressure conditions). Displacement is not imposed along the fault, but rather by defining an extension rate of 6 mm/yr which will give rise to motion along the fault plane, under control of the Diction law (elapsed time: 1 Myr, producing a 4% extension of the initial width);
Coulomb's law under Signorini's conditions is used to model Diction of the fault (Jean and Touzot [1988]);
The materials being modelled are considered elasto-plastic.