HR: 1330h
AN: G43B-10    [Abstracts]
TI: Changes in rock-mechanical properties, local stresses, and displacements during the evolution of strike-slip faults
AU: * Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Geoscience Centre, University of Gottingen, Goldschmidtstr. 3, Gottingen, 37077 Germany
AU: Geyer, A
EM: Ageyer@ija.csic.es
AF: Institute of Earth Science, Jaume Almera, Barcelona, Spain
AB: During their early evolution, strike-slip faults rapidly develop zones of rocks with widely different mechanical properties. These properties, in turn, largely determine the local stresses inside and around the fault zone and, thereby, the subsequent slips during fault rupture. It is common to distinguish between two main mechanical units: a fault core and a fault damage zone. The damage zone, which is normally much thicker than the core, contains some lenses of breccia, but is characterized by fractures of various types and frequencies. By contrast, the core is primarily composed of breccia and gouge. In an active seismogenic fault, the core is normally soft (with a low Young's modulus). The Young's modulus (stiffness) of the damage zone, however, depends on the fracture frequencies and trends in relation to the main direction of loading. As a rule, the higher the fracture frequency the lower is the effective stiffness in a direction perpendicular to the main fracture direction. Since fracture frequency tends to increase on approaching the core-damage zone boundary, it follows that the stiffness of a damage zone normally decreases toward that boundary. Field studies indicate that a damage zone can often be divided into several units based on fracture frequencies, each with a different stiffness. Observations also show clearly that the cores and particularly the fault-damage zones tend to grow thicker with increasing total fault displacement. We use these field results as a basis of numerical models of strike-slip faults using the boundary-element method. In these models we vary the stiffnesses of the cores and divide the damage zones into several units, each with a different stiffness. For active strike-slip faults, the stiffest units of the damage zone are at its contact with the host rock and gradually decrease in stiffness toward the contact with the core. In some of the models, the lateral tips of the strike-slip faults end inside, or near to, soft inclusions. Such inclusions include various soft rock bodies such as (for near-surface faulting) young volcanoes or (for transform faults) young volcanic zones or ridge segments. The three main results may be summarised as follows. First, when the fault tip is nearby, or inside, a soft inclusion, the slip, for given loading conditions and fault geometry, is much larger than when the tip is far away from the inclusion, or the inclusion is absent. Second, in models where soft inclusions are absent but the damage-zone thickness around the fault increases with time, the maximum displacement (u) on a strike-slip fault of a given length and with constant loading conditions gradually increases. It follows that for a strike-slip fault of a rupture (trace) length (L), the ratio u/L decreases with time. In other words, the slip in individual earthquakes in relation to the rupture length increases with time. Third, even when the fault slip in individual earthquakes increases, the displacement (slip) profile remains similar as regards shape. Thus, for this type of loading and with gradually increasing damage-zone thickness, the displacement profile remains a smooth, convex curve with a maximum slip at the fault center.
DE: 1206 Crustal movements--interplate (8155)
DE: 3210 Modeling
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8164 Stresses--crust and lithosphere
SC: Geodesy [G]
MN: 2005 Joint Assembly