An application of the slip model of brittle deformation to focal mechanism analysis in three different plate tectonic situations

Abstract

The “slip model” of brittle deformation ( Reches, 1983) imposes a set of constraints on the angular relationships between faults, their striations on slickensides and the type and orientation of the deformation ellipsoid. In focal mechanisms of earthquakes, these relationships can be used to determine which of the two nodal planes is the fault plane, and then to obtain the strain ellipsoid of regional tectonic deformation controlling seismicity. In this approach it is convenient to use pitch/dip diagrams ( De Vicente, 1988) and P/T diagrams ( Angelier and Mechler, 1977), both developed for fault population analysis. In this paper three regions in different plate-tectonic situations are analysed using these methods. These regions are a zone of intracontinental seismicity in the eastern Peruvian Andes, a zone of continental compressional deformation in the Iranian Plateau, and the Fenwei Graben of China. Nodal planes of focal mechanisms, published in different papers about seismicity in these regions, have been used, the following conclusions being reached. In the analysed zone of the Peruvian Andes, three regional deformation types appear, causing fault movements that range from slightly reverse strike-slip to reverse strike-slip. In addition, exclusively at depths in excess of 18 km, there is a deformation of an uniaxial shortening type. In all of these deformations the horizontal shortening axis is oriented from N87° to N93°. In the Iranian Plateau two horizontal shortening directions appear, N151° and N61°, even showing faults that imply an uniaxial deformation ellipsoid the shortening axis of which is at N61°. In the Fenwei Graben the ellipsoid of regional deformation has a stretching principal axis at N145°/150° and a shortening axis at N55°/60°, the strike-slip movement along faults being as important as the normal one.