Regional state of stress, fault kinematics and adjustments of blocks in a fractured body of rock: application to the microseismicity of the Rhine graben

Abstract

A simple mechanical model of a homogeneous state of stress and independent fault motions is generally used to interpret fault kinematics in terms of stress. However analysis of focal mechanisms of microearthquakes shows that, as a result of incompatibilities of block motions in an assemblage of rigid or elastic blocks, deformation of a fractured body of rocks is in general heterogeneous. Analyses of focal mechanism sets show that they are in general composed of a main group whose fault motions result from a mean state of stress (TM) in good agreement with the regional state of stress; the remaining complex fault motions are explained by a compressional (TC) and/or a tensional deviator (TE). An important observation is that within the computed uncertainties, these (TC) and (TE) deviators are co-axial with the mean state of stress (TM). These deviators statistically model the local fault-slips probably due to the incompatibilities of block motions and cannot be considered as other mean states of stress. Thus if the regional state of stress is known for a fault plane whose strike and dip are given, its kinematics cannot be surely predicted. Two types (TC and TE) of motions different from that (TM) predicted by the regional state of stress may exist; they are the kinematic instabilities. As a consequence all the motions shown by focal mechanisms or by striated faults are not significant of the regional tectonic regime. This is well illustrated by the microseismicity of the Rhine graben where it is shown that the compressional fault motions are kinematic instabilities, and the extensional fault motions are representative of a regional state of stress ( σ 1 vertical) with a N45°-trending σ 3 axis. Hence, the simple mechanical model of homogeneous state of stress and independant fault motions is valid only if there are no incompatibilities of block motions. Possibly, these incompatibilities may be accommodated by a pressure-solution-crystallization process for creep faulting or by internal cataclastic deformation of large blocks separated by major stick-slip faults.