Geology of the earthquake source: an introduction

Earthquakes arise from frictional ‘stick–slip’ instabilities as elastic strain is released by shear failure, almost always on a pre-existing fault. How the faulted rock responds to applied shear stress depends on its composition, environmental conditions (such as temperature and pressure), fluid presence and strain rate. These geological and physical variables determine the shear strength and frictional stability of a fault, and the dominant mineral deformation mechanism. To differing degrees, these effects ultimately control the partitioning between seismic and aseismic deformation, and are recorded by fault-rock textures. The scale-invariance of earthquake slip allows for extrapolation of geological and geophysical observations of earthquake-related deformation. Here we emphasize that the seismological character of a fault is highly dependent on fault geology, and that the high frequency of earthquakes observed by geophysical monitoring demands consideration of seismic slip as a major mechanism of finite fault displacement in the geological record