Earthquakes and Rock Deformation in Crustal Fault Zones

Abstract

The physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress. Although the possibility now exists that the nucleation regions for large earthquakes at depths of,..., 10 km may shortly become directly accessible by deep drilling, our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources. These include seismological studies, surface studies of fault zones and earthquake ruptures, geodetic information on modes of fault slip, ·geophysical constraints on fault zone structure and rheology, and information garnered from materials science and experi­ mental rock deformation. Studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones. Although descriptions of fault rocks are widespread in the geological literature (e.g. Spry 1969, Higgins 1971), it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths (Sibs on 1977, Watts & Williams 1979, Anderson et a11983, Wise et aI 1984). Such interpretations are still at an early stage, but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels, power dissipation, and seismic efficiency during earthquake faulting; on fluid pressure levels and episodic fluid flow accompanying