Chapter 13: Observations on Fracture and a Hypothesis of Earthquakes

Abstract

Experiments have shown that the deformation of rocks under high confining pressures cannot be described by terms based on ordinary experience. The writers believe there are only three fundamental macroscopic processes at work: extension fracturing, faulting, and uniform flow. The first involves separation across a plane of no shear normal to the direction of least principal stress and includes tensile fracture as a special case. Faulting involves a shearing displacement and occurs with or without loss of cohesion. It includes shear fracturing as a special case and may occur along a plane inclined at from a few degrees to 45° to the direction of maximum principal stress. From the standpoint of the stress-strain curve, faulting without loss of cohesion is indistinguishable from flow. (The three flow mechanisms-cataclasis, gliding, and recrystallization-cannot always be distinguished either.) Examples of experimental boudins are presented to illustrate these categories of deformation.The field evidence is that earthquakes are accompanied by shearing displacements and are therefore due to faulting in the general sense. Since stored elastic strain energy is released, there must be at least a momentary and local loss of cohesion. A crack then propagates at close to the speed of sound. For deep-focus earthquakes (down to 700 km) certainly, and most probably even for shallow disturbances (a few tens of kilometers), ordinary Coulomb fracture is impossible. The internal friction of dry rocks under tens or hundreds of thousands of bars pressure would demand impossibly high shearing stresses of many kilobars. The most reasonable mechanism of energy release at great depth is a phase change, and the most probable phase change is melting.Calculations suggest that, once a crack or a flaw exists, there is ample elastic energy to propagate the crack by shear melting even if the stress difference is only a few tens of bars. It is, of course, inconceivable that an open crack could exist at depth, so that the most baffling problem is the nature of a flaw of the Griffith type under these conditions. Although it is little more than speculation, we suggest that the flaw may be a pocket of already molten rock or of its more volatile constituents. This idea receives some support from the intimate association of earthquake epicenters and zones of volcanic activity. EXPERIMENTAL EVIDENCE CLASSIFICATION OF FRACTURE AND FLOWObservations by the writers and others indicate that the macroscopic deformation of rocks and minerals deformed at high confining pressures in the laboratory (that is, in triaxial compression tests) can be described in terms of three principal categories of behavior-extension fractures, faults, and uniform flow. How the material will deform appears to depend upon its relative ductility-that is, upon the amount of permanent deformation achievable prior to rupture. Ductility depends upon the composition and structure of the material and also upon the magnitudes of the con-•Publication No. 172,

The nucleation and evolution of Riedel shear zones as deformation bands in porous sandstone

“…Mandl et al 1977, Cundall 1989, and different strain mechanisms evolve including localized cataclasis (e.g. Griggs & Handin 1960). The changing strain mechanisms reflect the changing mechanical characteristic of the host medium.…”

Are deep focus earthquakes caused by a martensitic transformation?

“…The fundamental difficulties associated with the mechanism of fracture in deep seismic events were already pointed out in the late fifties (Griggs and Handin, 1960;Orowan, 1960): While the analysis of deep events indicates a predominance of a double-couple mechanism for the source, general arguments indicate that the magnitude of the friction force which may be expected for materials at great depths (subjected to high temperatures, large pressures, and low shear stress) is too great to allow for continuation of slip once fracture has initiated.…”

Comments on phase changes and shallow focus Earthquakes

“…Benioff (1964) still holds that the elastic rebound theory can account for shallow focus earthquakes and probably earthquakes of intermediate depth. Evison (1963) and Griggs and Handin (1960), however, question that even shallow focus earthquakes can be generated by elastic rebound. Evison (1963, p. 884) applying the theory of Knopoff (1958) to the San Andreas Fault was led to the conclusion that the maximum depth to which the sudden movement of 1906 penerated was only 3'2 km.…”

“…Arguments have been advanced by several writers that sudden fracturing and release of strain cannot be associated with deep focus earthquakes, because the frictional stress on fault planes at this depth must be much greater than the rock strength (Orowan, 1960;Griggs and Handin, 1960;Evison, 1963). Benioff (1964) still holds that the elastic rebound theory can account for shallow focus earthquakes and probably earthquakes of intermediate depth.…”