Experimental Deformation of Rock Samples at High Pressure and High Temperature and the Mechanical Properties of the Earth's Crust

Abstract

The deformation and the fracture of rock samples are discussed on the basis of recent experimental results by Griggs, Robertson, Heard and others. The fracture strength of rocks is appreciably influenced by confining pressure, temperature, strain rate, sample size and other factors. From these experimental results and some geodetic data, the strength of the earth's crust is estimated at various depths. According to this estimation, the strength remarkably increases up to the 20-30 km depth and it decreases gradually at deeper regions.
It is widely believed among seismologists that earthquakes are caused by brittle fracture of the earth's materials. However, at very high pressure and high temperature, brittle shear fracture cannot occur in rocks. This transition from brittle fracture to ductile fracture in rock samples is very interesting from the seismological point of view. New hypotheses on the earthquake mechanism at deep regions (at very high pressure and temperature) are reviewed.