Current laboratory studies pertaining to earthquake prediction

Abstract

Laboratory studies in three areas may prove useful in earthquake prediction. These studies deal with the source mechanism, with microfracturing and with changes in rock properties under stress. Of the various processes which produce sudden stress drops in rocks, brittle fracture and frictional stick-slip are being currently studied. Both are accompanied by numerous small elastic shocks which begin at stresses well below the fracture or sliding stress. Although the shocks in both cases are due to microfracturing, the pattern of microfracturing prior to brittle fracture differs in a characteristic way from that before frictional sliding. Just before fracture the number and intensity of shocks increases markedly, and the locations from which the larger shocks emanate cluster along the eventual fault plane. Neither feature has been observed prior to frictional stick-slip. Large changes in sound velocity have been observed at certain phase changes. Electrical resistivity and magnetic susceptibility are strongly affected by stress: in rocks partially saturated with water, electrical resistivity may decrease rapidly at low stress; in fully saturated rocks, it may decrease rapidly at high stress, just before fracture. Very little change takes place just before frictional sliding of saturated rocks.