SOME COMPARISONS BETWEEN MINING-INDUCED AND LABORATORY EARTHQUAKES

Although laboratory stick-slip friction experiments have long been reg arded as analogs to natural crustal earthquakes, the potential use of laboratory results for understanding the earthquake source mechanism h as not been fully exploited because of essential difficulties in relat ing seismographic data to measurements made in the controlled laborato ry environment. Mining-induced earthquakes, however, provide a means o f calibrating the seismic data in terms of laboratory results because, in contrast to natural earthquakes, the causative forces as well as t he hypocentral conditions are known. A comparison of stick-slip fricti on events in a large granite sample with mining-induced earthquakes in South Africa and Canada indicates both similarities and differences b etween the two phenomena. The physics of unstable fault slip appears t o be largely the same for both types of events. For example, both labo ratory and mining-induced earthquakes have very low seismic efficienci es eta = tau(a)/tauBAR, where tau(a) is the apparent stress and tau(BA R) is the average stress acting on the fault plane to cause slip; near ly all of the energy released by faulting is consumed in overcoming fr iction. In more detail, the mining-induced earthquakes differ from the laboratory events in the behavior of eta as a function of seismic mom ent M0. Whereas for the laboratory events eta congruent-to 0.06 indepe ndent of M0, eta depends quite strongly on M0 for each set of induced earthquakes, with 0.06 serving, apparently, as an upper bound. It seem s most likely that this observed scaling difference is due to variatio ns in slip distribution over the fault plane. In the laboratory, a sti ck-slip event entails homogeneous slip over a fault of fixed area. For each set of induced earthquakes, the fault area appears to be approxi mately fixed but the slip is inhomogeneous due presumably to barriers (zones of no slip) distributed over the fault plane; at constant tauBA R, larger events correspond to larger tau(a) as a consequence of fewer barriers to slip. If the inequality tau(a)/tauBAR less-than-or-equal- to 0.06 has general validity, then measurements of tau(a) = muE(a)/M0, where mu is the modulus of rigidity and E(a) is the seismically-radia ted energy, can be used to infer the absolute level of deviatoric stre ss at the hypocenter.