Impact fracture of granite at temperatures from 20 to 500 C

Earthquake nucleation and fracture propagation in deformed rocks generate elastic waves, within acoustic frequencies. Strain-induced acoustic waves appear both in field tectonic structures and in laboratory samples, thus making laboratory acoustic emission (AE) data from load tests suitable to interpret natural seismic processes. However, laboratory tests are commonly run at room temperature, while the natural rocks at the earthquake origin depths are as hot as hundreds of degrees centigrade. We report AE data for thermally and mechanically loaded granites subjected to impact fracture at different temperatures. The energy distribution in the time series of acoustic signals emitted from fine-grained granite fits a power law of the type of the Gutenberg-Richter relationship at temperatures from 20 to 500 C. Medium- and coarse-grained samples behave in this way only within 300 C but show a Poissonian (random) AE energy distribution above 300 C.