Stimulated IR Emission from the Surface of Rocks during Deformation

Studying the IR emission from rocks placed under stress may lead to a better understanding of the reported pre-earthquake "thermal anomalies", observed in mid-IR satellite images. Large blocks of granite and labradorite (45 x 30 x 7 cm3) were loaded in their center between two vertically oriented pistons (11.25 cm diameter). Smaller blocks of the same rocks plus gabbro (about 10 x 10 x 7 cm3) were loaded uniformly over their entire cross sections. We used a SATEC press (250 tons) and a BOMEN FT-IR spectrometer (3.3-14 micron), equipped with two black body sources (one at ambient temperature and another controlled at a higher temperature) for calibration. At 1 m from the samples, the spot size on the surface analyzed was about 5 cm in diameter. The blocks were loaded at a constant rate until failure. Typical runs lasted for 10-15 min. We monitored the IR emission from the front faces, which were either rough saw-cut or irregular broken. In the case of the large blocks the emitting surfaces were 20-25 cm from where the rocks were loaded. In the case of the smaller blocks the emitting surfaces were part of the rock volume that experienced increasing levels of stress and, at higher loads, underwent cracking and occasional spallation. We observed distinct changes in the IR emission spectra during press. The changes were specific of the rock and the loading geometry. They include a band at 11.7 micron, the position of which agrees with that of vibrationally excited O-O bonds, predicted to form when positive hole charge carriers, activated in the inner rock volume under stress, recombine at the rock surface.