Stimulated infrared emission from rocks: assessing a stress indicator

Abstract

To study the effect of stress-activated positive hole (p-hole) charge carriers on the infrared (IR) emission from rocks, we subjected a portion (10 vol.%) of a large (30607.5 cm3) block of anorthosite, a nearly monomineralic (Ca-rich feldspar) igneous rock, to uniaxial deviatory stress up to failure. We measured the IR emission from a flat surface 40 cm from the stressed rock volume over the 800-1300 cm1 (7.7-12.5μm) range. Instantly, upon loading, the emission spectrum and intensity change. At first narrow bands appear at 930 cm1 (10.75μm), 880 cm1 (11.36μm), 820 cm1 (12.4μm) plus additional narrow bands in the 1000-1300 cm1 (7.7-10.0μm) range. The 10.75-12.4μm bands are thought to arise from vibrationally excited O-O stretching modes, which form when p-hole charge carriers, which spread from the stressed rock volume into the unstressed rock, recombine at the surface. They radiatively decay, giving rise to ``hot'' bands due to transitions between excited states. Before failure the broad emission bands at 1170 cm1 and 1030 cm1 (8.7 and 9.7μm) also increase slightly in intensity, suggesting a small increase in temperature due to thermalization of the energy deposited into the surface through p-hole recombination. Stimulated IR emission due to hole-hole recombination and its follow on effects may help understand the enhanced IR emission seen in night-time satellite images of the land surface before major earthquakes known as ``thermal anomalies''.