Stress-Activated Electromagnetic Emission and Reflection from Gabbro and Gabbro-Diorite

Thermal Infrared (TIR) anomalies seen in night-time satellite images are thought to be a promising pre-earthquake signature, driven by the build-up of stress in the crust [1]. The reported radiative surface temperature increase is on the order of 2-6°C, occasionally higher. The cause of this effect is still debated. To produce TIR anomalies in the laboratory we stressed cut blocks of gabbro and boulders of gabbro-diorite using a hydraulic press and/or Bustar expanding cement. Using a Bruker Vertex 70 FT-IR spectrometer with an infrared emission accessory, we measured the IR emission off the rock surface far from where the stresses are applied. We also gathered information about the surface potential by measuring the voltage between the stressed rock and the rock surface at an unstressed part. In addition we monitored the concentration of airborne ions above the rock boulders and changes in radar reflectivity. Upon application of stress we record a positive surface potential, a slight increase in gray body emission, and IR emission bands in the 11-12.5 micron region. The increase in gray body emission indicates a temperature increase of the emitting rock surface. The 11-12.5 micron IR bands suggest a contribution from a non-thermal emission process. We record occasional bursts of air ions, mostly positive. All these observations, plus changes in radar reflectivity, are consistent with the activation of positive hole charge carriers in the stressed subvolume of the rocks and their flow to the surface [2]. At the surface the positive holes lead to a surface charge with an associated electric field, which can become high enough to cause air ionization [3]. The positive holes recombine at the surface, releasing some of their recombination energy as 11-12.5 micron IR photons, the rest as heat [4]. 1] D. Ouzounov et al., “Satellite thermal IR phenomena associated with some of the major earthquakes in 1999-2003,” Phys. Chem. Earth, 31, 154-163 (2006) [2] F. Freund et al., “Electric currents streaming out of stressed igneous rocks - A step towards understanding pre-earthquake low frequency EM emissions,” Phys. Chem. Earth, 31, 4-9, 389-396, (2006) [3] F. Freund et al., “Air ionization at rock surface and pre-earthquake signals,” J. Atmos. Sol. Terr. Phys., 71, 1824-1834, (2009) [4] F. Freund et al., “Stimulated thermal IR emission from rocks: Assessing a stress indicator,” eEarth, 2, 1-10, (2007)