Current and surface potential induced by stress-activated positive holes in igneous rocks

In order to model seismo-electromagnetic phenomena, we focus on one specific defect as an alternative source of charge carriers in igneous rocks. These charge carriers are defect electrons in the O2− sublattice that are chemically equivalent to O− in a matrix of O2− and are known as positive holes (p-holes). They are activated from peroxy defects: O3X–OO–XO3 (X = Si, Al, etc.) that are known as positive hole pairs (PHPs). Stressed igneous rocks behave like p-type semiconductors. In order to examine the contributions of p-holes to seismo-electromagnetic phenomena, we conducted two series of uniaxial loading tests using air-dry tiles of several types of rocks (granite, anorthosite, gabbro, limestone, and marble) and glass. We observed that the igneous rock tiles under central loading generated (1) a positive current, carried by holes and flowing from the central stressed volume through the surrounding unstressed volume to the edges of the tiles and (2) a negative current, carried by electrons and flowing from the central stressed volume into the load pistons. In addition, a positive potential appeared on the surface of the unstressed portion of the igneous rock tiles. On the other hand, limestone containing small amounts of silicate grains generated lower currents. Marble containing no silicate grains and glass generated much lower and no currents, respectively. The current generated per 1 m3 of stressed volume at a rapid loading up to 50 MPa was on the order of 10−5 A for granite. A slow increase of the stress level by 10 MPa superimposed on the 50 MPa added a 10−6 A of the current. Anorthosite and gabbro showed 10–50 times higher currents, and limestone and marble showed 1/10–1/100 times. The vertical electric field during a stress change from 50 MPa to 60 MPa was on the order of 102 V/m. We propose that, when stresses and strain change at or around a fault zone before and during an earthquake, similar currents and electric fields will be generated in and on the ground, respectively. Such currents and fields will be accompanied by changes in the conductivity of the rocks and may lead to abnormal electromagnetic fields and emissions and to ionospheric perturbations.