Mechanism of unipolar electromagnetic pulses emitted from the hypocenters of impending earthquakes

Abstract

Unipolar pulses (UPs) are short events characterized by outbursts of electromagnetic (EM) energy from deep within the Earth's crust. First recognized prior to the 2007 M = 5.4 Alum Rock earthquake in northern California, UPs can be as short as 150ms, followed by an overshoot in the opposite polarity direction or by undulations of the EM field lasting from to 2−20 sec. Near Lima, Peru, and Tacna, Peru, thousands of UPs in the 1−3 nT intensity range have been recorded, emitted from the 25−65km depth range, thought to arise in patches at the top of the Benioff Zone of the subducting Nazca Plate. To understand how these EM pulses can be generated deep in the rock column, we consider that rocks contain peroxy defects, typically O3Si−OO−SiO3, which, when subjected to increasing deviatoric stresses, break up in two steps. Step I: electrons in the tight non-bonding πnb molecular orbital decouple by transitioning into the antibonding σ*-level, where they occupy a significantly larger volume. This volume expansion is possible only, when the internal pressure in the stressed subvolume overcomes its confining pressure. This in turn requires that the number density of peroxy defects in the rock is high enough so that, during the πnb → σ* transition of the O−−O− bonds, the wave function of their decoupled O− states overlap, causing a solid plasma state with an internal electron degeneration pressure that can force the volume expansion against the load of the overlying rock column. Step II: once the σ*-level is reached, the decoupled O−−O− bonds can dissociate, generating highly mobile charge carriers, electrons e' and holes h•, which can burst out the stressed subvolume causing it to instantly contract again. Thus, UPs appear to be linked to an explosive expansion of stressed subvolumes of rocks against their lithostatic overload, followed by an outburst of electronic charge carriers and concomitant volume contraction.