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

Freund F.T.; Heraud J.A.; Centa V.A.; Scoville J. · European Physical Journal: Special Topics (2021) · DOI: 10.1140/epjst/e2020-000244-4

Resumen

Unipolar pulses (UPs) are short events characterized by outburstsof electromagnetic (EM) energy from deep within the Earth'scrust. First recognized prior to the 2007 M = 5.4 Alum Rock earthquakein northern California, UPs can be as short as 150ms, followedby an overshoot in the opposite polarity direction or by undulationsof 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 beenrecorded, emitted from the 25−65km depth range, thought to arisein patches at the top of the Benioff Zone of the subducting NazcaPlate. To understand how these EM pulses can be generated deepin the rock column, we consider that rocks contain peroxy defects,typically O3Si−OO−SiO3, which, when subjected to increasing deviatoricstresses, break up in two steps. Step I: electrons in the tightnon-bonding πnb molecular orbital decouple by transitioning into theantibonding σ*-level, where they occupy a significantly larger volume.This volume expansion is possible only, when the internal pressure inthe stressed subvolume overcomes its confining pressure. This in turnrequires that the number density of peroxy defects in the rock is highenough so that, during the πnb → σ* transition of the O−−O− bonds,the wave function of their decoupled O− states overlap, causing a solidplasma state with an internal electron degeneration pressure that canforce the volume expansion against the load of the overlying rock column.Step II: once the σ*-level is reached, the decoupled O−−O− bondscan dissociate, generating highly mobile charge carriers, electrons e'and holes h•, which can burst out the stressed subvolume causing it toinstantly contract again. Thus, UPs appear to be linked to an explosiveexpansion of stressed subvolumes of rocks against their lithostaticoverload, followed by an outburst of electronic charge carriers and concomitantvolume contraction.

Instituciones: Pontificia Universidad Católica del Perú