Time-resolved study of charge generation and propagation in igneous rocks

Abstract

Electrical resistivity changes, ground potentials, electromagnetic (EM), and luminous signals preceding or accompanying earthquakes have been reported many times, in addition to ground uplift and tilt and other parameters. However, no concept exists that would tie these diverse phenomena together into a physically coherent model. Using low- to medium-velocity impacts to measure electrical signals with microsecond time resolution, it is observed that when gabbro and diorite cores are impacted at relatively low velocities, ≈100 m/s, highly mobile charge carriers are generated in a small volume near the impact point. They spread through the rocks, causing electric potentials exceeding +400 mV, EM, and light emission. As the charge cloud spreads, the rock becomes momentarily conductive. When a granite block is impacted at higher velocity, ≈1.5 km/s, the propagation of the P and S waves is registered through the transient piezoelectric response of quartz. After the sound waves have passed, the surface of the granite block becomes positively charged, suggesting the same charge carriers as observed during the low-velocity impact experiments, expanding from within the bulk. During the next 2-3 ms the surface potential oscillates, indicating pulses of electrons injected from ground and contact electrodes. The observations are consistent with positive holes, e.g., defect electrons in the O2- sublattice, traveling via the O 2p-dominated valence band of the silicate minerals. The positive holes propagate as charge clouds rather than as classical EM waves. Before activation, they lay dormant in form of electrically inactive positive hole pairs, PHP, chemically equivalent to peroxy links, O3X/OO\XO3 with X=Si4+, Al3+, etc. PHPs are introduced into the minerals by way of hydroxyl, O3X-OH, which all nominally anhydrous minerals incorporate when crystallizing in H2O-laden environments. The fact that positive holes can be activated by low-energy impacts, and their attendant sound waves, suggests that they can also be activated in the crust by microfractures during the dilatancy phase. Depending on where in the stressed rock volume the charge carriers are activated, they will form rapidly moving or fluctuating charge clouds that can account for earthquake-related electrical signals and EM emission. Wherever such charge clouds intersect the surface, high fields are expected, causing electric discharges and earthquake lights. Copyright 2000 by the American Geophysical Union.