Tectonomagnetism

Abstract

The magnetization of crustal rock is influenced by stress by virtue of piezomagnetic effects in crystals of magnetic minerals. Crustal stress changes in tectonic zones are therefore expected to generate changes in the crustal component of the geomagnetic field and so provide a mechanism for monitoring stress. During the last quadrennium, rock magnetic studies further quantified our understanding of the stress sensitivity, but showed that while the standard rotation models explain observations in SD and PSD magnetic grains, a new model is required to describe MD effects. Observations of several tectonomagnetic effects were reported and the method of observation improved using predictive filtering techniques to remove the disturbing influence of ionospheric and magnetospheric activity. The possibility that fluid flow in the crust might generate magnetic fields comparable to those due to stress changes was evaluated. However supporting experimental evidence is lacking. Long term monitoring of the geomagnetic field has been continued at 29 stations (of the U.S. Geological Survey) on the San Andreas fault between San Francisco and the Mexican border. None of the fields has changed by more than 5nT since the study was initiated (which for five of the stations was 1974). Two earthquakes of moderate size (ML = 5.2, 5.9) have occurred within the array during this time. Although changes of several nT have been observed, apparently spatially and temporally related to these events, no clear pattern emerges as yet.