Tectonomagnetic effects

Measurements of the actual stress within the Earth and its changes with time are very difficult. It is much easier to monitor the direct effects of this stress, such as ground strain, or the indirect effects, such as changes in resistivity, strain, changes in seismic velocity or changes in magnetic field, and so forth. The latter technique is one of the more promising methods for monitoring substantial volumes of the Earth's crust near active faults with only a few instruments. It derives from the piezomagnetic properties of rocks (that is, the change in rock magnetization and, therefore, local magnetic field due to a change in applied stress). AS stress and strain fields along active faults vary, these variations should be reflected in changing local magnetic fields. In particular, rapidly changing fields might be expected to occur just before a large earthquake.

We know that earthquakes result from the sudden release of elastic strain, the end product, most likely, of a long period of gradual stress accumulation in the crust. At the U.S Geological Survey, our magnetic fields studies have two main directions. First, a determination of the magnetic behavior accompanying shallow earthquakes (seismomagnetic effects). Second, a long-term monitoring of the magnetic field in the vicinity of an active fault to detect magnetic changes due to general tectonic activity, particularly that which ultimately leads to earthquakes. These magnetic changes are generally termed "tectonomagnetic effects."