Sudden Field Changes in the Crust

Some rare changes in the Earth's structure or composition that occur within a short period of time can produce corresponding changes in the locally measured magnetic field. For their detection, at least two measurement sites are used, separated by a distance that is short with respect to the height of conducting upper-atmospheric layers near 100 kilometers (63 miles). This separation is designed to assure the observers that the flow of high-altitude localized variation currents and geomagnetic pulsations arising in the Earth's upper atmosphere are recorded similarly at the two locations, after adjusting for geological site differences. For example, a separation of one-tenth of the 100-km (63-mile) ionospheric height (see Chapter 3) gives a maximum suitable measurement separation of about 10 km (6 miles). Usually much shorter distances are used and the total-field component or the into-the-Earth component of magnetic field is measured with instruments sensitive to changes greater than 0.1 gamma. Then when different size signals are recorded at the two sites, a local effect is verified.

Two examples of small-amplitude (under 10 gammas) local field changes that have been reported as resulting from this tectonomagnetic effect are:

1. Magnetic signals arise from an alteration in the Earth's electrical conductivity. For example, field effects can be detected when there is a major change in the groundwater content at a deep subsurface fracture near one of the observatories but not the other, or when a highly conductive active magma chamber at a volcanic site moves (at different distances relative to the two observatories) before an eruption (Figure 2.16).

2. Movement of a rock's magnetic domain boundaries (or the rotation of the magnetization within the boundaries) under external stress can cause

Magnetic Field Signals

Magnetic Field Signals

FIGURE 2.16 ► A change in the difference of field measured for two locations near a volcano can disclose conductivity changes due to magma motion preceding an eruption.

changes in magnetization. For example, this piezomagnetic effect could occur as a result of the loading of rock surfaces as a major dam is filled or at a volcano as a result of a change in the magma chamber pressure on the surrounding rock material.

At the time of earthquakes, apparent signals have been generated by the physical vibration of the quake-site magnetometers, and such records have been misrepresented as a piezomagnetic event. Magnetic effects associated with the stress buildup preceding an earthquake have been sought as a quake-forecasting signal, but have yet to be conclusively found (see Section 2.2.3, p. 66).

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