Tectonic Stress Changes Research Articles

Synchronous precursors in the electrical earth resistivity and the geomagnetic field in relation to an earthquake near the Yamasaki fault, Southwest Japan.

Precursory changes both in th electrical earth resistivity and in the geomagnetic total force gradient were clearly observed in relation to the earthquake of M 5.6 which occurred near the Yamasaki fault on May 30, 1984. The observation site was at a distance of several kilometers from the epiceter. The precursors were recognized about 60-70 days before the event. First the resistivity gradually decreased during 10 days by 30% of its ordinary value. After keeping a low level for about 50 days, the resistivity began to increase, exceeding the ordinary value 2 days before the shock, and attained to a maximum of 400% after the shock. The extremely large value returned to its ordinary value within two weeks after the event. The geomagnetic total force gradient also showed about 10% increase synchronously with the resistivity change. Almost 2 days after the shock, the gradient recovered to its ordinary value. No coseismic change was recognized. Other precursor like changes of the resistivity possibly associated with several earthquakes in southwest Japan were also detected at the same observation site within the last year. We consider that the resistivity change may be caused by a small amount of change in the water content within the rocks or fissures around the fault and that the fault region is a place particularly sensitive to tectonic stress changes. A resistivity structure model to account for the observed precursors will be presented.

Theory of electrokinetic‐magnetic anomalies in a faulted half‐space

Fluid motion in the vicinity of a vertical fault separating regions of different streaming potential coefficient can produce an external magnetic field of observable magnitude. If tectonic stress changes along the fault produce fluid motion, the magnetic field changes would be indicative of stress changes, and might precede severe fault motion. The largest component of the magnetic field is oriented parallel to the strike of the fault. Magnetic field changes produced by this mechanism will have electric fields associated with them with similar time variation; this is in contrast to magnetic field changes produced by changes in susceptibility or remanent magnetization, which have no associated electric field. Experiments aimed at the detection of tectonomagnetic effects should include electric field sensors to help in determining the source of any observed variations. The hypothesis of Mizutani and Ishido (1976) that the magnetic field changes associated with the Matsushiro earthquake swarms were produced by electrokinetic currents seems reasonable.

Piezomagnetic response with depth, related to tectonomagnetism as an earthquake precursor.

Piezomagnetic field variations can result from tectonic stress changes in the focal zone of an impending earthquake. Interpretation of such observed tectonomagnetic effects requires modeling of the stress response of magnetic properties of the lithosphere. Calculations have been made to estimate the here piezomagnetic effect as a function of depth, by considering the effect of stress on the magnetization of magnetite (Fe3O4) with increasing temperature and hydrostatic pressure. The responsiveness of magnetization to seismotectonic stress is gauged by an appropriate balancing of magnetocrystalline and magnetoelastic anisotropy energies. The calculations indicate that magnetite becomes progressively more responsive to stress at depth increases. The rate of change depends on the local geothermal gradient. The upper 15 km of the lithosphere is likely to be the most important in yielding observable piezomagnetic field effects. Such shallow-focus earthquakes are expected to be best for monitoring tectonomagnetic anomalies for earthquake prediction.

Local magnetic events associated with displacement along the San Andreas fault (California)

The piezomagnetic properties or stress-dependence of the magnetic susceptibility and remanent magnetization of rock indicates that there should be a 1% decrease in the net magnetization in the direction of an applied compressive stress of 100 kg/cm 2. Through proper observation of the local geomagnetic field, it may be possible to utilize these effects for the remote observation of changes in tectonic stress. In an attempt to observe such phenomena, a telemetering array of five rubidium magnetometers was set up in a differential manner and has been operating for over two years on the San Andreas fault in California. Local events in the geomagnetic field were indeed observed having occurred 10 to 20 h before local creep displacements of the fault and at no other time.