ELECTROMAGNETIC PRECURSORS TO EARTHQUAKES IN THE ULF BAND - A REVIEW OF OBSERVATIONS AND MECHANISMS

Despite over 2 decades of international and national monitoring of ele ctrical signals with the hope of detecting precursors to earthquakes, the scientific community is no closer to understanding why precursors are observed only in some cases. Laboratory measurements have demonstr ated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stresse d. However, field experiments have had much less success. Many purport ed observations of low-frequency electrical precursors are much larger than expectations based on laboratory results. In some cases, no prec ursors occurred prior to earthquakes, or precursory signals were repor ted with no corresponding coseismic signals. Nonetheless, the field ex periments are in approximate agreement with laboratory measurements. M aximum resistivity changes of a few percent have been observed prior t o some earthquakes in China, but the mechanism causing those changes i s still unknown. Anomalous electric and magnetic fields associated wit h fluid flow prior to earthquakes may have been observed. Finally, pie zomagnetic signals associated with stress release in earthquakes have been documented in measurements of magnetic fields.