The time-dependence of electromagnetic response functions

Dynamic processes within the Earth are capable of modifying many of its physical properties. This review considers the modification of electrical resistivity mainly by time-changes in the stress­ field experienced by crustal materials. Laboratory experiments indicate that certain rock types, whose resistivity is controlled by the volume and geometry of microcracks, can undergo large resistivity variations in response to the typical strain accumulation and release that accompanies major earth­ quakes. Electromagnetic response functions are reviewed with regard to their ability to monitor time­ changes of resistivity in active regions. Some examples of the different experimental techniques used to investigate precursory changes in response functions prior to earthquakes are noted. The problems and successes of the subsequent correlation and interpretation of the derived response as a function of time are then discussed. Only relatively few controlled and integrated studies of time-dependent electromagnetic response functions exist. Those which are available, however, emphasise that changing stress-differentials occurring before major earthquakes can be monitored using suitable electromagnetic techniques.