Geoelectric potential changes: Possible precursors to earthquakes in Japan

Whether electromagnetic precursors to earthquakes (EQs) exist is an importa nt question not only for EQ prediction but also for understanding the physi cal processes of EQ generation. Slow transient geoelectric potential change s have been observed before several recent EQs in Japan. In most cases, the y appeared 1-19 days before the EQs, and their duration and intensity were several minutes to 1 h and 1-2 mV/100 m. The changes appeared before five o f all six EQs with magnitude greater than or equal to 5 that occurred withi n 20 km of our stations during the observation period. Changes were also de tected at greater epicentral distances (up to 75 km) before two other EQs, including one EQ of magnitude 4.7. which was preceded by a signal simultane ously recorded at three widely separated stations. These geoelectric potent ial changes have been distinguished through the following criteria from a m ultitude of other changes, which were noise of various origins. (i) The sel ected changes were proportional in amplitude to the length of the recording station's short (approximate to 100 m) dipoles and were simultaneously det ected also on long (1-10 km) dipoles when the latter were in operation. (ii ) No such changes occurred during the observation period that were not foll owed by EQs. Although the EQ precursory nature of these geoelectric potenti al changes is admittedly unproven, it seems that the present results warran t continued serious research into the occurrence, generation, and transmiss ion of these signals and their possible causal relationship to EQs.