RESULTS OF ULF MAGNETIC-FIELD MEASUREMENTS NEAR THE EPICENTERS OF THE SPITAK (M(S) = 6.9) AND LOMA-PRIETA (M(S) = 7.1) EARTHQUAKES - COMPARATIVE-ANALYSIS

The characteristics of the ULF magnetic field emissions measured at two magnetic observatories in the Republic of Georgia prior to and after the M(s) = 6.9 earthquake that occurred near Spitak, Armenia, on December 7, 1988, are compared with the apparently similar emissions associated with the M(s) = 7.1 earthquake that occurred near Loma Prieta, California, on October 17, 1989. The main features of the Spitak measurements, according to observations made at the Dusheti station (128 km to the Spitak epicenter), as compared with the Loma Prieta measurements, which were made at Corralitos, California (7 km to the Loma Prieta epicenter), are the following: (1) The intensity of ULF background activity started growing 3 to 5 days before the Spitak earthquake, whereas the corresponding increase in activity began 12 days before the Loma Prieta earthquake; (2) a substantial ULF emission burst was recorded at Dusheti starting 4 hours prior to the main shock; a similar large burst of ULF activity commenced 3 hours before the Loma Prieta event, and continued until the occurrence of the main shock-; (3) ULF activity remained high for about two weeks after the Spitak earthquake, and for several months after the Loma Prieta earthquake; (4) ULF noise bursts were observed 1 to 6 hours before powerful aftershocks at Spitak during the period of enhanced activity, but there was no conclusive link between the ULF noise at Corralitos and the aftershocks. A major difference in the ULF activity preceding the two earthquakes is a difference in amplitude (0.2 nT at Spitak and 5 nT at Loma Prieta), but this is easily explained as being caused by the different distances of the observation stations from the epicenters.