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

Abstract

The characteristics of the ULF magnetic field emissions measured at two magnetic observatories in the Republic of Georgia prior to and after the Ms = 6.9 earthquake that occurred near Spitak, Armenia, on December 7, 1988, are compared with the apparently similar emissions associated with the Ms = 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 after‐shocks. 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.

Changes in the ultra-low frequency wave field during the precursor phase to the Sichuan earthquake: DEMETER observations

“…Fraser-Smith et al (1990) reported ultra-low frequency (ULF) magnetic field activity in the frequency range 0.01-0.2 Hz that began around 12 days before the main shock of the Loma Prieta earthquake. Similar observations are also reported by Molchanov et al (1992) (M S = 6, Spitak), Hayakawa et al (2006) (M S = 8, Guam), and Prattes et al (2011) (M = 6.3, L'Aquila) amongst others. However, not all large earthquakes exhibit such phenomena.…”

Statistical features of seismoelectric signals prior to M7.4 Guerrero-Oaxaca earthquake (México)

“…Ramírez-Rojas (arr@correo.azc.uam.mx) 1996, 2002Eftaxias et al, 2004;Hayakawa et al, 1999Hayakawa et al, , 2000Hayakawa and Timashev, 2006;Smirnova et al, 2001;Telesca et al, 2001Telesca et al, , 2003Telesca et al, , 2005aTelesca et al, , 2005bTelesca et al, , 2007Ida et al, 2005;Ida and Hayakawa, 2006;Ida et al, 2007;. Furthermore, the presence of precursory signatures of EQs has been clearly identified in the ULF range for large magnitude (Ms>6) EQ's (Fraser-Smith et al, 1990;Molchanov et al, 1992;Kopytenko et al, 1993Flores-Marquez et al, 2007). In a previous work we performed a statistical analysis of the spectral exponent and the correlation time of geoelectrical signals associated to the Guerrero-Oaxaca EQ (Ms=7.4) occurred on 14 September 1995 in southern Mexico (Ramirez-Rojas et al, 2004).…”

Electromagnetic Radiation Phenomena before and after the 1995 Hyogo-ken Nanbu Earthquake.

“…Many results of observations of electromagnetic radiation (EMR) related to earthquakes have been reported (Gohkhberg et al, 1982;Yoshino et al, 1985;Oike and Ogawa, 1986;Fraser-Smith et al, 1990;Fujinawa et al, 1990;Yoshino, 1991;Molchanov et al, 1992;Hayakawa et al, 1993, Fujinawa andTakahashi, 1994; and so on). Oike and Ogawa (1986) reported their observation system that counted the number of spike-type electromagnetic noises at 163 kHz in LF range (LF noises) and the results of observations at Uji Station, Kyoto, Japan (shown in Fig.…”