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.
Abstract. Measurements of ULF electromagnetic disturbances were carried out in Japan before and during a seismic active period (1 February 2000 to 26 July 2000. A network consists of two groups of magnetic stations spaced apart at a distance of ≈ 140 km. Every group consists of three, 3-component high sensitive magnetic stations arranged in a triangle and spaced apart at a distance of 4-7 km. The results of the ULF magnetic field variation analysis in a frequency range of F = 0.002−0.5 Hz in connection with nearby earthquakes are presented. Traditional Z/G ratios (Z is the vertical component, G is the total horizontal component), magnetic gradient vectors and phase velocities of ULF waves propagating along the Earth's surface were constructed in several frequency bands. It was shown that variations of the R(F ) = Z/G parameter have a different character in three frequency ranges: F 1 = 0.1 ± 0.005, F 2 = 0.01 ± 0.005 and F 3 = 0.005 ± 0.003 Hz. Ratio R(F 3 )/R(F 1 ) sharply increases 1-3 days before strong seismic shocks. Defined in a frequency range of F 2 = 0.01 ± 0.005 Hz during nighttime intervals (00:00-06:00 LT), the amplitudes of Z and G component variations and the Z/G ratio started to increase ≈1.5 months before the period of the seismic activity. The ULF emissions of higher frequency ranges sharply increased just after the seismic activity start. The magnetic gradient vectors (∇B ≈ 1 − 5 pT/km), determined using horizontal component data (G ≈ 0.03 − 0.06 nT) of the magnetic stations of every group in the frequency range F = 0.05 ± 0.005 Hz, started to point to the future center of the seismic activity just before the seismoactive period; furthermore they continued following space displacements of the seismic activity center. The phase velocity vectors (V ≈ 20 km/s for F = 0.0067 Hz), determined using horizontal component data, were directed from the seismic activity center. Gradient vectors of the vertical component pointed to the closest seashore (known as the "sea shore" effect). The location of the seismic activity centers by two gradient vectors, conCorrespondence to: V. S. Ismaguilov (galina@gh5667.spb.edu) structed at every group of magnetic stations, gives an ≈10 km error in this experiment.
The present analysis was stimulated by previous findings on the possible influence of natural ultralow‐frequency (ULF; 0.001–10 Hz) geomagnetic field variations on the cardiovascular system and indications of an effect of man‐made ULF magnetic fields on the rate of myocardial infarction. In the present study, we considered the occupational health hazards of the strongest ULF magnetic fields in densely populated urban areas. Measurements of ULF magnetic field fluctuations produced by trains powered by DC electricity were performed by means of a computer‐based, highly sensitive, three‐component magnetometer. We found that the magnitude of magnetic field pulses inside the driver's cab of electric locomotives (ELs) could be ≥ 280 μT in the horizontal component perpendicular to the rails and up to approximately 130 μT in the vertical component, and, in the driver's compartment of electric motor unit (EMU) trains, they were approximately 50 and 35 μT, respectively. We have investigated the relationships between the occupational exposure to ULF magnetic field fluctuations produced by electric trains and cardiovascular diseases (CVDs) among railroad workers in the former Soviet Union. We have analyzed medical statistical data for a period of 3 years for approximately 45,000 railroad workers and 4,000 engine drivers. We have also analyzed 3 years of morbidity data for three subgroups of engine drivers (∼4,000 in each group) operating different types of trains. We find that EL drivers have a twofold increase in risk (2.00 ± 0.27) of coronary heart diseases (CHDs) compared with EMU drivers. Because our analysis of major CVDs shows that the examined subpopulations of drivers can be considered to have had equal exposure to all known risk factors, the elevated CHD risk among EL drivers could be attributed to the increased occupational exposure to ULF magnetic fields. © 1996 Wiley‐Liss, Inc.
Recent epidemiological studies suggest a link between transport magnetic fields (MF) and certain adverse health effects. We performed measurements in workplaces of engineers on Russian DC and Swiss AC powered (16.67 Hz) electric trains using a computer based waveform capture system with a 200 Hz sampling rate. MF in DC and AC trains show complex combinations of static and varying components. The most probable levels of quasistatic MF (0.001-0.03 Hz) were in the range 40 microT. Maximum levels of 120 microT were found in DC powered locomotives. These levels are much higher than the geomagnetic field at the site of measurements. MF encountered both in DC and AC powered rail systems showed irregular temporal variability in frequency composition and amplitude characteristics across the whole frequency range studied (0-50 Hz); however, more than 90% of the magnetic field power was concentrated in frequencies =16.67 Hz. In AC locomotives, such as the most popular engine Re 4/4 II, the major energy falls around the fundamental frequency 16.67 Hz, with an average magnetic field intensity of about 44 microT; moreover, a significant contribution (about 15%) is due to components below the fundamental frequency. In Russian DC powered trains amplitudes of field variations sharply decrease from static to approximately 3-4 Hz fields; for higher frequencies a tendency of slow decrease is observed up to 50 Hz. For frequencies higher than 1 Hz average amplitudes of magnetic field variations are less than 1 microT; maximum levels reach tens of microtesla. At frequencies lower than 15 Hz, the average magnetic field generated by Swiss AC powered locomotives was approximately 10 times greater than fields observed in Russian DC powered trains.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.