Abstract
Abstract. We present the results of ground-based Ultra Low Frequency (ULF) magnetic field measurements observed from June to August 2004 during the Bovec earthquake on 12 July 2004. Further we give information about the seismic activity in the local observatory region for an extended time span 2004 and 2005. ULF magnetic field data are provided by the South European Geomagnetic Array (SEGMA) where the experience and heritage from the CHInese MAGnetometer (CHIMAG) fluxgate magnetometer comes to application. The intensities of the horizontal H and vertical Z magnetic field and the polarization ratio R of the vertical and horizontal magnetic field intensity are analyzed taking into consideration three SEGMA observatories located at different close distances and directions from the earthquake epicenter. We observed a significant increase of high polarization ratios during strong seismic activity at the observatory nearest to the Bovec earthquake epicenter. Apart from indirect ionospheric effects electromagnetic noise could be emitted in the lithosphere due to tectonic effects in the earthquake focus region causing anomalies of the vertical magnetic field intensity. Assuming that the measured vertical magnetic field intensities are of lithospheric origin, we roughly estimate the amplitude of electromagnetic noise in the Earths crust considering an average electrical conductivity of <σ>=10−3 S/m and a certain distance of the observatory to the earthquake epicenter.
Highlights
At the beginning the instrumental characteristics of the magnetometers are described and an overview about the seismic activity during this epoch is given
Ultra Low Frequency (ULF) magnetic field observations are a promising technique of analysis associated to anomalies before earthquakes
The ULF disturbances related to earthquakes are generally weak and sophisticated signal processing methods and a lot of experience is required to evaluate the source of ULF emission (Harada et al, 2004; Hattori and Hayakawa, 2007)
Summary
At the beginning the instrumental characteristics of the magnetometers are described and an overview about the seismic activity during this epoch is given. It is suggested by many authors performing ULF seismic observations (Fraser-Smith et al, 1990; Hayakawa and Fujinawa, 1994; Hayakawa et al, 1996, 2007; Molchanov et al, 2002, 2004b; Kopytenko et al, 2004; Harada et al, 2004; Sorokin et al, 2004) to apply certain standard signal processing methods to magnetic field data to possibly extract seismogenic electromagnetic ULF emission
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