Abstract. Ionograms from Rome (41.8N, 12.5E) and Sofia (42.4N, 23.2E) ionospheric stations during earthquake (EQ) activity with magnitude (M) between 5 and 6 in Central Italy are analyzed. It is found that several ionospheric disturbances occur in the intermediate E-F region before the EQ shock. In fact, besides sporadic E (Es) layer development (of type h) of short duration (transients), fmin increase, trace gaps near the critical frequencies, and E region trace disappearance are also observed within one to three hours before the EQ shock. Before the EQ shocks we find that the F2 region parameters are practically undisturbed. The only exception is the so-called fork trace that appears mostly near the critical frequency of the F2 region. Acoustic gravity waves (AGW) are suggested as one of the possible sources of transients observed in the ionosphere before the EQ shock.
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.
Abstract. This work presents ground based Ultra Low Frequency (ULF) magnetic field measurements in the frequency range from 10–15 mHz from 1 January 2008 to 14 April 2009. In this time period a strong earthquake series hit the Italian Abruzzo region around L'Aquila with the main stroke of magnitude M = 6.3 on 6 April 2009. In the frame of the South European Geomagnetic Array (SEGMA), a European collaboration runs ULF fluxgate instruments providing continuously magnetic field data recorded in mid- and south Europe. The main scientific objective is the investigation of signal variations due to seismic activity and the discrimination between other natural and human influences. The SEGMA station closest to the L'Aquila earthquake epicenter is L'Aquila observatory located in the epicenter region. For the scientific analysis we extract the nighttime period from 22:00–02:00 UT and determine the power spectral density (PSD) of the horizontal (H) and vertical (Z) magnetic field components and the standardized polarization ratio (Z) over (H). To discriminate local emissions from global geomagnetic effects, data from three SEGMA stations in distances up to 630 km from the epicenter region are analyzed and further compared to the independent global geomagnetic ∑ Kp index. Apart from indirect ionospheric effects, electromagnetic noise could be originated in the lithosphere due to tectonic mechanisms in the earthquake focus. To estimate the amplitude of assumed lithospheric electromagnetic noise emissions causing anomalies in the PSD of the (Z) component, we consider magnetotelluric calculations of the electric crust conductivity in the L'Aquila region. Results found at L'Aquila observatory are interpreted with respect to the lithosphere electrical conductivity in the local observatory region, the ∑ Kp index, and further in a multi station analysis. Possible seismic related ULF anomalies occur ~2 weeks before the main stroke.
Abstract. Several investigations reported the possible identification of anomalous geomagnetic field signals prior to earthquake occurrence. In the ULF frequency range, candidates for precursory signatures have been proposed in the increase in the noise background and polarization parameter (i.e. the ratio between the amplitude/power of the vertical component and that one of the horizontal component), in the changing characteristics of the slope of the power spectrum and fractal dimension, in the possible occurrence of short duration pulses. We conducted, with conventional techniques of data processing, a preliminary analysis of the magnetic field observations performed at L'Aquila during three months preceding the 6 April 2009 earthquake, focusing attention on the possible occurrence of features similar to those identified in previous events. Within the limits of this analysis, we do not find compelling evidence for any of the features which have been proposed as earthquake precursors: indeed, most of aspects of our observations (which, in some cases, appear consistent with previous findings) might be interpreted in terms of the general magnetospheric conditions and/or of different sources.
