Unipolar magnetic field pulses as transient signals prior to the 2009 Aquila earthquake shock

📝 Abstract

Unipolar pre-seismic magnetic field pulses have been observed first by Bleier et al. (2009) and Villante et al. (2010) and Nenovski et al. (2013). In the present study a detailed analysis of the pulses is conducted looking for signatures of transient signals similar to that recorded at the 2009 Aquila earthquake main shock (Nenovski, 2015). Various magnetic field data around the Aquila earthquake provide an instrumental basis for such an analysis. In addition to fluxgate magnetometer data (already examined in previous studies), overhauser magnetometer data are involved. The result is a detection and discrimination of transient signals of diffusive form that appear prior to the earthquake main shock.

💡 Analysis

Unipolar pre-seismic magnetic field pulses have been observed first by Bleier et al. (2009) and Villante et al. (2010) and Nenovski et al. (2013). In the present study a detailed analysis of the pulses is conducted looking for signatures of transient signals similar to that recorded at the 2009 Aquila earthquake main shock (Nenovski, 2015). Various magnetic field data around the Aquila earthquake provide an instrumental basis for such an analysis. In addition to fluxgate magnetometer data (already examined in previous studies), overhauser magnetometer data are involved. The result is a detection and discrimination of transient signals of diffusive form that appear prior to the earthquake main shock.

📄 Content

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UNIPOLAR MAGNETIC FIELD PULSES AS TRANSIENT SIGNALS PRIOR TO THE 2009 AQUILA EARTHQUAKE SHOCK P. Nenovski UCSRT, Sofia University, Sofia, Bulgaria

Abstract. Unipolar pre-seismic magnetic field pulses have been observed first by Bleier et al. (2009) and Villante et al. (2010) and Nenovski et al. (2013). In the present study a detailed analysis of the pulses is conducted looking for signatures of transient signals similar to that recorded at the 2009 Aquila earthquake main shock (Nenovski, 2015). Various magnetic field data around the Aquila earthquake provide an instrumental basis for such an analysis. In addition to the fluxgate magnetometer data (already examined in previous studies), overhauser magnetometer data are involved. The result is a detection and discrimination of transient signals of diffusive form that appear prior to the earthquake shock.

Keywords: transient signal, diffusive form, pre-seismic and co-seismic, signal polarity, pulse width,

Introduction Attempts to examine electric and magnetic field effects caused by irreversible mechanical. processes as cracks, fractures and stick-slip events at the time of earthquakes have been conducted for long time. A lot of mechanisms of generation and release of electric charges in rocks and hence are considered, then electric currents are expected as a underlined mechanism of electric and magnetic fields generated prior and/or during earthquakes. For example, step- like changes in stress and strain in rocks should result in an enormous electrical. signal. because of its short duration at the time of indentation fracture (Enomoto and Hashimoto, 1990, 1992, Parrot, 1995). On the other hand, unipolar pulses recorded by search-coil (induction) magnetometers have been recorded preceding earthquakes (Bleier et al., 2009; Bortnik et al., 2010; Villante et al., 2010, Nenovski et al., 2013). During the weeks leading up to the M = 5.4 Al.um Rock earthquake of 30 October 2007, a magnetometer located about 2 km from the epicenter recorded unusual non-al.ternating magnetic pulses, reaching amplitudes up to 30 nT (Bortnik et al.., 2010; Bleier et al.., 2009). The incidence of these pulses increased as the day of the earthquake approached. Note that after the earthquake, the pulse count immediately falls back to normal. levels. These local.ized unipolar pulses were much stronger and lasted longer, and could not be seen at locations far from the EQ epicenter (Bleier et al.., 2009). Villante et al. (2010) have observed an event of unusual. clusters of unipolar pulses of amplitude ~ 1 nT and width up to several. seconds on March 18, 2009, 19 days before the Mw 6.1 2009 Aquila earthquake. Their mechanism however has not been considered. Earlier, on 14 and 16 February 2009, unipolar pulse activity also has been identified, it lasted up to several hours under night-time conditions (Nenovski et al., 2013). It was concluded that the nature of the pulse signatures prior to the L’Aquila earthquake is

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unknown and its possible relationship to the L’Aquila earthquake (it occurred 55 days later) is not easy to be proved. Until recently, field observations of electric and magnetic signals at the time of earthquakes however remains still rare. The only exception were the piezomagnetic changes in the total. magnetic field related to stress (Stacey, 1964; Johnston et al., 1994; Johnston, 2002). Recently, single signals of unipolar sense that started just after the rupture of the fault have been detected first by electric field measurements (Fujinawa et al., 2011) and very recently by magnetic field measurements (Nenovski, 2015). To be exact, co-electromagnetic signals just before the seismic wave arrival (about 10 seconds and ~4 seconds after the EQ shock time) have been recorded first by Belov et al. (1974). Belov et al. (1974) however have measured only ULF field intensity filtered in specified frequency diapason and have not studied their characteristics. Authentic electric field pulses have been detected in Japan by borehole antenna in the 0.01-0.7 Hz channel (Fujinawa et al., 2011). The observed pulse width seemingly exceeds the seismic wave periods, but lies within a minute and less (Fujinawa et al., 2011). The transient magnetic field signal. of 0.8 nT amplitude of unipolar type and pulse width that exceeds 1 minute and overal.l duration of up to 4(5) minutes was observed just after the earthquake shock. The latter was registered at ~10 km distance from the 2009 L’Aquila earthquake hypocenter (Nenovski, 2015).
In contrast to the co-seismic magnetic field signal. recorded at the Aquila earthquake, the magnetic field unipolar pulses preceding the earthquake shock were of much shorter duration and usually does not exceed several. seconds (Bleier et al., 2009; Villante et al., 2010, Nenovski et al., 2013; Scoville et al., 2015). Another specific feature of the observed magnetic field signals is their am

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