Ultra Low Frequency (ULF) European multi station magnetic field analysis before and during the 2009 earthquake at L'Aquila regarding regional geotechnical information

Abstract

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 K p index. Apart from indirect ionospheric effects, electromagnetic noise could be originated in the lithosphere due to tectonic mechanisms in the earthquake focus. To estimate Correspondence to: G. Prattes (gustav.prattes@oeaw.ac.at) 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 K p index, and further in a multi station analysis. Possible seismic related ULF anomalies occur ∼2 weeks before the main stroke.

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.…”

Review of unprecedented ULF electromagnetic anomalous emissions possibly related to the Wenchuan <i>M</i><sub>S</sub> = 8.0 earthquake, on 12 May 2008

“…Anomalous electromagnetic emissions were also observed before the great crustal M S = 6.9 EQ at Spitak, Armenia, on 7 December 1988 (f = 0.005-1 Hz, d = 120-200 km, A = 0.03-0.2 nT) (Molchanov et al, 1992;Kopytenko et al, 1993), about one month and a few days before the 8 August 1993 M S = 8.0 Guam EQ (f = 0.02-0.05 Hz, d = 65 km) Kawate et al, 1998) and before the great M W = 8.2 Biak EQ in Indonesia, on 17 February 1996 (f = 0.005-0.03 Hz, d = 80 km, A = 0.2-0.3 nT) (Hayakawa et al, 2000). Possible seismic related ULF (f = 0.01-0.015 Hz) anomalies occurred 2 weeks before L'Aquila M = 6.3 EQ with the distance up to 630 km (Prattes et al, 2011). Short-term geo-electric field signals of particular form and character precede EQs with magnitudes greater than 5 at distances up to several hundreds of kilometers (Varotsos and Alexopoulos 1984a, b;Varotsos and Lazaridou, 1991;Varotsos et al, 1993a, b;Nagao et al, 1996).…”

Comment on "Ultra Low Frequency (ULF) European multi station magnetic field analysis before and during the 2009 earthquake at L'Aquila regarding regional geotechnical information" by Prattes et al. (2011)

“…As a consequence, some authors (see Campbell, 2009;Thomas et al, 2009;Masci, 2010Masci, , 2011a have recently put into question well-known magnetic seismogenic precursors by showing that these anomalous signals are actually induced by normal geomagnetic activity. Prattes et al (2011), hereafter cited as P11, report the analysis of ULF magnetic data coming from the South European Geomagnetic Array (SEGMA) during the period [2008][2009]. See P11 for the location of the SEGMA stations.…”

“…Prattes et al (2011) report ULF magnetic anomalous signals claiming them to be possibly precursor of the 6 April 2009 M W = 6.3 L'Aquila earthquake. This comment casts doubts on the possibility that the observed magnetic signatures could have a seismogenic origin by showing that these pre-earthquake signals are actually part of normal global geomagnetic activity.…”