Search

© 2015 Elsevier Ldt. The Portuguese team acknowledges the support of the Portuguese Science and Technology Foundation (FCT) through the project Pest/OE/CTE/UI0078/2014 and the FCT/FEDER-COMPETE project EAC (PTDC/GEO-FIQ/4178/2012) FCOMP-01-0124-FEDER-029197. We are grateful to Samuel Bárias for the maintenance of the Portuguese radio antennas, to Elisa Cardoso for her help with wavelet theory, to Tommaso Maggipinto for instructing us on the MatLab® wavelet script and to Rui Salgado for the interpretation of the weather data. H.G.S is grateful to FCT for the postdoctoral fellowship SFRH/BPD/63880/2009 and to Calouste Gulbenkian Foundation (Portugal) for the award Estímulo à Criatividade e à Qualidade na Actividade de Investigação (2010). The Spanish team acknowledges the support of the Spanish Ministry of Economy and Competitiveness through the project CGL2014-62113-EXP. This work is part of the research activity of the Spanish Team “Grupo de Estudios Ionosféricos y Técnicas de Posicionamiento Satelital (GNSS)” financed by the Universidad Complutense de Madrid. The authors are very grateful to Luigi Ciraolo for his collaboration. They also want to thank the International GNSS Service, IGS, and EUREF Permanent Network (EPN) for providing the GNSS data; the World Data Center for Geomagnetism Kyoto for the Dst and Kp index values; and Intermagnet for the magnetic field data. AVVSO, the Russian Weather website and OMNIWeb Data Explorer from NASA, also provided very valuable data. Figures were drawn using the Generic Mapping Tools (Wessel and Smith, 1998). The final version of the article was prepared when M.H. was on sabbatical leave at the Abdus Salam International Centre for Theoretical Physics (Trieste, Italy) granted by the Spanish Ministry of Education, Culture and Sports. The authors are grateful to Dr. Kazimirowsky and two anonymous reviewers for their comments and suggestions, which improved the quality of the present manuscript., The present work integrates ground-based ionosphere measurements using very-low-frequency radio transmissions with satellite measurements of the total electron content to draw common conclusions about the possible impact that the Mw6.1 earthquake that took place in Greece on January 26, 2014, had on the ionosphere. Very-low-frequency radio signals reveal the existence of an ~4-day anomaly in the wavelet spectra of the signals received inside the earthquake preparation zone and a significant increase in the normalized variance of the signals prior to the earthquake (approximately 1 day before). Through total electron content analysis, it was possible to identify a clear anomaly from 15:00 until 20:00 UT on the day before the earthquake that appears again on the day of the earthquake between 07:00 UT and 08:00 UT. The anomalous values reach TEC*Sigma ~4.36 and 3.11, respectively. Their spatial and temporal distributions give grounds to assume a possible link with the earthquake preparation. The geomagnetic, solar and weather conditions during the considered period are presented and taken into account. This work is an initial and original step towards a multi-parameter approach to the problem of the possible earthquake-related effects on the ionosphere joining observations made from both ground stations and satellites. A well-founded knowledge of these phenomena is clearly necessary before dealing with their application to earthquake prediction purposes., Ministerio de Economía y Competitividad (MINECO), Fundación para la Ciencia y Tecnología (FCT), Portugal, COMPETE - Programa Operacional Factores de Competitividade, Portugal, Fondo Europeo de Desarrollo Regional (FEDER), Unión Europea (UE), Calouste Gulbenkian Foundation (Portugal), Universidad Complutense de Madrid, Depto. de Física de la Tierra y Astrofísica, Fac. de Ciencias Físicas, TRUE, pub

Radio Monte Carlo (MCO) is a long wave broadcasting station (43.47 deg N, 06.09 deg E) with a frequency of 216 kHz transmitting continuously with a mean power of 1,400 kW. The electric field strength has been measured since 1991 in central Italy at two sites named Assergi (AS) and Pietrasecca (PIE) located about six hundred kilometers from the MCO transmitter. Two anomalous decreases in the field strength of MCO, with a duration of several days happened, at the AS measurement site. On the occasion of May 1993 radio-anomaly, the estimated attenuation were of 22 dB for the sky wave and of 7 dB for the ground wave, while they were of 21 dB for the sky wave and 5 dB for the ground wave, on the occasion of the February 1994 radio-anomaly. After about fifteen days from the end of the May 1993 radio-anomaly, an earthquake with magnitude 4.3 occurred at about 65 km from MCO-AS path. Furthermore, after about four days from the end of the February 1994 radio-anomaly, a seismic sequence happened along the MCO-AS path at about 40 km from AS site, with magnitude ranging from 3 to 3.5. It must be noted that the epicenters are located where the sky wave is already in the troposphere. So, the attenuation observed, seem to be connected with some defocusing process of MCO signal related to preseismic phenomenology that occurs nearby the receiver, along the path from the transmitter to the receiver. Since this defocusing seems to regard only MCO signals or signals with similar frequency, the focal zone of the earthquakes can be assumed as a sphere with a radius ranging from 0.5 to 2.5 km, i.e., the same order of the wavelength of MCO radiowave. A possible explanation of the defocusing process can be a modification of the elliptical polarization state of the radiowaves due to the formation of a birefringent structure in the troposphere caused by the preparation phase of the earthquakes. The coincidence of a warm air advection during the anomalies can help the effect of the birefringent, ラジオモンテカルロ(MCO)は、周波数216kHz、平均出力1,400kWで連続的に送信している長波放送局(43.47 度N, 06.09 度E)である。その電場強度は、1991年以来、MCOの送信機から約600km離れた中部イタリアのAssergi(AS)とPietrasecca(PIE)の2箇所で測定されている。ASの測定場所で、数日間続く、MCO電場強度の異常減少が2回起った。推算された減衰は、1993年5月の電波異常の場合は上空波に対して22dB、地上波に対して7dBであり、1994年2月の電波異常の際は上空波に対し21dB、地上波に対して5dBであった。1993年5月の電波異常が終ってから約15日後、MCO-AS経路から約65kmでマグニチュード4.3の地震が起きた。さらに1994年2月の電波異常が終ってから約4日後にマグニチュード3-3.5の連続的な地震がASから約40kmのMCO-AS間の経路に沿って起った。震央の位置では上空波はすでに対流圏にあることが注目される。従って、観測された減衰は、送信機から受信機までの経路に沿って、受信機近くで起った地震前現象学に関係した、MCO信号のある種の焦点ぼけの過程に関連しているようである。この焦点ぼけはMCO信号か類似した周波数の信号だけに関係していると思われるので、地震の震源地帯は、MCO電波の波長と同程度の、半径0.5-2.5kmの球と仮定することができる。この焦点ぼけの過程は、地震の準備段階に起因する対流圏の複屈折構造形成による、電波の楕円偏波状態の一種と説明することができる。この異常期間中に同時発生した暖かい空気の移流は、MCO上空波に及ぼす複屈折構造の影響を助長したであろう。

The present work integrates ground-based ionosphere measurements using very-low-frequency radio transmissions with satellite measurements of the total electron content to draw common conclusions about the possible impact that the Mw6.1 earthquake that took place in Greece on January 26, 2014, had on the ionosphere. Very-low-frequency radio signals reveal the existence of an ∼4-day anomaly in the wavelet spectra of the signals received inside the earthquake preparation zone and a significant increase in the normalized variance of the signals prior to the earthquake (approximately 1 day before). Through total electron content analysis, it was possible to identify a clear anomaly from 15:00 until 20:00 UT on the day before the earthquake that appears again on the day of the earthquake between 07:00 UT and 08:00 UT. The anomalous values reach TEC∗Sigma ∼4.36 and 3.11, respectively. Their spatial and temporal distributions give grounds to assume a possible link with the earthquake preparation. The geomagnetic, solar and weather conditions during the considered period are presented and taken into account. This work is an initial and original step towards a multi-parameter approach to the problem of the possible earthquake-related effects on the ionosphere joining observations made from both ground stations and satellites. A well-founded knowledge of these phenomena is clearly necessary before dealing with their application to earthquake prediction purposes.