The European VLF/LF Radio Network: Advances and Recent Results

Since 2009 a network of VLF (20-60 kHz) and LF (150-300 kHz) radio receivers has been put into operation in Europe in order to study earthquakes precursors. At the moment the network consists of ten receivers three of which are located in Italy, two in Greece and one in Portugal, Romania, Malta, Cyprus and Turkey. The data (sampling rate of 1min) are downloaded automatically at the end of each day and are collected at the Department of Physics of the University of Bari (Italy) that is the central node of the network. A detailed study of the radio data collected in the radio network from July 2009 to September 2011 was performed, using different methods of analysis. In total 27 cases suitable for analyzing were found and successes, i.e. radio anomalies preceding the subsequent earthquake (Mw 5.0) and clearly related to the event, were obtained in 70% of the cases; but increasing the value of the Mw threshold for the earthquakes this percentage seems to increase. Among the different methods of analysis the Wavelet spectra appear to be the most sensitive ones. At the moment a system able to apply on the radio data the Wavelet analysis automatically at the end of each day is being developed. On May 20, 2012 an earthquake with Mw=6.1 occurred in north Italy (Emilia region); the epicenter is located inside the “sensitive” area of the network. The results obtained in such occasion are presented

Wavelet analysis of the LF radio signals collected by the European VLF/LF network from July 2009 to April 2011

In 2008, a radio receiver that works in very low frequency (VLF; 20-60 kHz) and LF (150-300 kHz) bands was developed by an Italian factory. The receiver can monitor 10 frequencies distributed in these bands, with the measurement for each of them of the electric field intensity. Since 2009, to date, six of these radio receivers have been installed throughout Europe to establish a 'European VLF/LF Network'. At present, two of these are into operation in Italy, and the remaining four are located in Greece, Turkey, Portugal and Romania. For the present study, the LF radio data collected over about two years were analysed. At first, the day-time data and the night-time data were separated for each radio signal. Taking into account that the LF signals are characterized by ground-wave and sky-wave propagation modes, the day-time data are related to the ground wave and the night-time data to the sky wave. In this framework, the effects of solar activity and storm activity were defined in the different trends. Then, the earthquakes with M ≥5.0 that occurred over the same period were selected, as those located in a 300-km radius around each receiver/transmitter and within the 5th Fresnel zone related to each transmitter-receiver path. Where possible, the wavelet analysis was applied on the time series of the radio signal intensity, and some anomalies related to previous earthquakes were revealed. Except for some doubt in one case, success appears to have been obtained in all of the cases related to the 300 km circles in for the ground waves and the sky waves. For the Fresnel cases, success in two cases and one failure were seen in analysing the sky waves. The failure occurred in August/September, and might be related to the disturbed conditions of the ionosphere in summer. © 2012 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserve

Anomalies observed in VLF and LF Radio Signals on the occasion of the western Turkey Earthquake (M=5.7) on may 19, 2011

Since 2009 a network of VLF (20 - 60 kHz) and LF (150 - 300 kHz) radio receivers is operating in Europe in order to study the disturbances produced by the earthquakes on the propagation of these signals. In 2011 the network was formed by nine receivers, of which three are located in Italy and one is in Austria, Greece, Portugal, Romania, Russia and Turkey. On May 19, 2001 an earthquake (Mw = 5.7) occurred in western Turkey, that is inside the “sensitive” area of the network. The radio data collected during April-May 2011 were studied using the Wavelet spectra, the Principal Component Analysis and the Standard Deviation trends as different methods of analysis. Evident anomalies were revealed both in the signals broadcasted by the TRT transmitter (180 kHz) located near Ankara and in a VLF signal coming from a transmitter located in Western Europe and collected by the receiver TUR of the network located in eastern Turkey. Evident precursor phases were pointed out. Some differences in the efficiency of the three analysis methods were revealed

Gravitační mapy litosférické struktury pod Indickým oceánem

Litosférická struktura pod Indickým oceánem je pravděpodobně nejsložitější, ale zároveň nejméně pochopená mezi světovými oceány. Výsledky tomografických, geochemických, magnetických a jiných průzkumů svědčí o jeho komplexní geologické historii. Seismické průzkumy byly primárním zdrojem informací o litosférické struktuře pod Indickým oceánem, ale tyto experimenty jsou soustředěny hlavně na místech s vysokým geofyzikálním zájmem. Údaje o mořské gravitaci získané zpracováním měření družicové výškoměry naopak poskytují podrobný obraz reliéfu celého mořského dna, čímž se dále rozšiřují znalosti o jeho tvorbě, tektonismu a vulkanismu. V této studii používáme údaje o gravitační, bathymetrické, mořské sedimentu a litosférické hustotě pro sestavení gravitačních map Bouguer a pláště. Poté použijeme obě gravitační mapy k interpretaci litosférické struktury pod Indickým oceánem. Bouguerova gravitační mapa odhaluje hlavní tektonické a sopečné rysy, které jsou prostorově korelovány s odchylkami tloušťky kůry. Mapa gravitačního pláště vykazuje hlavně tepelný podpis litosférického pláště. Gravitační minima v této gravitační mapě značí výrazně aktivní oceánské divergentní tektonické okraje podél středových, jihovýchodních a jihozápadních indických hřebenů, včetně také Carlsbergského hřebene. Gravitační minima se rozprostírá podél Rudého moře - Adenského zálivu a východoafrických trhlin, což potvrzuje spojení mezi středo oceánskými hřebeny šíření (v Indickém oceánu) a kontinentálními trhlinami (ve východní Africe). Kombinovaná interpretace Bouguerových a plášťových gravitačních map potvrzuje kolizní původ horských pásem podél kontinentálních trhlin ve východní Africe. Důkaz o jižním rozšíření východoafrického riftového systému a jeho spojení s jihozápadním indickým hřbetem v mapě gravitačního pláště chybí. Podobně se neprojevuje pokračující rozpad složené indo-australské desky. Chybějící tepelný podpis v gravitační mapě pláště na těchto dvou místech je vysvětlen skutečností, že jižní hranice Nubian-Somálska (tj. Lwandleova deska) a Indo-australská hranice (tj. Capricornova deska) jsou rozptýlené zóny konvergence, charakterizované nízkou deformací a seismicitou v důsledku velmi pomalých rychlostí relativních pohybů ubytovaných přes tyto hranice. Chybí také jasný projev tepelného podpisu horkých míst uvnitř destičky v mapě gravitace pláště. Toto zjištění souhlasí s důkazy z přímých měření tepelného toku, které nenaznačují přítomnost významné pozitivní teplotní anomálie ve srovnání s oceánskou litosférou podobného věku.The lithospheric structure beneath the Indian Ocean is probably the most complicated, but at the same time, the least understood among world’s oceans. Results of tomographic, geochemical, magnetic and other surveys provide the evidence of its complex geological history. Seismic surveys have been a primary source of information about the lithospheric structure beneath the Indian Ocean, but these experiments are mainly concentrated at locations of a high geophysical interest. Marine gravity data obtained from processing the satellite altimetry measurements, on the other hand, deliver a detailed image of the whole seafloor relief, advancing further the knowledge about its formation, tectonism and volcanism. In this study, we use gravitational, bathymetric, marine sediment and lithospheric density structure data to compile the Bouguer and mantle gravity maps. We then use both gravity maps to interpret the lithospheric structure beneath the Indian Ocean. The Bouguer gravity map reveals major tectonic and volcanic features that are spatially correlated with crustal thickness variations. The mantle gravity map exhibits mainly a thermal signature of the lithospheric mantle. Gravity lows in this gravity map mark distinctively active oceanic divergent tectonic margins along the Central, Southeast and Southwest Indian Ridges including also the Carlsberg Ridge. Gravity lows extend along the Red Sea–Gulf of Aden and East African Rift Systems, confirming a connection between mid-oceanic spreading ridges (in the Indian Ocean) and continental rift systems (in East Africa). The combined interpretation of the Bouguer and mantle gravity maps confirms a non-collisional origin of mountain ranges along continental rift systems in East Africa. The evidence of a southern extension of the East African Rift System and its link with the Southwest Indian Ridge in the mantle gravity map is absent. Similarly, the ongoing breakup of the composite Indo-Australian plate is not manifested. A missing thermal signature in the mantle gravity map at these two locations is explained by the fact that the southern Nubian-Somalian plate boundary (i.e., the Lwandle plate) and the Indo-Australian plate boundary (i.e., the Capricorn plate) are diffuse zones of convergence, characterized by low deformation and seismicity due to very slow rates of relative motions accommodated across these boundaries. The clear manifestation of the thermal signature of intraplate hot spots in the mantle gravity map is also absent. This finding agrees with the evidence from direct heat flow measurements that do not indicate the presence of a significant positive temperature anomaly compared to the oceanic lithosphere of a similar age