A VLF/LF facility network for preseismic electromagnetic investigations

International audienceEarthquakes are one of the most frequently occurring natural disasters. Many indications have been collected on the presence of seismo-ionospheric perturbations preceding such tragic phenomena. Radio techniques are the essential tools leading the detection of seismo-electromagnetic emissions by monitoring at very low frequency (VLF, 3-30 kHz) and low frequency (LF, 30-300 kHz) sub-ionospheric paths between transmitters and receivers (Hayakawa, 2015). In this brief communication, we present the implementation of a VLF/LF network for searching earthquake electromagnetic precursors. The proposed system is comprised of a monopole antenna including a preamplifer, a GPS receiver and a recording device. This system will deliver a steady stream of real-time amplitude and phase-measurements and a daily recording VLF/LF data set. A first implementation of the system was done in Graz, Austria, the second one will be in Guyancourt, France, a third one in Réunion, France, and a fourth one in Moratuwa, Sri Lanka. In the near future, we are planning on expanding of our network for enhanced monitoring and increased coverage

Analysis of Pre-Seismic Ionospheric Disturbances Prior to 2020 Croatian Earthquakes

International audienceWe study the sub-ionospheric VLF transmitter signals recorded by the Austrian Graz station in the year 2020. Those radio signals are known to propagate in the Earth-ionosphere waveguide between the ground and lower ionosphere. The Austrian Graz facility (geographic coordinates: 15.46 • E, 47.03 • N) can receive such sub-ionospheric transmitter signals, particularly those propagating above earthquake (EQ) regions in the southern part of Europe. We consider in this work the transmitter amplitude variations recorded a few weeks before the occurrence of two EQs in Croatia at a distance less than 200 km from Graz VLF facility. The selected EQs happened on 22 March 2020 and 29 December 2020, with magnitudes of M w 5.4 and M w 6.4, respectively, epicenters localized close to Zagreb (16.02 • E, 45.87 • N; 16.21 • E, 45.42 • N), and with focuses of depth smaller than 10 km. In our study we emphasize the anomaly fluctuations before/after the sunrise times, sunset times, and the cross-correlation of transmitter signals. We attempt to evaluate and to estimate the latitudinal and the longitudinal expansions of the ionospheric disturbances related to the seismic preparation areas

Satellite Measured Ionospheric Magnetic Field Variations over Natural Hazards Sites

Processes and threats related to natural hazards play an important role in the evolution of the Earth and in human history. The purpose of this study is to investigate magnetic field variations measured at low Earth orbit (LEO) altitudes possibly associated with earthquakes, volcanic eruptions, and artificial outbursts. We focus on two missions with well equipped magnetometer packages, the China Seismo-Electromagnetic Satellite (CSES) and ESA’s three spacecraft Swarm fleet. After a natural hazards survey in the context of this satellites, and consideration of external magnetospheric and solar influences, together with spacecraft interferences, wavelet analysed spatio-temporal patterns in ionospheric magnetic field variations related to atmospheric waves are examined in detail. We provide assessment of the links between specific lithospheric or near surface sources and ionospheric magnetic field measurements. For some of the diverse events the achieved statistical results show a change in the pattern between pre- and post-event periods, we show there is an increase in the fluctuations for the higher frequency (smaller scales) components. Our results are relevant to studies which establish a link between space based magnetic field measurements and natural hazards