{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,24]],"date-time":"2026-02-24T05:57:14Z","timestamp":1771912634471,"version":"3.50.1"},"reference-count":38,"publisher":"MDPI AG","issue":"3","license":[{"start":{"date-parts":[[2024,1,30]],"date-time":"2024-01-30T00:00:00Z","timestamp":1706572800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>We 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\u00b0E, 47.03\u00b0N) 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 Mw5.4 and Mw6.4, respectively, epicenters localized close to Zagreb (16.02\u00b0E, 45.87\u00b0N; 16.21\u00b0E, 45.42\u00b0N), 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.<\/jats:p>","DOI":"10.3390\/rs16030529","type":"journal-article","created":{"date-parts":[[2024,1,30]],"date-time":"2024-01-30T07:49:47Z","timestamp":1706600987000},"page":"529","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":12,"title":["Analysis of Pre-Seismic Ionospheric Disturbances Prior to 2020 Croatian Earthquakes"],"prefix":"10.3390","volume":"16","author":[{"given":"Mohammed Y.","family":"Boudjada","sequence":"first","affiliation":[{"name":"Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2584-8481","authenticated-orcid":false,"given":"Pier F.","family":"Biagi","sequence":"additional","affiliation":[{"name":"Department of Physics, University of Bari, 70126 Bari, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Hans U.","family":"Eichelberger","sequence":"additional","affiliation":[{"name":"Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7621-5014","authenticated-orcid":false,"given":"Giovanni","family":"Nico","sequence":"additional","affiliation":[{"name":"Institute for Applied Mathematics (IAC), National Research Council of Italy (CNR), 70126 Bari, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Patrick H. M.","family":"Galopeau","sequence":"additional","affiliation":[{"name":"LATMOS-CNRS, UVSQ Universit\u00e9 Paris-Saclay, 78280 Guyancourt, France"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9323-6087","authenticated-orcid":false,"given":"Anita","family":"Ermini","sequence":"additional","affiliation":[{"name":"Department of Industrial Engineering, University of Tor Vergata, 00133 Rome, Italy"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Maria","family":"Solovieva","sequence":"additional","affiliation":[{"name":"Institute of the Earth Physics, Russian Academy of Sciences, Moscow 123995, Russia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Masashi","family":"Hayakawa","sequence":"additional","affiliation":[{"name":"Hayakawa Institute of Seismo Electromagnetics, Co., Ltd. (Hi-SEM), Tokyo 182-8585, Japan"},{"name":"Advanced Wireless & Communications Research Center, The University of Electro-Communications, Tokyo 182-8585, Japan"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Helmut","family":"Lammer","sequence":"additional","affiliation":[{"name":"Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Wolfgang","family":"Voller","sequence":"additional","affiliation":[{"name":"Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Martin","family":"Pitterle","sequence":"additional","affiliation":[{"name":"Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2024,1,30]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Hayakawa, M. 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