ÖAW — AUSTRIAN ACADEMY OF SCIENCES SYNOPTIC VIEW ON SUB-IONOSPHERIC VLF/LF AMPLITUDE AND PHASE VARIATIONS AT THE GRAZ SEISMO-ELECTROMAGNETIC FACILITY Hans Eichelberger 1 , Konrad Schwingenschuh 1 , Mohammed Y. Boudjada 1 , Bruno P. Besser 1 , Daniel Wolbang 1 , Maria Solovieva 2 , Pier Francesco Biagi 3 , Manfred Stachel 1 , Özer Aydogar 1 , Martin Pitterle 1 , Cosima Muck 1 , Claudia Grill 1 , and Irmgard Jernej 1 1 Space Research Institute, Austrian Academy of Sciences, Graz, Austria, 2 Schmidt Institute of Physics of the Earth, RAS, Moscow, Russia, 3 Department of Physics, University of Bari, Bari, Italy This presentation discusses physical processes related to potentially seismic and non-seismic disturbances along VLF/LF paths measured with two different receivers located side by side at the Space Research Institute (IWF) facility in Graz, Austria. At the same time both systems are embedded in international networks which gives the unique opportunity to probe the waveguide cavity over a large area. In general, a variety of VLF/LF amplitude and phase variations are ubiquitous at wide scales throughout the cavity. We analyse such signals observed in the period 2018-2020 (solar minimum, i.e. less external forcing of the upper ionospheric boundary) in the time- and frequency- domain for several paths. In this attempt we aim to single out natural disturbances, characterise the source event, and figure out the lithosphere atmosphere ionosphere coupling mechanism. For known seismic events we consider the so-called Dobrovolsky Bowman relationship (Dobrovolsky etal, 1979; Bowman etal, 1998) allowing to estimate the pre-seismic zone crossed by the VLF/LF paths. The findings open up good prospects for an automated monitoring and characterisation of source phenomena who affect the electric field of VLF/LF sub- ionospheric links. VLF/LF NETWORK, EUROPEAN AREA Figure 1: Great circle paths (orange color) between transmitter and receiver for the UltraMSK system with four stations Paris (planned), Moscow, Sheffield, and Graz (yellow circles). INFREP system (paths/diamonds in blue/yellow) over Europe, see (Biagi etal, 2019). Credit map software: Generic Mapping Tools (GMT) Table 2: Transmitter received at the VLF/LF Graz facility, current settings in blue and green color, see (Schwingenschuh etal, 2011). SUMMARY Different types of variations in nighttime VLF/LF amplitude measurements for selected earthquakes are detected. A network is necessary to sample the waveguide over a large area. Further improvements can be achieved in complementary investigations. APRIL 2021 IWF — SPACE RESEARCH INSTITUTE, HUE@OEAW.AC.AT, EGU21-10123 REFERENCES ▪ Dobrovolsky, I.P., Zubkov, S.I., and Miachkin, V.I., Estimation of the size of earthquake preparation zones, PAGEOPH 117, 1025–1044, 1979. https://doi.org/10.1007/BF00876083 ▪ Bowman, D.D., Ouillon, G., Sammis, C.G., Sornette, A., and Sornette, D., An observational test of the critical earthquake concept, JGR Solid Earth, 103, B10, 24359-24372, 1998. https://doi.org/10.1029/98JB00792 ▪ Schwingenschuh, K., et al., The Graz seismo-electromagnetic VLF facility, NHESS, 11, 1121–1127, https://doi.org/10.5194/nhess-11- 1121-2011, 2011. ▪ Boudjada, M.Y., et al., Ray paths of VLF/LF transmitter radio signals in the seismic Adriatic regions, EGU General Assembly 2021, EGU21-7659, 2021. https://doi.org/10.5194/egusphere-egu21-7659 ▪ Biagi, P.F., et al., The INFREP Network: Present Situation and Recent Results, Open Journal of Earthquake Research, 8, 101-115, 2019. doi: 10.4236/ojer.2019.82007 http://www.infrep-network.eu Receiver: Graz, IWF, Elettronika [E] and UltraMSK [U] system, N 47°2‘40.38‘‘ O 15°28‘47.68‘‘ No. Acronym Frequency (kHz) GCP (km) Transmitter, Systems [U 1sec], [U 20sec], [E 60 sec] 1 JXN 16.40 2160 Aldra, Norway [U 1s] [U 20s] 2 GWU 18.30 980 Le Blanc, Rosnay, St. Assise, France [U 1s] 3 VTX 19.20 (17.00) 7240 Vijayanarayanam, India [U 1s] [U 20s] 4 GBS 19.58 1570 Anthorn, UK [U 1s] [U 20s] 5 NWC 19.80 12390 Exmouth, Western Australia [U 1s] 6 ICV 20.27 820 Tavolara, Sardinia, Italy [U 1s] [U 20s] [E 60s] 7 HWU 20.90 / 21.75 1080 Le Blanc, St. Assise, France [U 1s] [U 20s] [E 60s] 8 NPM 21.40 12380 Lualualei, Hawaii, USA [U 1s] [U 20s] 9 GBZ 22.10 1540 Skelton, UK [U 1s] [U 20s] [E 60s] 10 JJI 22.20 9140 Ebino, Kyushu, Japan [U 1s] 11 DHO 23.40 875 Rhauderfehn, Germany [U 1s] [U 20s] [E 60s] 12 NAA 24.00 6110 Cutler, Maine, USA [U 1s] [U 20s] 13 NLM 25.20 7820 LaMoure, North Dakota, USA [U 1s] 14 TBB 26.70 1445 Bafa, Turkey [U 1s] [U 20s] 15 NRK 37.50 2975 Keflavik, Iceland [U 1s] [U 20s] [E 60s] 16 JJY 40.00 9195 Mount Otakadoya, Honshu, Japan [U 1s] 17 NAU 40.80 7985 Aguada, Puerto Rico, USA [U 1s] 18 ITS 45.90 1105 Niscemi, Sicily, Italy [U 1s] [U 20s] 19 DCF 77.50 580 Mainflingen, Germany [U 1s] 20 RRO 153 790 Brasov, Romania [E 60s] 21 TDF (EU1) 162 (183) 1010 (700) Allouis, France (Felsberg-Berus, Luxembourg) [E 60s] 22 CH1 198 1900 Berkaoui/Ouargia, Algeria [E 60s] 23 RTL (MCO) 234 (216) 740 (820) Beidweiler, Luxembourg (Roumoules, Monte Carlo) [E 60s] 24 CZE 270 275 Topolna, Czech Republic [E 60s] LIST OF EVENTS & DATA PROCESSING The period of investigation is 3 years, from Jan. 1, 2018 to Jan. 1, 2021, in the geographical latitude/longitude range [30° ≤ lat ≤ 50°] and [10° ≤ long ≤ 40°], i.e. roughly a wider Southeast Europe area which is covered by sub-ionospheric VLF/LF propagation paths. For this temporal, spatial, and also moment magnitude M w ≥ 5.5 constraints the United States Geological Survey (USGS) earthquake database https ://earthquake.usgs.gov/ (as of April 2021) gives 25 earthquakes. From this list 4 events (Tab. 1) are selected according to VLF/LF path crossings and the size of the Dobrovolsky-Bowman relationship (the radius of the effective precursor manifestation zone ρ = 10 (0.43*M) km and log(R) ≈ M/2 km). Table 1: Selected earthquakes (and Figure 1, stars in yellow color). The VLF/LF amplitude data have 1 Hz temporal resolution and nighttime values are used (±2 hours around midnight). The values are smoothed with a low pass filter, the residuals are below 1 dB. Phase data are avoided due to higher variations on all paths. Selected Earthquakes, USGS database, time span 01.01.2018 – 01.01.2021, long/lat range: [10°-40°]/[30°-50°] No. Date, Time Long (°) / Lat (°) / Depth (km) Mag / ρ (km) Location 1 2020-12-29T11:19:54.762 +45.42 / +16.26 / 10.0 6.4 / 565 Petrinja, Croatia 2 2020-10-30T11:51:27.348 +37.90 / +26.78 / 21.0 7.0 / 1023 Néon Karlovásion, Greece 3 2019-08-08T11:25:31.104 +37.94 / +29.70 / 11.0 5.9 / 344 Baklan, Turkey 4 2019-03-20T06:34:27.835 +37.41 / +29.53 / 8.0 5.7 / 282 Acipayam, Turkey RESULTS ▪ Sub-ionospheric VLF/LF measurements (in particular during nighttime) are a valuable tool to characterise variations from the lithosphere up to the lower ionosphere ▪ A network structure is required in order to sample a large area and to distinguish seismic from non-seismic disturbances ▪ The lithosphere-atmosphere-ionosphere coupling is presumably via atmospheric waves (physical process) ▪ For robust statistics long term measurements are necessary ▪ Nighttime VLF/LF amplitudes have lower values before the Petrinja-EQ, Croatia, and show an increase for 8-10 days ▪ INFREP investigations for the Petrinja-EQs, Croatia, are discussed in (Boudjada etal, 2021) ▪ For the Turkey and Greece EQs the relevant TBB-GRZ path (16.70 kHz, 1445 km) show different types of modulations, e.g. an increase before the EQ und subsequent attenuation and variations within a constant envelope