INVESTIGATION OF IONOSPHERIC PRECURSORS OF EARTHQUAKES IN ROMANIA USING THE ROMANIAN GNSS/GPS NETWORK

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1 INVESTIGATION OF IONOSPHERIC PRECURSORS OF EARTHQUAKES IN ROMANIA USING THE ROMANIAN GNSS/GPS NETWORK EDUARD ILIE NASTASE 1, CHRISTINA OIKONOMOU 2, DRAGOS TOMA-DANILA 1, HARIS HARALAMBOUS 2, ALEXANDRA MUNTEAN 1, IREN ADELINA MOLDOVAN 1 1 National Institute for Earth Physics, PO BOX MG2, , Magurele, Romania, s: eduard_nastase@infp.ro; toma@infp.ro; muntean@infp.ro; irenutza_67@yahoo.com 2 Frederick Research Center, Filokyprou St.7, Palouriotisa, Nicosia, 1036, Cyprus, s: res.ec@frederick.ac.cy; eng.hh@fit.ac.cy Received August 31, 2015 We examine the lithosphere-atmosphere-ionosphere interaction with respect to earthquake events using Total Electron Content (TEC) data deriving from the Romanian permanent GPS network by applying three different techniques: a) estimation of TEC deviations from the mean state, b) Cross-Correlation Analysis and c) Spectral Analysis. The analysis concerns four seismic events that took place in Romania with magnitude ranging from 5.2 to 6.0. The aim is to identify and study possible ionospheric precursory phenomena linked to these seismic events. Key words: Ionospheric earthquake precursors, Total Electron Content (TEC), Romanian GNSS/GPS network. 1. INTRODUCTION The last two decades a hard effort is put to analyze and possibly predict seismic events through monitoring of ionosphere. That was possible mostly with the aid of technological developments such as ground based techniques to study the subionospheric propagation of VLF/LF radio waves [1] and the bottomside and the middle ionosphere with ionosondes, satellite based instruments to carry out investigations of the topside ionosphere. To complement these techniques, dense networks of GNSS receivers enabled monitoring of the full extent of the ionospheric plasma via Total Electron Content (TEC) and ionospheric tomography. Lately, the link between earthquakes and preseismic ionospheric perturbations has acquired significant attention. Possible earthquake precursors have been identified including ground deformations, radon/helium emissions, crustal stress, atmospheric thermal anomalies [2]. Rom. Journ. Phys., Vol. 61, No. 7 8, P , 2016

2 2 Investigation of ionospheric precursors of earthquakes using GNSS/GPS technology 1427 Gas emissions occurring prior to an earthquake cause the ionisation of the neutral atmosphere above the epicenter and thus the generation of anomalous electric field that penetrates the ionosphere leading to large-scale positive and negative anomalies of electron concentration in the vicinity of the epicenter. This electric field is not restricted only to the epicenter, but covers an area that is a function of the earthquake magnitude named as earthquake preparation zone [3]. In addition, Atmospheric Gravity Waves (AGW) are generated by pre-seismic activity of emanating gases. The most widely known long wavelength perturbations are travelling ionospheric disturbances (TIDs) which are associated to AGW and also infrasonic waves propagating upwards, amplified by the exponential density decrease of the atmosphere [4]. It has been statistically proved that ionospheric precursors are observed between 12 days to a few hours prior to the earthquake and that earthquakes should exceed the magnitude of 5 in order to provoke ionospheric disturbances. The duration of a seismically induced ionospheric deviation is short about 4 6 hours being compared with perturbations caused by geomagnetic storms and anomalies [2]. The objective of this study is to investigate possible pre-earthquake ionospheric anomalies that occurred prior to four earthquakes in Romania with magnitude Mw ranging from 5.2 to 6.0, following a multi-technique approach, by using TEC data obtained from the ground-based Romanian GNSS/GPS receiver network. 2. DATA AND METHODOLOGY The four seismic events selected in this study as well as their main characteristics are presented at Table 1. In order to detect possible ionospheric disturbances prior to earthquakes, TEC data were utilized as obtained from dualfrequency phase and code measurements made by the GPS receivers that are located in Romania and belong to the Romanian permanent GPS network which is operating since Figure 1 shows the map of the GPS network along with the earthquake epicenters and the preparation zone of each seismic event. In order to calculate the vertical TEC (vtec) we have processed RINEX files from the selected GPS stations with a calibration algorithm [5]. This processing technique assumes ionospheric thin shell model (located at 350km of altitude) to obtain vtec from slant total electron content (stec) at the Ionospheric Pierce Point (IPP). The elevation angle used here was 67. First we tried to identify possible significant disturbances of TEC data prior to earthquakes using the statistical envelope method: Under the assumption of normal distribution with mean m and standard deviation σ of TECs the expected values of upper bound and lower bound of envelope are m ± 1.34σ. If the observed TEC falls out of either the associated lower or upper bounds of such an envelope then an abnormal signal is detected at the confidence level of about 82% [6].

3 1428 Eduard Ilie Nastase et al. 3 Seismic events No. Table 1 List of the four seismic events in Romania studied here and their characteristics Year Month Day Latitude North ( ) Longitude East ( ) Magnitude Mw Depth (Km) Preparation Area Radius (Km) Fig. 1 Map of Romania showing: a) the preparation areas and epicenters (starts) of the 4 examined earthquakes, b) the Romanian GPS stations network. Pink colored boxes denoted GPS stations used for the 5.2 Mw event, red colored boxes show stations used for the 5.5 Mw event, dark red colored boxes correspond to the 5.7 event, and black colored boxes to the 6 Mw event. With grey colored boxes are noted the stations within the actual GPS network while with white color boxes the stations which are no longer in use are indicated. The GPS stations OROS and SOFI shown in the map belong to the EUREF network.

4 4 Investigation of ionospheric precursors of earthquakes using GNSS/GPS technology 1429 Then, we applied the cross-correlation method on TEC values using one measurement point located inside the earthquake preparation area and one or more points located outside it. The preparation area is the area where the ionosphere above it is affected by earthquake precursors and is defined as a circle with radius ρ = Μ km where M is the magnitude (Mw) of the earthquake [3]. Since ionospheric variability induced by seismic activity is generally lower than those variations related to geomagnetic storms, it is shadowed by storm-time variations. In order to surpass this problem the two measurement points should be in the same or very close geomagnetic latitude so as ionospheric variations registered at both points to show similar behavior during quiet and disturbed geomagnetic conditions. Therefore, their correlation coefficient will be high. On the opposite, during seismic activity their correlation coefficient is expected to drop since the station closer to epicenter (inside the preparation area) will be more sensitive to seismic variations than the station outside the preparation area. In addition, the longitude of the two sites should not be so different, as ionosphere is local time dependent. Finally, we performed the Spectral Analysis using differential TEC data, defined as the difference of stec measurement between two successive satellite epochs. The period and amplitude of differential TEC fluctuations were estimated. 3. RESULTS AND DISCUSSION In Fig. 2 (a, b, c and d) the diurnal TEC variations at the GPS stations during the interval 30 days prior, during and up to 7 days after the earthquake are depicted for all examined seismic events. In the same figures the geomagnetic conditions during each studying period are described through the daily variations of the geomagnetic index Dst which comprises a measure of the variation in the geomagnetic field due to the equatorial ring current. A Dst index of 50 or deeper indicates a geomagnetic storm-level disturbance. As it can be detected from the TEC time series within 25 days before the Mw = 5.7 crustal earthquake on 22 November 2014, positive anomalies occurred at around 10 hours before the earthquake only at the GPS stations located inside the preparation area and closer to the epicenter (VRAP, TINA, MARE, BICA, ROSU) (Figs. 2d and 3). These anomalies have short duration of about 3 hours (7:00 10:00 UT) which is in consistence with [2] who state that pre-seismic variations are comparatively short in time (around 4 6 hrs) relative to magnetic storm effects. On the opposite, positive or negative anomalies of TEC variations were not found at the GPS stations located outside the earthquake preparation zone (ROSP, BUZE) during the same time. Therefore, this positive anomaly observed on 22 November 2014 can be possibly associated to the impending earthquake, taking also under consideration that quiet geomagnetic conditions were prevailing during that day.

5 1430 Eduard Ilie Nastase et al. 5 Figure 2c shows the diurnal TEC variations at the Romanian GPS stations during the interval 23 days before and at the earthquake day on 6 October As it can be seen, TEC values are reduced compared to the mean values three days before the earthquake in all stations. This is attributed to the occurrence of a major geomagnetic storm which commenced at 2 October 01:55 UT, had its initial and main phase during the same day and its recovery phase during the 3-day period prior to the earthquake (2 5 October). Since ionospheric variability induced by seismic activity is generally lower than those variations related to geomagnetic storms, it is most probable that any TEC anomaly related to the earthquake during the storm period 2 5 October is shadowed by storm-time variations. In Figure 2b the daily TEC variations within one month before the Mw = 5.5 deep earthquake on 14 May 2005, as well as the daily Dst geomagnetic index variations are presented. As it can be observed from the Dst index values a major geomagnetic storm occurred 6 days before the earthquake. The initial phase of the storm started at 19:20 UT on 7 May and was followed by its main phase that took place during 8 May and its recovery phase which lasted from 9 to 13 May, causing long-lasting high TEC anomalies observed at all GPS stations. Thus, similar to the previous seismic event no earthquake induced TEC anomaly could be detected. Inspection of the TEC variations at the GPS stations during the interval 30 days prior to the deep Mw = 6 earthquake on 27 October 2004 reveals a high positive TEC anomaly occurring on daily basis at the last 6 days before the earthquake which lasts more than 6 hours (Fig. 2a). Though no geomagnetic storm occurred during that period, a sequence of solar C- and M-flares was observed (ftp://ftp.swpc.noaa.gov/pub/warehouse/2004/, NOAA, Space Weather Prediction Center) which could be partially responsible for this daily increase of TEC values [7]. Therefore, it was not possible to detect any earthquake induced TEC deviation. Another possible cause for this could be that fact that this earthquake, though it is of high magnitude, it has a large depth. According to [8] and [9] who have statistically studied pre-earthquake ionospheric anomalies using different methods the anomalies occurring before deep focus earthquakes have smaller intensity and occurrence rates. It has become obvious that TEC deviations from the mean state that could be linked to earthquake were identified only for the crustal Mw = 5.7 earthquake on 22 November 2014 around 10 hours before the earthquake during also geomagnetically quiet conditions, unlike the other seismic events which were either deep (27 October 2004) or had low magnitude and their preparatory period occurred during the development of major geomagnetic storms (6 October 2013, 14 May 2005). For this purpose and in order to obtain additional confidence and confirmation about the association of the observed TEC anomalies on 22 November 2014 with the Mw = 5.7 seismic event we applied two additional methods, the Cross-Correlation and the Spectral Analysis.

6 6 Investigation of ionospheric precursors of earthquakes using GNSS/GPS technology 1431 a. b. Fig. 2

7 1432 Eduard Ilie Nastase et al. 7 c. d. Fig. 2 (continued) Diurnal vtec variations (red) and corresponding upper and lower bounds (black) fixed at m ± 1.34σ are depicted for selected GPS stations for 30 days before, during and up to 7 days after the four examined seismic events. Pink shaded areas show the geomagnetically disturbed periods and yellow vertical line shows the moment of earthquake.

8 8 Investigation of ionospheric precursors of earthquakes using GNSS/GPS technology 1433 Fig. 3 Same as Fig. 2 but for 3 days before and during the earthquake on

9 1434 Eduard Ilie Nastase et al. 9 First we have correlated TEC values recorded at the closest to the earthquake station (VRAP) during the period November 2014 with all the remaining stations. The correlation coefficient was calculated for each of these three days for four separate intervals: 00 24, 00 08, and hours (Table 2). As it can be seen, though the correlation coefficient values at entire day level are high and do not show disruption patterns, the analysis for the 8 hours intervals (especially for the h UT, which is during night time considering the local time) reveals low correlations, mostly in the day before the earthquake ( ). Since the anomalies that we seek to find for a medium magnitude earthquake can't be considerable and can be influenced by the diurnal variability, we aim to believe that the night time low correlations that have been found could be somehow connected to the seismic activity within the epicentral area. Indeed, correlations in all stations except TINA station show in general a decrease of the correlation with the distance from the epicenter. The fact that TINA station which is closest to VRAP station shows the lowest correlation coefficient values instead of the highest as it is expected is most probably due to technical problems at the station that time that led to incorrect TEC values. Table 2 Correlation coefficient values between VRAP station and remaining stations ordered by distance from VRAP, during four time intervals: 00 24, 00 08, and hours for the 5.7 Mw earthquake on 22 November 2014, 19:14:17 UT GPS STATIONS 20 24h correl. with VRAP h correl. with VRAP h correl. with VRAP h correl. with VRAP TINA ROSU TUDO BUCE BICA MARE COTI HIST MALI ROSP BUZE Then, the Spectral Analysis was performed using differential stec values estimated for the propagation path between the close to the epicenter GPS receiver

10 10 Investigation of ionospheric precursors of earthquakes using GNSS/GPS technology 1435 stations BUCU and BACA and several satellites passing close and above the earthquake preparation area during the day where large TEC disturbances have been detected ( ) (Figs. 2d and 3). In Fig. 4 high periodic sinusoidal stec fluctuations can be identified at the same time that TEC anomalies on the 5.7 event have been noted (7 10 UT) for all satellites, having period of oscillations around minutes. These results confirm that the observed TEC disturbances in the case of the 5.7 Mw seismic event can be produced by the penetration of Atmosphere Gravity Waves (AGW) into the ionosphere as described by the Lithosphere-Atmosphere-Ionosphere coupling theory. Other authors have also observed preseismic VLF/LF radio signals with propagation path passing close enough to the epicenters to have similar periods of about min [10, 11]. Fig. 4 Fluctuations of differential TEC obtained from measurements of 7satellites passing over the area of interest at times of the highest TEC anomalies observed at at GPS stations close to epicenter. The power spectrum of the normalized amplitude is also shown. Map shows the number and position of satellites IPP (blue asterisks), the position of the GPS receiver station BUCU (pink triangle) and the earthquake epicenter (green asterisk). 4. CONCLUSIONS The analysis of four seismic events in Romania with magnitude Mw 5.2 to 6.0 by utilizing TEC data obtained from the Romanian permanent GPS station network has shown that ionospheric precursory phenomena can be observed one day up to few hours prior to the crustal Mw = 5.7 earthquake and to closest to the epicenter

11 1436 Eduard Ilie Nastase et al. 11 stations, whereas preseismic TEC anomalies were not identified in case of deep or of low magnitude earthquakes and during the occurrence of major geomagnetic storms. In addition, this study demonstrates that in order to increase the credibility on the presence of ionospheric precursory phenomena associated with an earthquake and to provide more safe conclusions, it is of high importance to simultaneously apply different techniques such as the Cross-Correlation Analysis and the Spectral Analysis. The Romanian GPS stations network has proven a useful tool for the investigation of ionospheric precursory phenomena related to earthquakes. Acknowledgements. This paper was partially funded by grants of the Romanian National Authority for Scientific Research: (i) Capacity Program, Module III Projects supporting Romania's participation in international research projects, Bilateral cooperation programs Romania Cyprus, , project number 759/2014, and (ii) Nucleu Program PN 09 30/2009 and the project Investigation of earthquake signatures on the ionosphere over Europe-ΔΙΑΚΡΑΤΙΚΕΣ/ΚΥ- ΡΟΥ/0713/37which is co-financed by the Republic of Cyprus and the European Regional Development Fund (through the ΔΕΣΜΗ of the Cyprus Research Promotion Foundation). REFERENCES 1. I.A. Moldovan, A.P. Constantin, P.F. Biagi, D. Toma Danila, A.S. Moldovan, P. Dolea, V.E. Toader, T. Maggipinto. The Development of the Romanian VLF/LF Monitoring System as Part of the International Network for Frontier Research on Earthquake Precursors (INFREP), Rom. Journ. Phys., 60, 7 8, (2015) 2. S.A. Pulinets and K.A. Boyarchuk. Ionospheric precursors of earthquakes, Springer, Berlin (2004). 3. I.R. Dobrovolsky, S.I Zubkov and V.I. Myachkin. Estimation of the size of earthquake preparation zones. Pageoph, 117, (1979). 4. J. Artru, T. Farges and P. Lognonn e. Acoustic waves generated from seismic surface waves: propagation properties determined from doppler sounding observation and normal-modes modeling. Geophys. J. Int., 158, (2004). 5. L. Ciraolo. Evaluation of GPS L2-L1 biases and related daily TEC profiles, in: Proc. of the GPS/Ionosphere Workshop, 90 97, Neustrelitz (1993). 6. P.I. Nenovski, M. Pezzopane, L. Ciraolo, M. Vellante, U. Villante and M. De Lauretis. Local changes in the total electron content immediately before the 2009 Abruzzo earthquake. Adv. In Space Res. 55, (2015). 7. V. Yu, Yasyukevich, S.V. Voeykov, I.V. Zhivetiev and E.A. Kosogorov. Ionospheric Response to Solar Flares of C and M Classes in January February Cosmic Research, 51, (2013). 8. Y. He, D. Yang, J. Qian and M. Parrot Response of the ionospheric electron density to different types of seismic events. Nat. Hazards Earth Syst. Sci., 11, (2011). 9. H. Le, J.Y. Liu and L. Liu A statistical analysis of ionospheric anomalies before 736 M6.0+ earthquakes during , J. Geophys. Res., 116, A02303 (2011). 10. M. Hayakawa. On the fluctuation spectra of seismo-electromagnetic phenomena. Nat. Hazards Earth Syst. Sci., 11, (2011). 11. A.A. Rozhnoi, M.S. Solovieva, P.F. Biagi, K. Schwingenschuh and M. Maekawa. Low frequency signal spectrum analysis for strong earthquakes. Annals of geophysics, 55 (2012).