WDS'1 Proceedings of Contributed Papers, Part II, 189 193, 21. ISBN 978-8-7378-14-8 MATFYZPRESS Statistical Study of the Ionospheric Density Variation Related to the 21 Chile Earthquake and Measured by the DEMETER Satellite D. Píša, O. Santolík Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. M. Parrot LPC2E/CNRS, Orléans, France. Abstract. The presented study shows the results of uncommon variations of the plasma density observed by the DEMETER spacecraft in the vicinity of a very powerful earthquake in Chile. This earthquake of magnitude 8.8 occurred on February 27, 21 with an epicenter located at 3.8 S, 72.72 W. Data recorded a few days before the main shock along an orbit close to the future epicenter show increasing plasma densities. This increase is not only observed above the rupture zone, but also at the position of the same geomagnetic field line as the epicenter was located. Consequently the systematic study has been performed using several years of DEMETER data (27-21) to monitor density variations in the vicinity of the epicenter (at distances less than 1 degrees) at the same local time (night time orbits) and seasonal conditions (from the beginning of January to the end of March). This study shows that a large increase of the plasma density is very uncommon at this location and at this time, and that the increase observed several days before the main shock could be considered as a possible short-term precursor of this powerful earthquake. Introduction The observation of phenomena related with the seismic activity is a very actual topic because a series of strong and damaging earthquakes recently occurred around the World. Many efforts are done to find some effects which can be used as possible short-time precursors. Some authors have recently discussed the electromagnetic wave perturbations possibly connected with the seismic activity using for theirs ground-based data [Tate and Daily, 1989; Asada, 21] and satellite data [Parrot and Mogilevsky, 199; Larkina, 1989; Parrot, 1994]. In general, it is very difficult to show effects connected with earthquakes because they are weak and often superposed to more powerful signals for example related to lightning strokes. Therefore some authors tried to eliminate these effects and have show statistical studies of electromagnetic field variations which can remove the other ambient noises [Parrot, 1999; Němec et al., 28; Němec et al., 29]. The purpose of this paper is to present a statistical analysis of the ionospheric density observed by the DEMETER satellite at the time of a large earthquake in Chile. The area along the Chile coast is one of the most active seismic zones around the World. It is due to the very fast relative motion of the Nazca and the South American tectonic plates. The M8.8 Chile earthquake which occurred on February 27, 21 is the second largest event since the launch of DEMETER. Its epicenter was located at 3.8 S, 72.72 W and the depth was 3 km. The fact that this powerful earthquake occurred during and after a long period of very low solar activity provides a unique opportunity to check its effects on the ionosphere and to perform a statistical analysis. 189
PÍŠA ET AL.: CHILE EARTQUAKE 21 The experiment DEMETER is a low-altitude satellite (71 km) launched in June 24 onto a polar and circular orbit which measures electromagnetic waves and plasma parameters all around the globe except in the auroral zones [Parrot, 26]. The altitude of the satellite was decreased to 66 km in December 2. Due to technical reasons data are only recorded at invariant latitudes less than 6. The orbit of DEMETER is nearly sun-synchronous and the up-going half-orbits correspond to night time (22.3 LT) whereas the down-going half-orbits correspond to day time (1.3 LT). The variations of the electron density are studied with a Langmuir probe named ISL (Instrument Sonde de Langmuir). Details of this ISL experiment can be found in Lebreton [26]. A different instrument named IAP (Instrument Analyseur de Plasma) is devoted to the measurement of the ion density. IAP is fully described in Berthelier [26]. The observation of the ionospheric perturbations and their interpretation as possible short-term precursors of earthquakes is the main scientific objective of DEMETER. The systematic study For our study, we use earthquake data from the USGS service (http://www.usgs.gov) and Kp index sum obtained from the website of the University in Kyoto (http://wdc.kugi.kyotou.ac.jp). We use DEMETER data recorded on orbits which have a trace at less than 1 degrees (11km) from the position of the main shock (3.8 S, 72.72 W). The restriction for the distance condition agrees to the earthquake preparation zone reported by the Dobrovolsky et al. [1979]. As a next criterion, night time orbits have been used because during day time the ionospheric F layer is denser and acts as a shield for the perturbations which could come from the Earth s surface. The data have also been selected at the same season only (from the beginning of January to the end of March). When these criteria for the position and time are satisfied all data provided by the IAP and ISL instruments on board the DEMETER satellite have been selected during 4 years. It corresponds to about 3 orbits, and to 6 minutes of data for each of them. For each chosen orbit we have made an array of ion density where each bin corresponds to the geographic latitude of the satellite with regard to the latitude of the epicenter (noted as on two last panels of Figure 1 and each panel of Figure 2). The white gaps (for panels with the densities) delimit position in which the given criteria were not satisfied and we do not have any data or values were lower than chosen threshold. These arrays are displayed as function of the days before and after the earthquake day (February 27) noted as zero on the time axis (this is same for each panel on both Figures). The difference between absolute value of ion sum densities from IAP and electron density from ISL is caused by unresolved calibration problems. This fact does not prevent to say that qualitative behaviors for both densities are same (see Fig 1.). Therefore only ion density variation has been presented in Fig. 2. An analysis for year 21 is showed in Fig.1. The first panel (from the top) represents sum of Kp index for the given day indicates increasing geomagnetic activity. The second panel (from the top) shows geographic distribution of the earthquakes in the vicinity of the epicenter. The position for all earthquakes with magnitude greater than is plotted and related in time and position to the main shock. The geomagnetic activity and earthquakes could be one pair of possible sources of density fluctuations in the lower ionosphere. The last two panels display the total ion density and electron density. The range for the absolute values is different due to calibration problems, as it is mentioned above. The relative behavior of densities is qualitatively similar for both data sets. Therefore, we only use variations of the total ion density in the following analysis. A systematic study of the ion density in the vicinity of the earthquake epicenter has been done to check if similar perturbations could be found when no seismic activity occurs. The results of the systematic observation of the ion density are displayed in Fig. 2. In the Ion composition dominates oxygen ion. Each panel of Figure 2 corresponds to a different year and the top panel is related to the earthquake year. Among the four years of data, it can be observed that the ion density presents maxima only during days preceding the Chille earthquake. 19
PÍŠA ET AL.: CHILE EARTQUAKE 21 Kp sum 6 Kp sum 4 2 1 - -1 Earthquakes 8 7 magnitude 6 1 - -1 IAP 1 4 4 1 4 3 1 4 2 1 4 1 1 4 1 - -1-4 -2 2 time [days] ISL 7 1 4 6 1 4 1 4 4 1 4 3 1 4 N e Figure 1. Panels represent observation during year 21 in the vicinity of the epicenter of the Chille earthquake. The x axis is the same for all panels and represents time corresponds to the main shock of the earthquake (February 27). On the first panel from top is plotted sum of the Kp indexes for given day. The second panel represents geographical distribution of the earthquakes in the vicinity (distance less than 1 degrees) of the epicenter. The y axis represents different of the geographical latitude corresponds to the latitude of the epicenter (3.8 S). Colors scale represents magnitude for the plotted earthquakes. Last two panels at the bottom show ion sum (upper) and electron (lower) density. The densities depend on time and position to the main shock. Discussions and conclusions The presented statistical analysis agrees with previous studies that reported local increasing in particle density in time and position close to coming earthquakes (see for example, (Liu et al., 2, 24a, 24b, 26, 29). The seismic effect is of course all the more important as the magnitude is large [Hobara and Parrot, 2]. DEMETER often registers such anomalies before earthquakes. It already demonstrated a statistical link between the appearance of a similar kind of perturbations and the earthquakes with magnitude larger than (Němec et al., 28). There are many theories involving a lithosphere-atmosphere-ionosphere coupling to explain these ionospheric perturbations. They can be found in Pulinets and Boyarchuk (24). The statistical study over 4 years has shown that such high increase of particle density is only observed a few days before the main shock of this powerful Chile earthquake. We also 191
PÍŠA ET AL.: CHILE EARTQUAKE 21 1 Feb 27 21 1 4 4 1 4 - -1 3 1 4 2 1 4 1 1 4 1 29 1 4 4 1 4 - -1 3 1 4 2 1 4 1 1 4 1 28 1 4 4 1 4 - -1 3 1 4 2 1 4 1 1 4 1 27 1 4 4 1 4-3 1 4 2 1 4-1 -4-2 2 time [days] 1 1 4 Figure 2. Representation of the data recorded during 4 years in the vicinity of the epicenter of the M8.8 Chille earthquake. Each panel is related to one year and represents the plasma density for each day from January 1st to March 31th. The day on the x axis corresponds to February 27 each year (the day of the main shock in 21). The latitude on the y axis corresponds to the latitude of the epicenter (3.8 S). Only data which are at a distance less than 1 degrees ( 11km) from the epicenter are shown. The value of the density is color coded according to the color scales on the right. These color scales are identical for all years. The black vertical line delimits February 27 (day of the main shock). show other possible sources of the density variations. The values of the Kp index sum are during critical period (few days before earthquake) quite low. Also the geographic distribution shows no possible other source, because there is only one earthquake before and this earthquake is quite weak. We have not discovered any other possible source of variations. But, for several reasons, it remains difficult to perform prediction from this short-term precursor signal. In general, it is impossible to estimate the time of the earthquakes because these perturbations occur between a few hours and a few days before (even more than a week for earthquakes with very large magnitude). If we consider the current state of our understanding of the observed perturbations as possible precursors of seismic activity, the uncertainties on the predicted position and on the magnitude of the future earthquakes are also very large. There are many scientists over the world working with the DEMETER data. Their objective is to characterize these anomalies and 192
PÍŠA ET AL.: CHILE EARTQUAKE 21 the kind of seismic events they are associated with, to learn how to automatically detect them in the data flow, to compare their occurrence with the seismic activity in order to understand their origin, and to define criteria which can be used in the future to make predictions. This is a long term goal of our research, but the signal observed before the M8.8 Chile earthquake will contribute to this task. References Asada, T., H. Baba, M. Kawazoe, and M. Sugiura (21), An attempt to delineate very low frequency electromagnetic signals associated with earthquakes, Earth, Planets Space, 3, 62. Berthelier, J.J., Godefroy, M., Leblanc, F., Seran, E., Peschard, D., Gilbert, P. Artru, J., IAP, the thermal plasma analyzer on DEMETER, Planet. Space Sci., 4(), 4871, 26. Dobrovolsky, I. R., S. I. Zubkov, and V. I. Myachkin (1979), Estimation of the size of earthquake preparation zones, Pure Appl. Geophys., 117, 12 144. Hobara, Y, and M. Parrot, Ionospheric perturbations linked to a very powerful seismic event, Journal of Atmospherics and Solar-Terrestrial Physics, 67, 677-68, 2. Larkina, V. I., V. V. Migulin, O. A. Molchanov, I. P. Kharkov, A. S. Inchin, and V. B. Schvetcova (1989), Some statistical results on very low frequency radiowave emissions in the upper ionosphere over earthquake zones, Phys. Earth Planet. Int., 7, 1 19. Lebreton, J. P., et al. (26), The ISL Langmuir probe experiment and its data processing onboard DEMETER: Scientific objectives, description and first results, Planet. Space Sci., 4, 472 486. Liu, J. Y., Y. I. Chen, S. A. Pulinets, Y. B. Tsai, and Y. J. Chuo (2), Seismo-ionospheric signatures prior to M 6. Taiwan earthquakes, Geophys. Res. Lett., 27, 3113 3116, doi:1.129/2gl1139. Liu, J. Y., Y. I. Chen, H. K. Jhuang, and Y. H. Lin (24a), Ionospheric fof2 and TEC anomalous days associated with M. earthquakes in Taiwan during 19971999, Terr. Atmos. Oceanic Sci., 1, 371 383. Liu, J. Y., Chuo, Y. J., Shan, S. J., Tsai, Y. B., Pulinets, S. A., and Yu, S. B.: Pre-earthquake-ionospheric anomalies registered by continuous GPS TEC, Annales Geophysicae, 22, 18-193, 24b. Liu, J. Y., Y. I. Chen, Y. J. Chuo, and C. S. Chen (26), A statistical investigation of pre-earthquake ionospheric anomaly, J. Geophys. Res., 111, A34, doi:1.129/2ja11333. Liu, J. Y., Chen, Y. I., Chen, C. H., Liu, C. Y., Chen, C. Y., Nishihashi, M., Li, J. Z., Xia, Y. Q., Oyama, K. I., Hattori, K., and Lin, C. H: Seismoionospheric GPS total electron content anomalies observed before the 12 May 28 Mw7.9 Wenchuan earthquake, J. Geophys. Res., 114, A432, doi: 1.129/28JA13698, 29. Němec, F., O. Santolík, M. Parrot, and J. J. Berthelier (28), Spacecraft observations of electromagnetic perturbations connected with seismic activity, Geophys. Res. Lett., 3, L19 Němec, F., O. Santolík, and M. Parrot (29), Decrease of intensity of ELF/VLF waves observed in the upper ionosphere close to earthquakes: A statistical study, J. Geophys. Res., 114, A433 Parrot, M., and M. M. Mogilevsky (1989), VLF emissions associated with earthquakes and observed in the ionosphere and the magnetosphere, Phys. Earth Planet. Inter., 7, 86 99. Parrot, M. (1994), Statistical study of ELF/VLF emissions recorded by a low-altitude satellite during seismic events, J. Geophys. Res., 99, 23,339 23,347. Parrot, M. (1999), Statistical studies with satellite observations of seismogenic effects, in Atmospheric and Ionospheric Electromagnetic Phenomena Associated with Earthquakes, edited by M. Hayakawa, pp. 68 69, Terra Sci., Tokyo. Parrot, M., (Ed.), First results of the DEMETER micro-satellite, Special Issue of Planet. Space Sci., 4(), 26. Pulinets, S.A., Boyarchuk, K.A., 24. Ionospheric Precursors of Earthquakes. Springer, Hedelberg, New York. Tate, J., and W. Daily (1989), Evidence of electro-seismic phenomena, Phys. Earth Planet. Int., 7,1 1. 193