Several papers have been published to show examples of perturbations of ionospheric parameters in relation with earthquakes (Parrot et al., 2006; Bhattacharya et al., 2007a, 2007b; Pulinets, 2007; Sarkar et al., 2007). One must also notice another series of papers which deals with the change of signal intensity emitted by the ground-based VLF transmitters and received onboard Demeter (Molchanov et al., 2006; Rozhnoi et al., 2007; Muto et al., 2008).
But the more important is the statistical analysis of the intensity of waves measured by the Demeter electric antennaes as function of the seismic activity which has been implemented by the superposed epoch method (Nemec et al., 2008). In this method, all earthquake occurrences are at the time zero. The results are presented as function of the frequency under the form of relative intensity normalized by the standard deviation. The Figure 1 corresponds to the electric field measured during night time for earthquakes with magnitude larger than 4.8 (left) and 5 (right), with a depth less than 40 km, and a distance with the epicentre less than 3° (the aftershocks have not been considered). The figures show a decrease of the wave intensity measured by Demeter (more intense when the magnitude is larger) in a frequency range between 1 and 2 kHz which starts a few hours before the earthquakes. The Figure 2 shows the same results but as function of the distance with the epicenter.
Important variations of electron and ion densities have been registered a few days before earthquakes of large magnitude.
The Figure 3 shows a Demeter observation 7 days before the Samoa earthquake (Magnitude 8) which occurred on September 29, 2009 at 17.48.11 UT (epicentre at 15.51°S, 187.97°E). From the top to the bottom, the panels display the electron density, the electron temperature, the O+ ion density and the earthquakes which occurred along the corresponding satellite orbit. The red triangles indicate the position of the epicentre and of the numerous aftershocks. One can observe an increase of the ion and electron densities and a decrease of the temperature which are well localised above the fore coming earthquake (see the orbit in the figure 4).
The Figure 5 displays a Demeter observation 3 days before the HAITI earthquake (magnitude 7) which occurred on January 12, 2010 at 21.53.09 TU (epicentre at 18.451°N, 72.445°W). The top panel shows the variation of the night time electron density measured at the satellite altitude. The red triangle in the bottom panel indicates the time when the satellite is just above the future epicenter. A decrease of the density is locally observed around this location (see the orbit in the Figure 6).
Figure 5: Variation of the electron density observed by Demeter.
Figure 6: Orbit of Demeter which corresponds to the Figure 5. The star indicates the position of the earthquake epicenter. The variation is observed at the place where the line is thick.
The Chile earthquake which occurred on February 27, 2010 at 06:34:14 UT (35.85°S, 72.72°W) is the second more powerful earthquake (M = 8.8) registered after the Demeter launch. Its depth is about 35 km. Numerous ionospheric perturbations have been observed. The Figures 7, 8, and 9 show examples of these variations recorded along the orbits shown in figure 10.