Workpackage 1 - VLF/LF signal from ground transmitters received on the DEMETER satellite
The main objectives of the WP1 were:
- The development of data processing methods for VLF/LF signals received on board a satellite in order to search for precursory seismic effects.
- The correlation of satellite anomalies above seismo-active regions with ground VLF/LF observations. This objective is connected with objective 2 of WP5.
This work package used the data from electric field receiver ICE in two frequency bands 20Hz – 20kHz and 3.3kHz – 3.3MHz for orbits in the night-time sector.
Satellite observations of seismo-induced electromagnetic effects in the ionosphere were first reported by Migulin et al. (1982). Since then, there have been many publications dealing with satellite observations of wave-plasma disturbances possibly associated with individual earthquakes or several strong earthquakes (reviews in Parrot et al., 1993, Hayakawa, 1997; Molchanov et al., 2002). However, satellite observations are not so easy to interpret. It is difficult to estimate the reliability of such case studies in the sense that there is generally no way to reject the hypotheses of pure coincidences while taking into account the possibility of the large number of internal plasma instabilities that can occur. Statistical studies seem to be obviously more reliable than case studies. However, due to differences in the data selection, the parameters to extract, the way to estimate them, and the validity tests the results are partial and difficult to compare. This may explain the present controversies on the interpretation of statistical studies using data from several low-altitude satellites.
A targetted opportunity to investigate perturbations of ionospheric plasma related to seismicity arose with the advent of the DEMETER mission. The major scientific objectives of the French micro-satellite DEMETER were to study ionospheric disturbances in relation to seismic activity and to examine pre- and post-seismic effects (Parrot, 2002). The first paper, showing examples of unusual ionospheric observations made by the DEMETER satellite over seismically active regions, was published by Parrot et al., (2006). Recent statistical investigations, using data from more than 3.5 years of measurements, confirm the existence of a very small but statistically significant decrease of wave intensity at a frequency around 1.7 kHz (Nemec et al., 2009) a few hours before an earthquake.
DEMETER observations also provide a very interesting possibility to analyze ground based transmitter signals that may be detected onboard the satellite above seismic regions. Such observations have been undertaken on many satellites for the investigation of VLF wave propagation and VLF wave interaction with ionospheric plasma (e.g. Aubrey, 1968; Inan and Helliwell, 1982). However, in the application of VLF signals to long-time seismic effects special data processing is necessary. Therefore, it can be considered as a new method of ionospheric sounding in association with seismicity. The first results of such analysis have been reported by Molchanov et al. (2006) for several strong earthquakes that occurred in 2004. The method estimated changes in the reception zone of the transmitters signal using the signal to noise ratio. An evident effect that occurred before and during the great Sumatran earthquakes with long-time duration of the order of one month has been confirmed later by Solovieva et al. (2009). After the first publication, several effects were observed in the transmitter signals received onboard DEMETER during periods of seismic activity (Slominska et al., 2008; Muto et al., 2008; Boudjada et al., 2008; YuFei at el., 2009).
However, the method of reception zone changes does not allow for separating pre-seismic and post-seismic effects. Therefore a new method of satellite data processing has been developed (difference method) which is similar to the data processing used at ground stations. The difference method revealed the appearance of significant pre-seismic effects over several days for seismic events near Japan (Rozhnoi et al., 2007b). The procedure of control path for ground data and control area for satellite data has been applied. The first results have shown good coincidence between ground and satellite observations.
Within the SEMEP project further development of the simultaneous analysis methodologies were made. Simultaneous satellite and ground observations provided two independent monitoring methods. Such simultaneous analysis provides a cross validation of the results and can be more reliable in the study of earthquake precursors. Such anomalies of geophysical parameters, which have measured simultaneously by satellite and groundbased systems, can be interpreted as possibly connected with lithosphere processes. The detailed investigation of such anomalies with simultaneous ground based measurements has provided valuable information about the mechanisms by which seismic energy penetrates into the upper ionosphere. Such results are of fundamental significance for future studies of lithosphere-ionosphere coupling theory. top
Task 1.3 - Data processing and analysis and correlation with groundbased observations
This task employrd the methodology developed in the preceding tasks to search for anomalies in the data. Results are shown from both a case study and statistical study performed as part of this task.
Simultaneous analysis of ground-based and satellite VLF/LF transmitter signal have been made for the Simushir earthquake (November 15, 2006). Data from the VLF/LF station in Petropavlovsk-Kamchatsky and the ICE receiver on board DEMETER were used for the analysis, using data collected between 1st of October 2006 and January 2007. The satellite observations focused on the signal from the Australian NWC transmitter (19.8 kHz) whilst ground based observations also include signals from two Japanese transmitters, JJI (22.2 kHz) and JJY (40 kHz). The NWC transmitter signal is the most powerful in the VLF range, enabling the analysis of signals from a large area. Signals from Japanese transmitters are local and can only be used for satellite analysis if the epicenters of earthquakes are located within the maximum signal zone.ß
Figure 1.1: Statistics of the electric field recorded by DEMETER in the VLF range.
A comparison of the results from satellite and ground observations is presented in Figure 1.1. Here we use the differences averaged over night time for the ground reception and differences averaged along the section of the satellite orbit as it traversed the seismic area. We notice an evident decrease in the amplitude of VLF/LF signals both in the ground and in the satellite data in association with seismicity. Amplitude anomalies are always negative both for magnetic storms and seismic activity due to loss of signal caused by ionosphere irregularities during propagation. Phase anomalies can be both positive and negative. It depends on the length of the path. In the present case, the anomalies in the phase of the JJY signal are positive.
Statistics of the electric field recorded by DEMETER in the VLF range
The availability of almost 6.5 years of measurements provides a good opportunity to analyze a unique data set with global coverage and perform a statistical study of the intensity of VLF electromagnetic waves observed in the upper ionosphere. This analysis is based on survey mode VLF data from DEMETER and a list of earthquakes from the USGS earthquake catalogue (http://earthquake.usgs.gov). Altogether, about 9000 earthquakes with magnitude M>=5.0 and depth D <= 40 km were included in the study.
Figure 1.2 shows the frequency-time dependence of the normalized probabilistic intensity obtained for distances lower than 440 km from the epicenter. One can see that the main observed feature is a decrease of the normalized probabilistic intensity at the frequency of about 1.7 kHz shortly (0 — 4 hours) before the time of the main shocks. The frequency of 1.7 kHz at which the decrease is observed corresponds approximately to the cut-off frequency of the first TM mode (i.e. transverse magnetic mode; electromagnetic wave lacks magnetic field component in the direction of propagation) of the Earth–ionosphere waveguide during the night time. An increase of this cut-off frequency would therefore necessarily lead to the decrease of the power spectral density of electric field fluctuations observed by DEMETER in the appropriate frequency range. Such an increase of the cut-off frequency would correspond to a decrease of the height of the ionosphere. Our results could therefore indicate that the height of the ionosphere is statistically lower above the epicenters of imminent earthquakes.
Figure 1.2: Frequency-time dependence of the normalized probabilistic intensity (see text) obtained from the night-time electric field.
Task 1.4 - Results of the combined analysis of the data recorded in the ionosphere and on groundBetween December 2008 and February 2009 a number earthquakes with magnitude M > 5 occurred in the Kurile-Kamchatka region. However, their epicentres lay outside the sensitivity zone of the JJY-PTK propagation path and so did not disturb the transmitter signal. Some earthquakes did occur within the sensitivity zone of the JJI-PTK path but were too weak to perturb the signal. The two largest events, however, did perturb the signal of the NWC transmitter that was detected by the DEMETER satellite. To compare observations by both groundstations and DEMETER, this study was extended to include all large (M>6.8) earthquakes that occurred in the Kurile-Kamchatka region during the DEMETER mission. A total of 8 events were observed, of which 6 were analysed using both groundbased and space based observations. These observations are summarised in Table 1.1.
Table 1.1: Presence of anomalies in VLF-LF signals before strong earthquakes in the Far East Region
|Date (dd.mm.yyyy)||Time (UT)||M||Depth (km)||Presence of anomalies||Comment|
|14.11.2005||21:38||7.0||11||Yes||Missed data in the satellite|
|20.07.2008||02:39||7.0||36||No||Yes||Outside sensitivity zone|
|15.01.2009||17:49||7.4||36||No||Yes||Outside sensitivity zone|
The results in the table demonstrate that there is a good coincidence between observations of anomalies in VLF/LF transmitter signals from both groundbased and space-based sources. Groundbased observations, however, seem more reliable provided that the epicenter lies within the sensitivity zone of the signal propagation path. The satellite data can be used to confirm the groundbased observations that the anomalies are real effects.top
|D1.1||DEMETER satellite data above specific regions.|
|D1.2||Software and models of monthly and seasonal reception zone of VLF/LF signals from DEMETER database.|
|D1.3||Identification of ionospheric perturbations connected to seismicity from the analysis VLF/LF signals on the DEMETER satellite.|
|D1.4||Results of the combined analysis of the data recorded in the ionosphere and on ground.|