Page: 1 of 11 SEMEP Search for ElectroMagnetic Earthquake Precursors Identification of ionospheric perturbations connected to seismicity from the analysis VLF/LF signals on the DEMETER satellite Deliverable 1.3 Prepared by Rozhnoi Alexander, IPE 18/04/2012
Page: 2 of 11 Document Status Sheet Issue Date Details 1.0 April 10 th, 2012 Initial draft 1.4 July 30 th, 2012 Initial submission to EC 2.1 February 19 th, 2013 Resubmission to EC Document Change Record Issue Date Author Details 1.0 10/04/12 AR Initial Draft 1.2 13/03/12 SW Suggestions for updates 1.3 18/04/12 AR Implementation of updates 1.4 24/05/12 SW Minor English updates, added summary, enhanced conclusions 2.0 31/01/13 SW Updated document status/change records 2.1 13/02/13 SW Updated text based on reviewers comments. Table of Contents 1 SUMMARY... 3 2 INTRODUCTION... 3 2.1 Purpose... 3 2.2 Context... 3 3 IONOSPHERIC TURBULENCE FOR THE SIMUSHIR EARTHQUAKE (NOVEMBER 15, 2006)... 4 3.1 Data processing... 4 3.2 Results... 5 4 IONOSPHERIC TURBULENCE DURING SEISMIC ACTIVITY IN JUNE-JULY 2008.. 10 4.1 Data sources... 10 4.2 Results... 11 5 CONCLUSIONS... 12
Page: 3 of 11 1 SUMMARY Groundbased and satellite measurements of the VLF signals from groundbased transmitters have been analysed to determine if their properties (e.g. amplitude and phase) are modified if their propagation path lies close to a seismically active region. Two regions of seismic activity in the region of Simushir Island (October 2006 to January 2007) and the Honshu region of Japan (June- August 2008) are considered. During both periods groundbased receivers and the low orbiting DEMETER satellite observe a reduction in the amplitude of groundbased transmitter signals for propagation paths that lie close to the region of seismic activity. These changes in the signal strength began 1-2 days (Honshu) and two weeks (Simushir) before the main earthquake activity. 2 INTRODUCTION 2.1 Purpose This document provides an overview of the analysis of groundbased VLF/LF signals observed by the DEMETER satellite during the periods of seismic activity. The analysis of the signal was carried out as part of Workpackage 1, deliverable 3, due for delivery in month 17 of the project. 2.2 Context Task 1.3 involves the processing and analysis of groundbased VLF/LF transmitter signals observed by the DEMETER satellite. This analysis compares the observed signals with artificial ones generated as outlined by the results of Task 1.2. This report described results of analysis for two periods of seismic activity in November 2006 and June-July 2008.
Page: 4 of 11 3 IONOSPHERIC TURBULENCE FOR THE SIMUSHIR EARTHQUAKE (NOVEMBER 15, 2006) 3.1 Data processing Data from the VLF/LF station in Petropavlovsk-Kamchatsky and the ICE receiver on board the DEMETER satellite covering the period 1 st October 2006 until the end of January 2007 have been used for the analysis. During this period, a very strong earthquake with M=8.3 took place near Simushir island in the Central Kuril region (Russia) on November 15, 2006. Following this, the series of strong aftershocks (M=5-6.5) was observed lasting for several months. The location of the receiving station in Petropavlovsk-Kamchatsky (PTK) and the VLF/LF transmitters JJI, JJY, and NWC together with the epicenter of the earthquake and its aftershocks are shown in Fig.1. The earthquake epicenter lies within the sensitivity zones of the wave paths JJY-PTK, JJI-PTK and NWC-PTK. The DEMETER data analysed were measured during nighttime passages of the satellite above the active seismic region as indicated by the satellite track in Fig. 1. The zone of analysis has a width of 25, resulting in at least one orbital passage by DEMETER per day. The dynamic spectra were computed using 2s averages which provides a spatial resolution along the orbit of around 10-15 km. The ground and satellite data were processed using a method based on the difference between the real signal and that of a model. Computation of the models for ground signals was described in report D5.3 and that for satellite observations in report D1.2. Figure 1. A map showing the position of the station in Petropavlovsk- Kamchatsky (PTK) and VLF/LF transmitters (JJI and JJY) together with epicenter of the earthquake on November 15, 2006 and its aftershocks (catalogue USGS). The ellipses show the sensitivity zones for the
Page: 5 of 11 transmitters JJI, JJY and NWC. Part of a night time orbit passing above the earthquake region is indicated. The pink rectangle highlights the section of the DEMETER orbit in which signal anomalies are observed. 3.2 Results Fig 2 shows the diurnal amplitude variations along the wave paths JJY-PTK (left) and NWC- PTK (right) at frequencies of 40 and 19.8 khz respectively for the time period October 1 st 2006 until January 31 st 2007. The red arrow marks the time of the earthquake. It can clearly be seen that enhancements in the amplitude begin around 2 weeks before the onset of the earthquake and continue during the period in which the aftershocks are observed until the middle of December. The effect is more evident in the signal 40 khz. Fig. 3a shows the spectra of VLF signals recorded by the DEMETER satellite along the nighttime orbit as it passed above the active seismic zone on November 2, 2006. On this particular day enhancements in the signal amplitudes were observed. For comparison Fig 3b shows similar measurements on a quiet day October 29, 2006 before the signal amplitude enhancements occurred. The signals from several powerful VLF transmitters - Russian Alfa (11.8, 12.6 and 14.88 khz), Australian NTS (18.6 khz) and NWC (19.8 khz) - are clearly seen. A signal at around 18 khz can be seen in Fig. 3b, but was out-of-operation during the period shown in Fig. 3a. As the satellite passes above the region of seismic activity (149-152 E, 45-50 N ) significant changes are observed in the signals from the transmitters NTS and NWC. Both signals exhibit a decrease of their amplitudes and a broadening in their spectral peaks. The same effect is observed in dynamic spectra of VLF signals (Fig. 4) in which the transmitter signals are easily seen as horizontal lines. Spectral broadening of the NWC and NTS signals can be seen above the earthquake region.
Page: 6 of 11 Figure 2. Contour map of the VLF/LF signal amplitude diurnal variations in Petropavlovsk-Kamchatsky during October 1, 2006 January 31, 2007. Left panel for the wave propagation path JJY (40 khz) - PTK, right panel - for the wave path NWC (19.8 khz) - PTK.
Page: 7 of 11 Figure 3. VLF signal variations recorded by the DEMETER satellite along part of orbits for the frequency range 11-20 khz. The upper panel shows a disturbed day, 2 nd of November 2006. The data shown in the lower panel was recorded on a quiet day, 29 th of October 2006. Left axis X is longitude and right axis X is latitude of points of orbit.
Page: 8 of 11 Figure 4. Dynamic spectra of VLF signal variations for part of orbit passing above earthquake area for frequency band 10 20 khz. Disturbed day November 2, 2006 is represented here. Fig. 5 presents a comparison of the results of both satellite and ground observations. The top panel shows the magnitude of earthquakes and aftershocks that occur within the seismic region during the period of interest. The second panel shows the interplanetary Kp index to indicate the onset of any geomagnetic activity. The third panel shows the differences in the satellite signal amplitude (dsnr) when compared to the model signal representing the expected signal variation during undisturbed periods. The lower four panels show the amplitude (da) and phase (dp) differences of the groundbased transmitter signals as recorded by the receiver PTK. There is an evident decrease in the amplitude of VLF/LF signals both in the ground and in the satellite data in association with seismicity. The amplitude anomalies are always negative. This signature can result from the effects of either magnetic storms or seismic activity and is due to the losses in the signal in ionosphere irregularity 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.
Page: 9 of 11 Figure 5. Comparison of ground and satellite observations during October 2006 January 2007. For the satellite observations averaged along part of the orbits VLF signal differences from the reception of NWC transmitter signal are shown. For ground observations averaged through nighttime VLF/LF signal differences are shown for the wave paths: JJY-PTK (amplitude and phase), JJI- PTK and NWC- PTK. The X-axis shows the number of days from the 1 st of October 2006. Two upper panels represent earthquake magnitude and Kp index of magnetic activity.
Page: 10 of 11 4 IONOSPHERIC TURBULENCE DURING SEISMIC ACTIVITY IN JUNE-JULY 2008 4.1 Data sources Data from NWC transmitter received by the DEMETER satellite and data from JJI transmitter received in PTK were used for the analysis. The period of analysis was from the 1 st June to the end of August. Two strong earthquakes occurred during this period in the Honshu region of Japan. The first earthquake with magnitude 6.9 took place on June 13 th, 2008 and the second earthquake with M=7.0 happened on July 20 th, 2008. The epicenter of the first earthquake lay within sensitivity zone of the propagation path JJI-PTK, the epicenter of the second earthquake was outside sensitivity zones for any path for ground observations. Fig.6 shows a model of the variation in the NWC signal, recorded by the DEMETER satellite together with the epicenters of the earthquakes with M>5.5 for the period of analysis. The width of the analysis zone is 25 degrees. Figure 6. Model of NWC transmitter reception zones together with EQs positions (M 5.5).
Page: 11 of 11 4.2 Results The results of the satellite and ground observations are presented in Fig.7. As in the previous case differences between the observed averaged night time signals and synthetic, model generated quiet time signals are plotted. It is clearly seen that there is a decrese in the observed signal during the period 1-2 days before the earthquakes. Figure 7. Satellite and ground observations during June August 2008. For the satellite observations averaged along part of the orbits VLF signal differences from the reception of NWC transmitter signal are shown. For ground observations averaged through nighttime VLF signal differences are shown for the wave paths: JJI-PTK. Axis X shows the days beginning from the 1 st of June 2006. Red dotted lines show the 2σ level. Two bottom panels represent earthquake magnitude and Dst index of magnetic activity.
Page: 12 of 11 For the first earthquake we see this decrease in both satellite and ground observations. For the following earthquakes the effect is observed only in the satellite data. 5 CONCLUSIONS An analysis of the propagation of VLF/LF signals observed during two periods of seismic activity has been carried out. A comparison between ground and satellite observations shows similar results. Such simultaneous analysis provides a cross validation of the observations and hence a higher reliability in the results. In both cases measurements of groundbased VLF signals whose propagation path passes close to a region of seismic activity exhibit a decrease in the amplitude of the signal that begin a couple of weeks before the onset of the main earthquake in the case of the Simushir event and a few days before the Honshu events. The DEMETER satellite also observed similar effects as it monitored VLF signals from groundbased transmitters during passes in the vicinity of the seismically active regions. As well as decreasing in amplitude, the DEMETER measurements also show evidence for the spectral broadening of the transmitter signals. From the analysis, these effects appear to due to the increase in seismic activity at some point along the signal propagation path. It should, however, be noted that not all anomalies are associated with increases in seismic activity. Similar anomalies may also result from geomagnetic activity. Discrimination between these two sources will be discussed further in the D5.4. These propagation anomalies may result from the interaction of atmospheric gravity waves (AGW) with the ionospheric plasma. In a seismically active region AGW may be generated by the release of water and gasses from the crust. This release results in the generation of AGW that subsequently propagate into the ionosphere. The energy that these waves carry with them perturbs the ionospheric plasma. These changes in the ionosphere then modify the propagation characteristics of the signals from terrestrial transmitters as they pas over the seismically active zone.