Title: Study of anomalous VLF perturbations in possible relation to seismic activity.

Where use has been made of the work of others, this is duly acknowledged in the text. The ionosphere is the electrically charged region of the upper atmosphere that reaches up to approximately 500 km.

INTRODUCTION 2

The Earth-Ionosphere Waveguide

If the medium is vertically homogeneous for at least a few km, such assumptions are valid for low frequencies. Kumar et al. [2008] collected and showed how various parameters of VLF wave propagation in a waveguide can be calculated.

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Anomalous Ionospheric Perturbations Observed Prior to Seismic ActivitySeismic Activity

• Variations of F-region Critical Frequency Prior to Earthquakes
• Unexpected VLF wave Perturbations Observed Prior to Seismic Ac- tivitytivity

Clear excursions of foF2 below the lower limit of the monthly mean between 1200–1800LT are given in Liu et al. These data compare well with Liu et al.[2000], where precursors were observed 4, 3 and 1 days before the earthquake, see Figure 1.4.

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INTRODUCTION 8

Significant irregular disturbances of the lower ionosphere can only be expected for strong M ≥6 and shallow earthquakes, but the threshold for seismic-related features to be observed compared to other sources of ionospheric disturbance is M = 5.5 [Horie et al ., 2007 ; Kasahara et al., 2010]. The preparation area, which is the circular area of ​​the ground affected by the earthquake with the central point being the epicenter, is determined using the Dobrovolsky formula.

Figure 1.7: An example of diurnal runs of amplitude variations in VLF amplitudes for the propagation path between the Chofu receiver and Omega transmitter stations, located at longitudes 139 ◦ 32’ E and 129 ◦ E respectively [Shvets, 2004].

INTRODUCTION 10

The first frame shows a scatter spot (red circled area) formed over the central seismically active zone, whereas the second frame, recorded just after the earthquake activity, shows no decrease in the SNR distribution for the same area. Working up through the earthquake magnitudes, theM = 9 2004 Sumatra earthquake showed a VLF dip for a diameter of about 5000 km around its epicenter.

INTRODUCTION 12

Possible Mechanisms for Anomaly Generation

The enhanced atmospheric vertical electric field can penetrate into the lower ionosphere and be transported along highly conductive geomagnetic field lines into the F-region. Once at F-zone altitudes, the increased electric field can cause ionospheric anomalies [Rios et al., 2004].

INTRODUCTION 14

Solar Influences on the Ionosphere

Large fluctuations in the solar wind are of major concern in contaminating ionospheric data when trying to isolate seismogenic responses from the ionosphere. Any disturbance that could possibly be caused by a source other than a seismic event is disregarded immediately [ Liu et al ., 2000 ].

INTRODUCTION 16

Data

Details such as longitude, latitude, time, depth and magnitude of earthquakes were given. It can be seen that most of the transmission paths are confined to the northern hemisphere with little or no seismic activity occurring in the Dobrovolski area. Unfortunately for the JJI transmitter path, the dates of the two relevant earthquakes occurred before the data set dates.

For the NAU transmission path, the M = 7.4 earthquake occurred just southeast of the transmitter, so it seems an ideal event to study since no other earthquakes occurred near the transmission path. Another method of data extraction was the actual spatial distribution of earthquake epicenters from the surface projection of propagation paths. Using Dobrovolski's formula to calculate the radius of earthquake preparation zones, earthquakes that could affect readings on a specific transmission path were identified.

Because of the strictly regional zone of the ionosphere affected by a nearby earthquake, only the zone of the ionosphere above the preparation zone would have a significant chance of being disturbed.

Figure 2.1: Locations of VLF receivers (diamonds) and transmitters (squares); propagation paths (red lines); Earthquakes (stars): 5.5< M <5.9 blue, 6< M <6.9 green, 7< M <7.9 black, 8< M <9 -yellow.

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Budapest Observations

Other days were not included in the discussions, as the profiles either followed their respective quiet-day curves without showing any significant amounts of fluctuation, or the data for such days were corrupted and could not be analyzed.

Figure 2.2: Locations of European VLF receivers (diamonds) and 19.8kHz NWC transmitter (square);

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Diurnal Anomaly Analysis

Case study 1: 12 September 2007 Earthquake swarm

However, for the night of 24 July as well as for 29 July, the dA values ​​fall below -σ and continue until the following mornings. The amplitude then gradually varied between -76 and -75dB until 18h00LT when it increased to -62dB, similar to the amplitude at the morning terminator time. One at around 10h00LT and a later one in the afternoon at 15h00LT, where the signal amplitude decreased, then rapidly decreased twice at 10h00LT and 15h00LT.

This occurred eleven days before the start of a very powerful series of earthquakes, ranging in magnitude from 5.5 to 8.5. For September 11, daytime amplitudes appear to be consistently higher than the quiet day curve, while the remaining days followed the quiet day curve roughly well. Sharp peaks around the terminator times are interpreted as a shift in the terminator time for that day.

But on September 1, another distinctive, well-defined dip was observed in the mid-morning hours (10h00LT) 11 days before the earthquakes.

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Case study 2: 25 July 2007 Earthquake

Unfortunately, the July data set started on July 22, so the quiet day curve consists of a fairly large amount of fluctuation due to the lack of available data. Thus, the quiet day curve was constructed from days with low VLF fluctuations before and after the period of seismicity. Figures 2.11 and 2.13 show the VLF profiles for the period leading up to the July 25 earthquake (black curve - calm day, red curve - daily VLF profile).

The profiles for 23 and 24 July did not immediately reach the quiet day curve, but fell to higher amplitudes, increased due to multimode interactions, but then decreased sharply by 10h00LT. It is a structure very similar to those seen in late August 2007, and may be the first concerns regarding the two earthquakes of August 8, 2007. This may be attributed to the effect of the earthquake on the ionosphere as it occurred.

Terminator dA peaks are seen for 28 July, again showing a temporal shift in the profile relative to the quiet conditions.

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Case study 3: Nighttime anomalous signals recorded between 12–23 October 2007October 2007

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Case study 4: Nighttime anomalous signals recorded between 20–26 December 2007

There were seven days in December 2007 with deviations from the corresponding quiet-day curve over the disturbed period, as shown in Figures 2.25 and 2.27. The anomalous signals for December 2007 were observed as sharp increases in the signal recorded during the late night hours from 19h00LT to 24h00LT. There were five consecutive days, December 21 - 25, towards the end of the month that showed stair-like departures from the quiet conditions during the afternoon and late night periods.

From 00:00 to 06:00 LT on the first three days, the signals started and continued below, until reaching the quiet day curve at or just before 06:00. The December 22 signal showed a large enhancement in daytime amplitudes between terminators over the same respective time period as for the December 21 step-down perturbations. It should be noted that the quiet day curves for December 26, 27, and 28 differ from the quiet day curve used for the earlier December dates.

This is due to the observation that there appeared to be a seasonal change in the VLF profiles starting on 26 December, so a new calm day curve had to be constructed for dates from 24 December onwards to make the comparisons realistic .

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Anomaly - seismic activity summary

Tihany observations

• Case study 5: Highly perturbed VLF period observed prior to seismic activity during February 2008activity during February 2008

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Diurnal Anomaly Analysis

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VLF profiles for 23.-25. February 2008, showing signal amplitudes returning to the quiet day curve (black) after highly disturbed period, 19–23 February 2008.

Figure 2.33: VLF profiles for 19-22 February 2007 (red). Increase of terminator minimum amplitude evindent, as well as drop in evening and latenight amplitudes

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However, only ten of these were identified as having epicenters close enough to the propagation path to facilitate VLF signal propagation, based on their size and depth. The temporal distribution of the anomaly days, shown in Figure 2.35, shows that the anomaly days started on February 14, six days before the first earthquake. Although the anomaly days extend from February 14 to 24, it must be stated that the anomalies observed before the onset of seismic activity, February 14 to 19, showed mostly very noticeable latent fluctuations over a period of a few hours.

The proximity to the propagation path, as well as the shallow epicentral depth and large magnitudes of the earthquakes are strong evidence that this may be the case. The week leading up to the earthquake showed nocturnal fluctuations in the signal amplitude that became more prominent with each successive day leading up to the earthquake day. From the day after the first earthquake leading up to the next one on February 25, the nightly anomalous structures were absent, but there were more well-defined structures that appeared just after the terminator time dip.

The Dst and ΣKp indices show low levels of geomagnetic activity through mid-February 2008.

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The purpose of this thesis is to determine if anomalous VLF signals observed at receiver stations have some relationship with seismic activity occurring close enough to the transmitter path propagating between the 19.8kHz NWC transmitter located in Australia and the recipient in Budapest, Hungary. The procedure for searching for possible deviations in the received signals has been outlined and can now be compared with the relevant seismic activity. The earthquake magnitudes and quantity per day are compared with the days of anomalies present.

The first case study covered events that were probably caused by the earthquake swarm that began on September 12, 2007. Clear VLF anomalies also appeared 11, 6, and 1 days before the onset of the swarm. However, the anomalous behavior of September 12 may be due to the direct effect of earthquakes on the ionosphere.

Leaving the fact that the VLF anomalies observed at the end of August 2007 and on September 1, 6 and 11, 2007 could be seismogenic as a valid argument.

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In this thesis, VLF waves from the Australian NWC 19.8 kHz transmitter received at Hungarian VLF narrowband receiving stations were investigated for anomalous disturbances possibly due to seismic activity in an attempt to help predict earthquakes. Three used data obtained from the Budapest station and the fourth from the Tihany station. No anomalies were recorded during the period of seismic activity and geomagnetic influences were excluded based on relevance.

The late-night dips in VLF amplitude seen for 12–23 October are possibly due to the 24 October 2007 earthquake, omitting the anomalies on 19 and 23 October due to a geomagnetic storm and the possible direct effect of the earthquake on the day respectively. Late night anomalies observed prior to the December 26, 2007 earthquake were considered possible precursors. Such fields are transmitted in the atmosphere to the ionosphere, affecting the ion distribution within the D region.

Hayakawa (2010), Ionospheric perturbations associated with Asian earthquakes as seen by subionospheric propagation from NWC at Japanese stations, Natural Hazards and Earth System Science doi:10.5194/nhess Ionospheric Signatures of Solar Flares, Msc-y NWC Thesis.