Patterns in the daily periodicity of low magnitude seismicity in Kamchatka

ISSN 0742�0463, Journal of Volcanology and Seismology, 2011, Vol. 5, No. 2, pp. 129–143. © Pleiades Publishing, Ltd., 2011.Original Russian Text © V.A. Gavrilov, V.I. Zhuravlev, Yu.V. Morozova, 2011, published in Vulkanologiya i Seismologiya, 2011, No. 2, pp. 60–75.

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INTRODUCTION

The history of the study of daily periodicity in lowmagnitude seismicity that reaches its maximum in thenighttime dates back more than 100 years [Davison,1893, 1896; Knott, 1908; Schuster, 1897]. Among laterstudies one notes Shimshoni [1971] in the first place;this author analyzed more than 15000 earthquakes withM ≥ 2.0 to demonstrate that the rate of nighttime earth�quakes exceeds that of daytime events by about 17%.

To quote an example of recent publications devotedto this topic, we mention Ivanov�Kholodnyi et al.,[2004] who did a statistical analysis of about 360000earthquakes worldwide with magnitudes 2.9–4.9 for theperiod 1973–2003. It has been shown that there is anundoubted effect of daily behavior in the seismicity ratethat reaches its maximum at about midnight and itsminimum at about noon local time (Fig. 1) and that theeffect is occurs worldwide. The authors state that thephysical mechanism that is responsible for the effect isnot clear. Searching among possible causes, they rejecttidal excitation, since the time series of small earth�quakes are nearly devoid of the 12�h component and arenot affected by geographic latitude. In the opinion ofthese authors, one improbable cause, which cannot becompletely ruled out, is the possible influence ofhuman�induced noise; this would reduce the number ofsmall earthquakes that are identifiable during the day�time. The conclusions reached in the publicationsreferred to above largely reflect the opinions of manyresearchers on the daily behavior of small earthquakes.

The effect is recognized as being real in the over�whelming number of publications. However, opinion isdivided as to the physical causes. Some workers thinkthat the nighttime maximum in the daily periodicity ofseismicity is due to exogenous factors. To take an exam�ple, Deshcherevskaya et al. [2004] relate the effect ofdaily seismicity periodicity as recorded at the GarmGeophysical Test Site to increased wind noise observedduring daytime. Several papers mention industrialactivities as a possible cause [Zotov, 2007]. Someauthors [Aoki et al., 1997; Tyupkin, 2002; Morgunovet al., 2005; Yurkov and Gittis, 2005; ] think the effectin question is due to the lunar–solar tides. This hypoth�esis is opposed in [Rydelek et al., 1992; Tsuruoka et al.,1995; Vidale et al., 1998].

The present authors try to find answers to the mostimportant questions connected with the daily periodic�ity in application to the Kamchatka region: Is theresuch an effect for the Kamchatka earthquakes and whatcould the physical causes be if the effect does exist?

AREA OF STUDY

The Kamchatka region is one of the most seismicallyactive regions in the world; the annual seismic energyrelease, as averaged over 42 years, is 6.2 ⋅ 1014 J [Saltykovand Kravchenko, 2004]. More than 70% of the energyis released in the Benioff zone of Kamchatka where thePacific plate is subducted under the Sea�of�Okhotskplate and where about 70% of all seismic events occurwith energy classes K ≥ 8.6. (Here and below we use

Patterns in the Daily Periodicity of Low Magnitude Seismicity in Kamchatka

V. A. Gavrilova, V. I. Zhuravlevb, and Yu. V. Morozovaa

a Institute of Volcanology and Seismology, Far East Division, Russian Academy of Sciences, Petropavlovsk�Kamchatskii, 683006 Russia

e�mail: [email protected] Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, 123995 Russia

e�mail: vzhtvertsa@mtu�net.ruReceived March 26, 2010

Abstract—A daily periodicity in small (K ≤ 8.0) Kamchatka earthquakes has been detected, with the maxi�mum occurring during the nighttime. The effect was not observed throughout the area of study, but only inseveral zones. We show that the results are not affected by human and weather factors. A hypothesis is put for�ward to explain the physical causes of the effect, viz., that the daily periodicity of small earthquakes could bedue to natural VLF electromagnetic radiation acting on the geologic medium. It is pointed out that the effectis related to the previously identified effect of natural electromagnetic radiation modulating the intensity ofgeoacoustic emission from rocks.

DOI: 10.1134/S0742046311020035

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class scale according to S.A. Fedotov [1972] forthe energy classification of earthquakes.) The hypo�centers of the bulk of events (about 80%) are situated atdepths of 0–70 km [Levina et al., 2008].

Our analysis of the daily periodicity of small earth�quakes is based on the final data as supplied in theregional catalog of Kamchatka and Commander Is.earthquakes by the Kamchatka Branch (KB) of theGeophysical Service (GS) of the Russian Academy ofSciences (RAS) for the period January 1, 1998 throughDecember 31, 2006 and the area within 51.0°–58.5°N,156.0°–165.0°E. The aftershocks have not been elimi�nated from the catalog. A total of 37792 earthquakeswere recorded in the area of study during the periodindicated, with 14756 earthquakes having energy classK ≤ 7.0. A map of the epicenters is given in Fig. 2a. Thestations of the Kamchatka seismometric network withsolid lines outlining the areas of complete reporting canbe found in Fig. 2b.

Our results from the processing of small earthquakeswere compared with continuous borehole geoacousticmeasurements that were conducted from August 2000at the G�1 borehole (53.05°N, 158.63°E) situated in thearea of Petropavlovsk�Kamchatskii [Gavrilov et al.,2006, 2008]. The borehole is 2542 m deep, is filled withwater, and has a casing throughout its length. Geoacous�tic measurements were carried out using a three�compo�nent geophone with magnetoelastic crystalline ferromag�netic sensors [Belyakov, 2000] installed at a depth of1035 m. Three�octave bandpass filters were applied to thesignal at the output of each sensor to have four frequencybands centered at 30, 160, 560, and 1200 Hz. The signalat the output of each filter is proportional to the meaninput signal when averaged over a 1�min interval. It wasshown by Gavrilov et al. [2006] that geophone deploy�ment at a sufficiently great depth reduces human cul�

KSF68 tural noise by more than two orders, so that the natural

geoacoustic background noise can be measured withsignal amplitudes (when converted to ground motion)less than 1 ⋅ 10–10 m. According to the results thusobtained, the time series of geoacoustic emission(GAE) level in the 30 and 160 Hz bands contain a well�pronounced daily component (24.0 h) peaking duringthe nighttime, provided the measurements were con�ducted during time intervals in a quiet seismic environ�ment. The times that the minimum GAE levels go fromthe mean values to the maximum and vice versa corre�spond with the times that the terminator line passesthrough the observation site (the times of sunset andsunrise) (Fig. 3a). The GAE daily behavior was bestseen on the vertical component with filters centered at30 Hz and 160 Hz [Gavrilov et al., 2006].

The daily periodicity in GAE level is also clearly seenin GAE spectra in the 30 Hz and 160 Hz channels (seeFig. 3b). (In fact, we use periodograms rather than spec�tra. For example, we refer to the 24.0�h componentrather than the frequency 1.157 ⋅ 10–5 Hz. The term“spectrum” is used by these authors as being morefamiliar to most readers without detriment to the mean�ing of the results obtained.)

A word of explanation is in order. The 12.0�h spec�tral component seen in Fig. 3 is the second harmonic ofthe 24�h component. It is due to the steepness of thefronts in the daily variations of GAE level. The level ofthe 7�day spectral component that is typical of man�made noise is about twice as high as the noise level forthe vertical GAE vertical component at 30 Hz [Gavrilovet al., 2006]. For the 160�Hz channel, the level of the7�day component does not exceed the noise level (seeFig. 3b).

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PATTERNS IN THE DAILY PERIODICITY 131

DATA ANALYSIS METHODS

The daily periodicity of small earthquakes was iden�tified using several devices, some of which can be foundin [Zhuravlev and Sidorin, 2005a, 2005b, 2006;Zhuravlev et al., 2006]. A spectral resolution of at least4 ⋅ 10–2 Hz is required for reliable identification of the

24.0�h periodicity observed upon the background ofadjacent spectral components, e.g., the К1 tidal compo�nent with a period of 23.93 h. The common practice isto assume that the spectral resolution is approximatelyequal to the inverse of the observation time [Marple,1987]. By this criterion the 9�year length of our catalog

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Fig. 2. A map of Kamchatka earthquake epicenters with for the period 1998–2006 in the zone confined within the coor�

dinates 51.0°–58.5°N; 156.0°–165.0°E (from the regional catalog for Kamchatka and the Commander Is.) (a); A map of the Kam�chatka seismometric network with contours of complete reporting (b): (1) epicenters of K ≤ 7.0 earthquakes occurring from Jan�uary 1, 1998 to December 31, 2006, (2) axis of Kamchatka volcanic belt, (3) trench axis, (4) seismic station, (5) isoline of Kmin.

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gives a spectral resolution on the order of 3 ⋅ 10–9 Hz;this is far more than adequate for the problem in hand.A similar conclusion can be reached with regard to theseries of GAE level, whose lengths are also sufficient forthe required spectral resolution.

As a narrow range of periods between 1 h and 48 h issufficient for dealing with our problem (the identifica�tion of the daily periodicity and its possible high fre�quency components), we used a simpler algorithm forspectral calculation than the Fast Fourier Transform(FFT) or other techniques for spectrum calculationswhere a uniform data step is required. In the case underconsideration, each seismic event in the earthquakecatalog was represented by the delta function.

For each case we computed the absolute values of A

where tj is the time of seismic event j to within 1 sec�ond; the ωk are frequencies calculated from selectedperiods Tk as ωk = 2π/Tk. The quantity Aj is alwaysunity for seismic events and equals the value of the sig�nal at a time of interest for GAE levels.

When the spectrum is calculated as described, theamplitudes of the extrema in the case of seismicity anal�ysis will be proportional to the number of earthquakesinvolved in determining the extremum of this har�monic.

The general scale of periods starts from 30 min at astep of 1 min. In addition to the periods quantized atthis step, we also examined periodicities that are notmultiples of 1 minute; these were the most intensiveharmonic components of the lunar–solar tide close to24.0 h: K1 23.934 h, O1 25.819 h, P1 24.066 h, J1

23.098 h, and M1 24.833 h. This was done to test thehypothesis that the daily periodicity could be due to thelunar–solar tide.

It should be noted that the time series of seismicityand geoacoustic emission level generally possess all thecharacteristics of series that involve a rather intensiverandom component. Therefore, in order to be able tocompute spectral characteristics for such series it is rec�ommended to use additional averaging procedures tomake the periodicities revealed by the analysis morereliable. The general assumption is that if a spectralextremum identified persists and the noise componentis reduced, this supports the assertion that the periodic�ity in question is not accidental. To assess whether theprocedure is reasonable, in several cases we calculatedmore detailed spectra at a step of 30 s in the immediate

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vicinities of the extrema of interest. The spectra werethen averaged over the amplitudes and the respectiveperiods in a moving window of five or ten adjacent peri�ods. The results showed that the averaging proceduredid not yield positive results in our particular case. Thiswas due in the first place to the high Q of the 24.0�hspectral extremum, because in that case the width of theidentified extremum turns out to be below the scanningstep and so narrow an extremum is lost on averaging in agreat number of adjacent extrema with lower Q’s. Sincethe overwhelming percentage of cases involved the24.0�h extremum at least two or three times the adjacentamplitudes of the noise component, it was decided to dowithout this additional averaging procedure.

In addition to spectra (periodograms) for the timeseries of seismicity, we also used the method of epochsuperposition to calculate plots of daily behavior inorder to examine the shape of the identified daily peri�odicity on the time scale.

THE RESULTS

Identification of the daily periodicity in the occur�rence of Kamchatka earthquakes. Figure 4 presentsspectra of earthquake time series and plots of the dailybehavior based on the catalog for the period January 1,1998 through December 31, 2006 for the entire area ofstudy (51.0°–58.5°N; 156.0°–165.0°E). The spectra ofearthquake series and the plots of the daily periodicityare given both for the complete catalog (without selec�tion by class and hypocentral depth) and for selectionswith K ≤ 8.0 and K ≤ 7.0. We also show results for twodepth ranges based on the K ≤ 8.0 selection.

From these plots one can see a well�pronouncedextremum at a period of 24.0 h for the first three selec�tions (without restrictions imposed on hypocentraldepths). The ratio of the 24.0�h extremum to the noisecomponent of the spectrum noticeably increases as thecutoff energy class is decreased in these selections. Theresults shown in Figs. 4g–4j demonstrate that the dailyspectral component has not been detected in the upper(0–10 km) depth range.

The 24.0�h periodicity, as identified, is certainly sig�nificant, especially for smaller earthquakes (K ≤ 8.0).The exceedance of the nighttime earthquake rate overthe daytime rate for the K ≤ 8.0 earthquakes is about20%, which is close to the results given in [Ivanov�Kholodnyi et al., 2004; Shimshoni, 1971].

The detailed spectra around 24.0 h, as presented inFig. 5 for the K ≤ 7.0 earthquakes, shows that the 24.0�hextremum is very well identified upon the background

Fig. 4. Spectra normalized by maximum amplitude (left column) and plots of daily behavior (right column) of Kamchatka earth�quake time series for 1998–2006: n is the daily number of earthquakes, local time is plotted along the horizontal axis in the plotsof daily behavior; a, b are based on the complete catalog (without restrictions on energy class K and hypocentral depth H); c, dare for the selection of K ≤ 8.0 earthquakes; e, f are for the selection of K ≤ 7.0 earthquakes; g, h are for the selection of K ≤ 8.0earthquakes in the depth range H = 0–10 km; i, j are for the selection of K ≤ 8.0 earthquakes and depths H > 10 km.



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of periodicities around it, having amplitudes of at leastthree times theirs. Also, the width of this extremumdoes not exceed the quantization step over time, thusindicating a high Q of the extremum. We conclude thatthe periodicity as identified is exactly at 24.0 h. It shouldbe emphasized that no extrema, however significant,have been detected around 24.0 h, except exactly at24.0 h. This also applies to the responses to harmonicsin the lunar–solar tide.

Results for various zones. Considering that the areaof study is extensive, we sought to somewhat refine theresult that a daily seismicity periodicity is present byprocessing the catalog when divided into several zones.At first we examined the presence of a daily periodicityby dividing the area of study into six zones at a step of 1°

along geographic latitude. Later, we refined the resultsby dividing the 52.0°–54.0°N area into smaller zones ata step of 0.5° along geographic latitude. The results areshown in the table.

Figure 6 shows spectra and plots depicting the dailyseismicity periodicity for some of these zones.

Figure 7 summarizes the location of the zones withthe identified daily periodicity of small earthquakes.

To sum up, it is only for two zones, zone 3 (52.5°–53.0°N) and zone 9 (56.0°–57.0°N) for the K ≤ 7.0earthquakes that we can reliably detect both the 24.0�hextremum and the daily pattern that peaks during thenighttime.

An identical daily behavior is identified as well inzone 8 (55.0°–56.0°N), although the 24.0�h extremum

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Fig. 5. Detailed spectrum normalized by maximum amplitude around 24.0 h for the K ≤ 7.0 earthquakes. Arrows indicate thepositions of the main lunar–solar periodicities [Melchior, 1968].

Results from the analysis of daily periodicity in small earthquakes in several zones

Zone # Coordinates of zone, north latitude, degrees, N

Number of events N ≤ 7.0

Presence of 24.0�h extremum

Presence of daily behavior

1 51.0°–52.0° 647 None None2 52.0°–52.5° 1929 None None3 52.5°–53.0° 3465 Yes Yes4 53.0°–54.0° 3555 None None5 53.0°–53.5° 1990 None None 6 53.5°–54.0° 1565 None None7 54.0°–55.0° 1931 None None8 55.0°–56.0° 1821 None Yes9 56.0°–57.0° 1393 Yes Yes



is not detected in the spectrum for that zone. Actuallythat statement means that the daily periodicity is cer�tainly present in that zone, but is not dominant amongthe other components.

Assessing the possible influence of the noise. As notedin the Introduction, a lively debate is still occurringconcerning the causes of the daily periodicity in thetime series of seismic events. Some researchers are ofthe opinion that the daily periodicity is merely an arti�fact due to changed actual sensitivity in the seismologi�cal observation network, which stems from the fact thatthe noise level due to various factors (human� or wind�induced noise) is higher during the daytime, thus mask�ing many of the weaker seismic signals.

Generally speaking, the quality of seismological datacan certainly be affected by wind�induced noise.According to the data in Kondratyuk [1983], the windvelocity is highest at Petropavlovsk�Kamchatskii in 15–18 h year round. The maximum number of days withstrong winds occurs during the autumn and winter.However, the daily behavior of wind velocity is barelynoticeable. In the winter the variations in the meandaily amplitude of wind velocity do not exceed 0.3 m/s;the summer variation is 2.4 m/s. Therefore, it seemsunlikely that such small variations in the daily ampli�tudes of wind velocity could have significantly affectedseismogram quality.

In any assessment of the influence of differenthuman factors it should be kept in mind that most seis�mic events, as recorded in the catalog of Kamchatkaearthquakes, are based on data from automatic teleme�try seismic stations that are installed at uninhabitedlocations that are far from cities and villages. For exam�ple, for the 52.5°–53.0°N zone that contains Petropav�lovsk�Kamchatskii only the Petropavlovsk station canpossibly be affected by human factors (industrial activi�ties and traffic), while the other stations that providerecords are situated outside of the zone of the possibleinfluence of these factors. The idea that the results can�not be affected by manmade factors is also corroboratedby the results from calculations of spectra for the timeseries of the K ≤ 8.0 earthquakes in the 10�day range(Fig. 8).

As appears from Fig. 8, the 7�day component thatcharacterizes the level of man�induced excitation doesnot exceed the noise level. As well, one can adducethe data shown in Figs. 4g–4j to support the assertionthat the possible human noise in this case has nobearing on the daily behavior effect. It follows fromthese data that the daily component is not identifiedfor the upper (0–10 km) depth range where thehuman noise is the highest.

Comparison with the data of geoacoustic boreholemeasurements. In our opinion, the decisive argumentthat supports the hypothesis that there is no connectionbetween the daily periodicity of small earthquakes thatpeaks during nighttime and noise due to various causes

is the comparison of the series of small earthquakes withthe series of multiyear geoacoustic measurements in theG�1 deep borehole. According to the results of theseborehole measurements, the GAE series contain a well�pronounced daily component (24.0 h) that peaks duringthe nighttime (see Fig. 3). Gavrilov et al. [2006] showedthat a geophone installed at a sufficiently great depthwill record human noise reduced by more than twoorders; this kind of deployment also nearly completelyremoves the influence of weather conditions on themeasurements. Figure 9 shows the plots of daily varia�tions in GAE level at the G�1 borehole at 1035 m depththrough the seasons calculated by the method of epochsuperposition as compared with similar plots for small(K ≤ 7.0) earthquakes. It appears from these data thatthere are some general patterns in the daily variations inGAE level measured at the deep borehole and in thedaily behavior of small earthquakes.

This primarily concerns the plots for summer andautumn for which the absolute maxima of the correla�tion coefficient between the respective time series areclose to 0.9. The respective values for winter and springare 0.62 and 0.66.

RESULTS AND DISCUSSION

The results presented in Fig. 9 suggest that the pat�terns observed in the daily variations in the rate of smallearthquakes and the patterns in the daily variations inGAE level are due to some physical causes that arecommon to both. Previously it was found that the dailyvariations in GAE level are due to the modulating influ�ence of natural electromagnetic radiation (NEMR) onearth materials based on data from synchronous geoa�coustic and electromagnetic measurements, as well ason the results of special experiments with rock speci�mens [Gavrilov et al., 2006; Gavrilov and Bogomolov,2008; Gavrilov, 2007]. It was found from measurementsconducted at the G�1 borehole that the daily variationsin GAE level and the variations in NEMR level ofatmospheric origin in the ranges 30 Hz and 160 Hz dur�ing seismically quiet periods were nearly identical, withthe correlation coefficient ρ being on the order of 0.80–0.99 (Fig. 10). (The correlation coefficient decreased tovalues on the order of 0.001–0.34 before large earth�quakes (a day or longer), as well as during relaxationperiods [Gavrilov, 2007]. The physical causes of thiseffect are not discussed in the present paper.)

The mechanisms that are responsible for the radia�tion and propagation of VLF radio noise have beenstudied in detail [Aleksandrov et al., 1972; Dolukhanov,1972]. The natural electromagnetic radiation in therange 30 Hz to 30 kHz for northeastern Russia mostlycomes from distant storm sources [Aleksandrov et al.,1972]. The daily variations in the level of electromag�netic field in this range are due to a dramatic deteriora�tion of radio wave propagation during the daytime in the

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Comparison between the plots of daily variations inthe rate of small Kamchatka earthquakes and changesin the level of the NEMR electrical component throughthe seasons as calculated by the method of epoch super�position (Fig. 11) suggests that the effect of NEMRmodulating influence on the geologic medium can beclearly seen in the time series of small earthquakes aswell.

The data presented in Fig. 11 clearly show that theshapes of the plots of daily variations in the rate of smallearthquakes are largely controlled by the variation inNEMR level. The absolute maxima of the correlationcoefficients between the corresponding time series forsummer and autumn are 0.88 and 0.83, respectively.The absolute maxima of the correlation coefficientsbetween the series for winter and spring are 0.6 and0.77, respectively. In all cases the variations in the rate ofsmall earthquakes occurred with some delays relative tothe NEMR variations.

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Fig. 7. Positions of zones with an identifiable daily periodicity of small earthquakes: (1) zones with the 24.0�h extremum andthe daily behavior, (2) zone with daily behavior only, (3) boundary of the area of study, (4) axis of Kamchatka volcanic belt, (5)trench axis.

Fig. 6. Spectra normalized by maximum amplitude (left column) and plots of daily behavior (right column) obtained from timeseries of Kamchatka earthquakes for several zones. n is the daily number of earthquakes; local time is plotted along the horizontalaxis in the plots of daily behavior.

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This relationship between the processes is consistentwith the results that were obtained during experimentsthat investigated the influence of electromagnetic exci�tation (EME) on rock specimens [Sobolev and Pono�marev, 2003; Zakupin et al., 2003, 2006; Gavrilov andBogomolov, 2008]. A typical example is presented inFig. 12 showing results from an experiment with a gra�nodiorite specimen under mechanical load [Zakupin etal., 2007]. The compressive force was 91% of thestrength limit. The specimen was subjected to electricalexcitation by means of electrodes attached to the sidefaces, which were used to transmit rectangular electricalpulses of amplitude 35 V, duration 20 μs, and frequency2.2 kHz.

It appears from Fig. 12 that this electrical excitationresulted in a considerably increased acoustic activity ofthe specimen in the form of more frequent acousticemission pulses. The acoustic response to the action ofelectrical pulses followed with considerable delays.Such a delay has been noted in nearly all experiments ofthis kind. According to results from these experiments,the typical delay times for monolith specimens were15–40 min [Zakupin et al., 2003]. It may be noted thata considerable amount of in�situ measurements andresults from experiments on rock specimens have beenaccumulated by now, so we are in a position to reach a

reliable conclusion as to seismic (acoustic) activation ofrocks under variable electromagnetic fields. The effectwas observed for physical processes within a very wideenergy range, from geoacoustic emission to M ≥ 2.2earthquakes. The intensity of the electrical field thatproduced these effects also varied in a wide range. Togive an example, for borehole geoacoustic measure�ments the level of the NEMR electrical component thatacts on rocks at a depth of 1000 m at frequencies ofabout 160 Hz is 5 to 6 orders of magnitude below theintensity of the electrical field in experiments with rockspecimens [Gavrilov and Bogomolov, 2008].

The data presented above suggest that the daily peri�odicity in the time series of small earthquakes is mostlydue to the action of the VLF natural electromagneticfield on the geologic medium. At the same time, severalquestions have to be answered, as these arise in the stud�ies of correlative relationships between the series ofsmall earthquakes and the NEMR. Among other issues,one notes that the correlation coefficient between theseries of small�earthquake daily variations and theNEMR level is significantly lower during the winterthan during the other seasons. One can suggest two pos�sible factors that could cause this effect. The first is thatthe processing for the winter period might use fewerevents. To investigate this issue, the authors calculated

1.00

0.75

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250200150100500

24.0 h7 days

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Fig. 8. Spectrum calculated from the time series of K ≤ 8.0 earthquakes and normalized by maximum amplitude in the range10 days for the entire area of study.



seasonal changes in the mean daily rates of small (K ≤7.0) earthquakes. The results showed that the meandaily rates of events for the winter and summer areapproximately equal, although the summertime corre�lation coefficient is much higher. The other factor thatcould cause a relatively low correlation coefficient forwinter is more real. This is the longer duration of thedark period of the day (intervals of increased NEMRlevel, Fig. 13). To convince oneself that the effect is real,one might look at the data shown in Fig. 12. It is clearlyseen that the acoustic response of rocks is much shortercompared with the electrical excitation. As noted byZakupin et al. [2003], responses involving this charac�teristic feature are typical of moderate mechanicalloads. Without further specifying the physical mecha�nism that is responsible for the effect, it may be conjec�

tured that the cause lies in a limited number of sourcesthat produce the response of rocks to electrical excita�tion (e.g., the number of opened microcracks). Whenthe electrical excitation acts during sufficiently longintervals of time (which is primarily the case for thewinter period) and such sources are few in number, therelevant response of the geologic medium will come toan end before the electrical excitation does. As anexample of such an effect for a real geologic medium,Fig. 13 presents results from geoacoustic measurementsat the G�1 borehole at a depth of 200 m compared withdata from electromagnetic measurements conducted atthe same site. It follows from these data that it was onlyduring the first halves of the NEMR intervals that a sig�nificant GAE response was recorded. It should be notedthat a similar pattern is observed also for the plot of daily

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summer autumn

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Fig. 9. Comparison between daily variations in the rate of small Kamchatka earthquakes and the daily variations in GAE level asmeasured in the G�1 borehole at a depth of 1035 m through the seasons. Local time is plotted along the horizontal axis in all plots:(1) plot of daily periodicity for K ≤ 7.0 earthquakes, (2) plot of daily periodicity in GAE level, vertical component Z, the 160 Hzchannel.

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variations in the rate of small earthquakes during thewinter period, viz., considerable jaggedness affectingthe second half of the plot (see Fig. 11). In the authors'opinion, this is due to the same physical factors that actduring “abortive” GAE responses to NEMR excitation.

A detailed discussion of issues that arise withregard to the physical mechanism that is responsiblefor the stimulating action of NEMR on earthquake�generating processes is reasonable to conduct in aseparate publication.

According to our results, the daily periodicity ofsmall Kamchatka earthquakes is reliably detected in twozones only, zone 3 (52.5°–53.0°N) and zone 9 (56.0°–57.0°N). The data given in the table for the distributionof the number of seismic events over zones show thatthese results could not have been due to insufficientsample size (the number of earthquakes) for any zone.It seems natural to relate the results in separate zones todifferences in geological structure and geophysicalcharacteristics between zones. Considering that thedaily variations in the rate of small Kamchatka earth�quakes show a high correlation with changes in the levelof the NEMR electrical component, it may be hypoth�esized that the differences can largely be due to differ�ences in the electrical characteristics of the rocks thatcompose these zones. Since most of the K ≤ 7.0 earth�

quakes involved in the processing are shallow events,one primarily thinks of differences in the electricalcharacteristics for the upper crust.

Comparison of the results obtained for the Kam�chatka region with those for other regions reveals bothsome common features and certain differences. Forexample, comparing the results given in this paper withthose found in Zhuravlev et al. [2006] for (formerSoviet) central Asia, one notes the following. For bothregions we see the daily (exactly 24.0 h) periodicity inthe series of small earthquakes. However, the CentralAsian results also involve the presence of a semidiurnal(exactly 12.0 h) component, while that component isabsent from the time series of Kamchatka earthquakes.In addition, unlike the situation for Kamchatka, thedaily behavior of the seismicity rate for some areas inCentral Asia peaks during the daytime rather than thenighttime. A possible explanation of the two effects issought by Zhuravlev et al. [2006] in the influence ofconsiderable human factors, such as the noise fromwater falling from electric power station dams and thehydro�generators. Such noise does not occur in Kam�chatka and this might explain the difference in theresults. Another feature common to the two regions isthe absence of a response to tidal excitation in the seriesof small earthquakes.

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MLH = 5.9H = 39 kmR = 357 km

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Fig. 10. An example of synchronous measurements of GAE level and the level of NEMR electrical component (G�1 borehole)[Gavrilov, 2007].



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Fig. 11. Comparison between daily variations in the rate of small Kamchatka earthquakes and the daily variations in the level ofNEMR electrical component through the seasons. Local time is plotted along the horizontal axis for all plots: (1) plot of dailyperiodicity for the K ≤ 7.0 earthquakes, (2) plot of daily periodicity for NEMR level, the 160 Hz channel.

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AE

rat

e, 1

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Fig. 12. Variation in the acoustic activity of a granodiorite specimen during electrical pulses. Filled horizontal bar marks the interval ofpulse activity [Zakupin et al., 2007].

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CONCLUSIONS

(1) A daily periodicity was detected in the occur�rence of small (K ≤ 8.0) Kamchatka earthquakes for thearea within 51.0°–58.5°N; 156.0°–165.0°E. The maxi�mum of the daily seismicity rate occurs in the interval 0to 4 h LT, depending on the season. The exceedance ofthe seismicity rate during the nighttime over the corre�sponding daytime value is about 20% for the K ≤ 8.0earthquakes. The ratio of the 24.0�h spectral extremumto the noise spectral component noticeably increases asthe cutoff energy class is decreased in the earthquakeselections.

(2) Additional processing with a subdivision of thearea of study into east–west zones showed that it wasonly for the 52.5°–53.0°N zone and the 56.0°–57.0°NN zone that both the 24.0�h spectral extremum and thedaily behavior peak during the nighttime were reliablydetected in the series of small earthquakes. An identicaldaily behavior was found also in the 55.0°–56.0° Nzone, although the 24.0�h extremum was not detectedfor this zone. For the rest of the area of study no dailybehavior of seismicity was detected.

(3) We showed that the results were not affected byhuman and weather factors. An analysis of our data per�mits the assertion that the daily periodicity of smallearthquakes could not have been caused by tidal excita�tions.

(4) We put forward an explanation of the physicalfactors responsible for the effect. We suggest that thedaily periodicity of small earthquakes may have been

caused by very low frequency natural electromagneticradiation acting on the geologic medium. We note thatthis effect is related to the previously identified modu�lating influence on the part of natural electromagneticradiation that affects the intensity of geoacoustic emis�sion from rocks.

ACKNOWLEDGMENTS

We authors are grateful to M.V. Rodkin, Interna�tional Institute of Earthquake Prediction Theory andMathematical Geophysics, Russian Academy of Sci�ences and to L.M. Bogomolov, Institute of MarineGeology and Geophysics, Far East Division, RussianAcademy of Sciences for valuable remarks.

This work was supported by the Russian Foundationfor Basic Research, project no. 09�05�98543�p_vostok_a and by the Far East Branch, Russian Acad�emy of Sciences, project no. 09�III�A�08�420).

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GAE level,

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January 29, 2009 January 30, 2009

Fig. 13. An example of response by the geologic medium to NEMR excitation during decaying GAE daily behavior.

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Documents

Patterns in the daily periodicity of low magnitude seismicity in Kamchatka