directional properties of van’s ses and ulf mt signals at ...physics of the earth and planetary...

Ž .Physics of the Earth and Planetary Interiors 105 1998 153–166

Directional properties of VAN’s SES and ULF MT signals atIoannina, Greece

M. Uyeshima a,), W. Kanda a, T. Nagao b, Y. Kono c

a Earthquake Research Institute, The UniÕersity of Tokyo, Yayoi, 1-1-1, Bunkyo, Tokyo 113, Japanb Earthquake Prediction Research Center, Tokai UniÕersity, 3-20-1 Orito, Shimizu, Shizuoka 424, Japan

c Department of Earth Science, Kanazawa UniÕersity, Kakuma, Kanazawa 920-11, Japan

Received 16 March 1996; received in revised form 4 Novenber 1996; accepted 4 November 1996

Abstract

ŽIn order to examine the validity of the earthquake prediction technique developed in Greece by VAN Varotsos,. Ž .Alexopoulos and Nomicos and elucidate the underlying physical processes, we performed a magnetotelluric MT recording

Ž . Ž .of the ULF band 3–200 s in the vicinity of VAN’s sensitive station, Ioannina IOA . By comparing MT electric field ofŽ .ionospheric or magnetospheric origin, electric field of lightning origin and VAN’s seismic electric signals SES , the

Ž .following facts were revealed. 1 The MT electric field always varies in the ENE–WSW direction, indicating existence ofŽ .significant electrical conductivity contrast near the site. 2 The lightning electric field orientation is somewhat more

scattered but still holds the same preferred direction as that of the MT electric field. The scatter is larger for larger lightningfield, probably due to closer location of lightning stroke to the station andror due to no-linear saturation of the recording

Ž .signals. 3 The SES electric field orientation is even more scattered than that of the lightning origin. These observationssuggest that the sources of the SES are in the neighborhood of the station. It is still unclear how the SES, associated withdistant earthquakes, are emitted from the sources near the IOA station. q 1998 Elsevier Science B.V.

Keywords: Earthquakes; SES; Greece; Magnetotelluric recording; Electric field

1. Introduction

Since the early 1980s, Varotsos and co-workershave continuously monitored the electric field of theearth at several stations in Greece and reported thatthey actually succeeded in short-term earthquakepredictions by using transient changes in the electric

Ž .field, which they term Seismic Electric Signals SESŽe.g., Varotsos and Alexopoulos, 1984a,b; Varotsos

) Corresponding author.

.and Lazaridou, 1991; Varotsos et al., 1993b . Severaldifferent mechanisms have been proposed for the

Žcause of the SES Varotsos et al., 1993a; Park et al.,.1993; Park, 1994 . These mechanisms can be roughly

classified into two types: Local and hypocentral. Thelocal mechanisms require generation of the SES nearthe recording sites in response to precursory strain or

Ž .stress Dobrovolsky et al., 1989; Bernard, 1992 ,whereas the hypocentral mechanisms require genera-tion of current in the preparation zone of the earth-

Žquake Varotsos and Alexopoulos, 1986; Slifkin,.1993 and transmission of electromagnetic energy to

0031-9201r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0031-9201 97 00088-5

( )M. Uyeshima et al.rPhysics of the Earth and Planetary Interiors 105 1998 153–166154

Žthe recording site Varotsos et al., 1993a; Utada,.1993; Honkura and Kuwata, 1993 . At the present

stage, however, no proposed mechanisms can com-pletely explain the observational characteristics ofthe SES. In order to examine the validity of the VANearthquake prediction method or to understand theunderlying physical processes, it is essential to eluci-date the physical properties of the SES on whichtheir method is based.

For this purpose, a series of studies in Greece hasbeen performed by Japanese research groups since

Ž .1991. In these studies, VAN’s Ioannina IOA sta-tion was chosen as the target area since IOA isreported as one of the most useful stations in thesense that the VAN-group repeatedly have issuedmany predictions based on the SES detected at this

Ž .station Dologlou, 1993; Varotsos et al., 1993b .Ž .First, Nagao et al. 1996 acquired all the anoma-

lous electric disturbances from VAN’s analog strip-chart records of 3 NS short dipoles, 4 EW short

Ž .dipoles and 1 NS long dipole see Fig. 1 for theperiod from April 30, 1988 to September 15, 1990,and selected the candidate SES. The selection wasdone by applying VAN’s criteria for the selection ofSES entirely independently of the VAN-group. Themost important part of the VAN’s criteria is termedDVrL test and they select only the anomalous elec-

Žtric signals as the candidate SES whose DVrL volt-.age difference normalized by dipole length values

are same with reasonable allowance on all dipolesindependent of dipole length. Anomalous electricalvariation originated from their recording system in-cluding electrodes and electric circuits can be dis-criminated as noise after performing the DVrL test.Electrical disturbances due to the sources in the areaof dipole arrays can also be discriminated as noise.In the 28.5 month period, 41 changes were recog-nized as possible SES out of more than 3000 anoma-lous changes by applying the VAN’s criteria. Acomplete list showing the amplitudes of all the se-lected SES for all the dipoles are included in Nagao

Ž .et al. 1996 . During the period, the VAN groupissued 18 predictions. 14 of the SES used by theVAN group were identical to the SES which Nagao

Ž .et al. 1996 selected, while 4 were judged as noiseŽ .by Nagao et al. 1996 . In spite of some discrepan-

cies, it is remarkable that two independent operationsidentified the same 14 changes as the SES out ofmore than 3000 changes, clearly showing the objec-tivity of the VAN’s criteria for noise rejection.

In this paper, we report results from a magnetotel-Ž . Ž .luric MT survey of the ULF band 3–200 s in the

vicinity of the IOA station. We measured both elec-tric and magnetic fields independently from theVAN’s system in August, 1994. Unfortunately, dueto the short survey period, we did not observe anySES in our records. Besides determining the sub-surface electrical conductivity distribution, some as-pects of the physical character of SES have beenrevealed by comparing the field orientations of theSES with those of our EM field records.

2. Field operation in the IOA station area

The Ioannina area is in the northwestern part ofŽ .Greece Fig. 1 . The valley, whose width is about 5

km, runs in the NW–SE direction with lake Ioanninain its center. VAN’s IOA station is located on aGreek military base at the northeastern border of thevalley and 3 NS and 4 EW short span electricdipoles with various dipole lengths are installed on afan deposit of this border. In addition, the VAN-grouphas installed two long dipoles with which they mea-sure the voltage difference between the IOA station

Ž .and the Perama town 2.1 km southward , and thatbetween the IOA station and the center of the IOA

Ž .city 5 km southward . The measurement on the longdipole connected to the IOA city, however, was notoperated in the duration of the study by Nagao et al.Ž .1996 . All above records are transmitted to the

Fig. 1. Distribution of short dipoles installed by the VAN group and location of MT site: S001 installed by ourselves at the IOA station ofŽ . Ž .VAN together with the index map of Greece top left and that of the Ioannina area top right , where two long dipoles of the VAN group

are also shown and mountainous areas higher than 2000 ft are shaded.

( )M. Uyeshima et al.rPhysics of the Earth and Planetary Interiors 105 1998 153–166 155


central station at Athens in real time and are dis-played on analog strip chart recorders without digitalstorage.

In the central area of the VAN site, which hasmany of the short dipoles in the vicinity, a magne-totelluric site, S001, was installed using dipole

Ž . Ž .Fig. 2. a Sets of 1-h EM time series of MT origin with 1 s. sampling at site S001 upper two . Hx and Hy denote magnetic fields ofŽ . Ž . Ž . Ž .NS Nq and EW Eq component, and Ex and Ey denote electric fields of NS Nq and EW Eq component, respectively. Time is inŽ . Ž . Ž .UT. b EM field orientations of MT origin. The diagrams for magnetic lower left and electric lower right fields are produced from the

sets of 1-h EM time series depicted in Fig. 2a. In the diagram for the electric field, the least square regression line is also added.


Ž . Ž .lengths of 80.9 m NS and 71.3 m EW during theperiod from August 17 to 19, 1994. A portable MTrecording system recorded the two dipoles and a

three component fluxgate magnetic sensor. In thesystem, signals are first filtered by a one-pole 3–180s band pass filter of 6 dBroctave and the data values

Ž . Ž . Ž .Fig. 3. a The same 1-h EM time series as in Fig. 2a except for pre-thunderstorm period upper two . b EM field orientations forŽ . Ž .pre-thunderstorm period. The diagrams for magnetic lower left and electric lower right fields are produced from sets of 1-h EM time

Žseries depicted in Fig. 3a. In the diagram for electric field, the regression line showing the preferred direction of the MT electric field Fig..2b is also added.


Ž . Ž . Ž .Fig. 4. a The same 1-h EM time series as in Fig. 2a except for the period of the midst of thunderstorm upper two . b EM fieldŽ . Ž .orientations for the period of the midst of thunderstorm. The diagrams for magnetic lower left and electric lower right fields are produced

from sets of 1-h EM time series depicted in Fig. 4a. In the diagram for electric field, the regression line showing the preferred direction ofŽ .the MT electric field Fig. 2b is also added.


Ž . Ž . Ž .Fig. 5. a The same 1-h EM time series as in Fig. 2a except for post-thunderstorm period upper two . b EM field orientations forŽ . Ž .post-thunderstorm period. The diagrams for magnetic lower left and electric lower right fields are produced from sets of 1-h EM time

Žseries depicted in Fig. 5a. In the diagram for electric field, the regression line showing the preferred direction of the MT electric field Fig..2b is also added.


were stored in 16 bit binary format every second.The ranges for magnetic and electric records areplus–minus 32.768 nT and 10 mV, respectively.

3. Comparison among MT-signals, lightning-sig-nals and SES

During the observation period, no SES wererecorded either by the VAN-group or by ourselves.Instead, in addition to the normal MT-signals of

Ž .ionospheric or magnetospheric origin Fig. 2 , manyspike-type EM disturbances were recorded as shownin Figs. 3–5. These spikes were probably due tolightning activities which were identified during theperiod of the Figs. 3–5 by the observers who were atthe University of Ioannina about 10 km south of therecording site. At first, we thought that the lightningspikes were merely a nuisance as they severelycontaminated our EM records as shown especially inFig. 4. After the inspection of these sets of timeseries, however, we became aware of the followingfacts.

First, the electric field variations of MT origin inNS and EW plane show a distinct directionality,whereas no such directionality is detected for the

Ž .magnetic field Fig. 2 . Second, even for the light-ning spikes, such directionality is still detected for

Ž .the electric field Figs. 3–5 . Third, for the SESŽ . Žselected by Nagao et al. 1996 see table 2 of Nagao

.et al., 1996 , as shown in Fig. 7, such a directionalityis much less pronounced.

For the MT disturbance, the electric field variesonly in the preferred direction trending ENE–WSW,whereas the magnetic field varies in all directionsŽ .Fig. 2 . This preferred direction of the electric fieldis 248 north of geographical east with correlationcoefficient between Ex and Ey of 0.996. Similarresults have been reported by Varotsos et al.Ž .1996a,b . The existence of such a significant pre-ferred direction for the MT electric field is wellknown in areas where a significant contrast of elec-trical conductivity such as land–sea boundary exists

Žnearby e.g., Park et al., 1991; Ranganayaki and.Madden, 1980 . When the magnetic field variation

occurs in the direction perpendicular to the strike ofthe boundary, which means that the external voltageexcitation is parallel to the strike, the electric fieldson both sides of the boundary should be continuous.On the other hand, when the magnetic field varies inparallel to the strike, the electric field perpendicularto the strike is discontinuous across the strike. In thiscase, the electric field on the resistive side is en-hanced and that on the conductive side is reduced.The ratio between the electric fields on both sides isequal to the ratio of the resistivities on both sidessince the electric current density should be continu-ous across the boundary. Therefore, although themagnetic field varies in all directions, the electricfield trajectory on resistive side tends to be perpen-dicular to the strike and that on conductive side tendsto be parallel to the strike. In the Ioannina case, thepreferred direction may be caused by the ocean–con-

Ž .tinent boundary e.g., Park et al., 1991 andror morelocalized contrast between a resistive mountain body

Ž .and conductive valley sediments Uyeda, 1996 . Inboth cases, the station is located on the resistive sideof the boundary.

The electric field trajectory generated by the light-ning shows a little more scatter than that of the MTorigin but still follows the preferred direction. As isdepicted in Figs. 3 and 5, before the thunderstormcame to the area and after it went away, the electricfield trajectory still followed the MT preferred direc-tion although many lightning spikes were detected intime series and the amplitude ratio between electricand magnetic field variation was larger than that of

Ž .the MT disturbance Fig. 2 . This enhancement ofthe amplitude ratio of electric over magnetic field is

Žan indication of the near-field effect Goldstein and.Strangway, 1975 . When a thunderstorm occurred in

the observation area, as is shown in Fig. 4, theelectric field trajectory deviated more from the MTpreferred direction as the amplitude of the electricfield became larger. This may indicate that the morescattered orientation occurs when the source is nearer

Ž . Ž . Ž .Fig. 6. Time series of voltage differences on various VAN’s dipoles including a Isolated-type SES top and b Continual SES activitiesŽ .bottom , where SES are indicated by arrows. These time series charts were made by tracing the analog records of VAN. Five smallest gridscorrespond to 1 cm for amplitude axis and two smallest grids correspond to 1 cm for time axis in the actual strip-chart records of VAN.


to the site. Since the lightning phenomenon is thecharge transportation between the cloud and the earthor in the cloud, the direct current source on the earthtakes the form of monopole. As the source is nearerto the station, the effect of a monopole field configu-ration more significantly overcomes the effect ofelectrical conductivity distribution mentioned above.Another reason for the scatter is due to no-linearsaturation of the recording signals. Since the full-scales for Ex and Ey components are 124 mVrkmand 140 mVrkm, respectively, some spikes reachedthis extremal values especially for Ey, which mayresult in steepening the gradient of the electric fieldtrajectory compared with that of MT. Therefore, ifthere was no such saturation effect, even the trajec-tory of lightning generated electric field trajectoryshould follow the MT preferred direction better inspite of its near field nature.

As for the electric field orientation of the SES,however, such signals as is depicted in Fig. 6a wereobserved on both the NS and EW dipoles, whereas

Fig. 7. Electric field orientation of the candidate SES selected bythe study with the strip-chart records of VAN-group for the period

Ž .from April 30, 1988 to September 15, 1990 Nagao et al., 1996 ,where "0.3 smallest grids are assumed to be reading errors ofrespective SES amplitudes. The regression line showing the pre-

Ž .ferred direction of the MT electric field Fig. 2b is also added tothe diagram.

continual electrical activity in Fig. 6b, which wasalso recognized as SES by VAN group, was recordedonly on the NS dipoles. In the electric field orienta-tion diagram for all the signals selected by Nagao et

Ž . Ž .al. 1996 Fig. 7 , some signals are on the preferreddirection of the MT field, but their deviation fromthe MT preferred direction is more significant thanthat of the lightning generated electric field trajec-

Ž .tory. Since Nagao et al. 1996 only read the ampli-tude of the isolated signals as in Fig. 6a or the singlemaximum amplitude for such electric activities as inFig. 6b from VAN’s analog chart, the diagram inFig. 7 is not equivalent with the diagrams in Fig. 2b,Fig. 3b, Fig. 4b and Fig. 5b. Then if all the timeseries of all the signals are digitized and their trajec-tories are plotted on the same diagram, their devia-tion from the MT preferred direction will be moreremarkable.

4. Discussion

In the previous section, the directional propertiesof the electric fields of the MT origin, lightningorigin and VAN’s SES were shown and compared toeach other. The first two were similar and the elec-tric fields tended to vary only in the ENE–WSWdirection regardless of the direction of the magneticfield variation, reflecting the regional andror localelectrical conductivity distribution. So far we do notknow the location and the source configuration ofthe SES. However, if the sources of the SES are farenough for the electromagnetic field to be treated asplane wave like MT disturbances, their directionalproperties should also follow the MT one. Consider-ing that the electric field generated by nearby light-ning with monopole configuration still follow theMT preferred direction, the sources of some SESshould be located near the IOA station so that theeffect of the source field configuration may over-come the effect of the regional andror the localelectrical conductivity distribution. A similar conclu-

Ž .sion for the SES at IOA was obtained by Park 1994from the fact that no magnetic change was associatedwith the SES and that a magnetotelluric cancellationscheme successfully preserved the SES in the resid-

Ž .ual electric fields Hadjioannou et al., 1993 .


Ž .Nagao et al. 1996 demonstrated the objectivityand usefulness of the VAN’s criteria for the selectionof the SES including the DVrL test. The graphic

presentation of the DVrL test based on their data isshown in Fig. 8. As shown in Fig. 8a, the electricfields of the candidate SES, which survived the test,

Fig. 8. Graphic presentation of the DVrL test for the electric field variations of the SES and those of MT origin observed on the variousdipoles at IOA of VAN. The SES were selected by the study with the strip-chart records of VAN-group for the period from April 30, 1988

Ž .to September 15, 1990 Nagao et al., 1996 . The diagrams for the MT disturbances are made by digitizing the analog records of VAN everyŽ .5 min for the period of from 2100 to 2300 UT on April 4, 1988 circles with errors and the period from 000 to 0200 UT on July 9, 1989

Ž . Ž .dots with errors . The definitions of error bars are the same as those of Fig. 7. a Comparison between SES DVrL values for EW48Ž . Ž . Ž . Ž . Ž .X-axis and EW181 Y-axis upper left . b Comparison between SES DVrL values for a long dipole X-axis and those composed from

Ž . Ž . Ž . Ž .the values for the NS- and EW-short dipoles Y-axis upper right . c The same comparison between short dipoles as a except for MTŽ . Ž . Ž . Ž .disturbances lower left . d The same comparison between long and short dipoles as b except for MT disturbances lower right .


take almost constant values for any pairs of shortdipoles parallel to each other, and the electric fieldvalues on short dipoles are almost identical with

Ž .those on a long dipole Fig. 8b . We primarilythought that this indicated the uniformity of the SESfield in the area where the VAN’s short and long

Ž .dipoles were deployed several km times several kmand that we could exclude the possibility that thesource of the SES was located near the study area,also assuming the uniform electrical conductivitydistribution. However, the same comparison betweenlong and short dipoles for the MT electric field asshown in Fig. 8d indicates that the electric fieldcomposed from the two components of the shortdipoles is systematically larger than that actuallymeasured on the long dipole, whereas, the electricfields on short dipoles parallel to each other take

Ž .identical values also for MT origin Fig. 8c . Theabove difference between the electric fields of thelong and short dipoles for the MT electric field canbe explained by considering possible surface electri-cal conductivity distribution around the site as fol-lows. Since, with the long dipole, the VAN groupmeasures the potential difference between the Ioan-nina station on the resistive mountain body and thePerama town on the conductive valley sediment, theelectric field on the long dipole is averaged withelectric fields on the resistive and conductive areas.On the other hand, the synthetic electric field iscomposed from the electric fields of short dipolesinstalled only on the resistive body. The electric fieldon the resistive area is larger than that on the con-ductive area due to the static effect. If so, why arethe electric fields of SES distributed apparently uni-form in the area? This question cannot be fullyanswered so far since there have been only restrictednumber of observations in the Ioannina area. Whatwe can say from the present study is that the SESelectric fields have a different nature from that ofMT origin in the sense of DVrL test between longand short dipoles and that this may also be caused bythe differences in the locations and the configura-tions of the respective source fields. In order tolocate the source of the SES, more extensive electricfield monitoring with dipoles of a grid configurationin wider area together with the determination of thedetailed sub-surface electrical conductivity distribu-tion is necessary. A trial for observing the EM field

pattern around the site from the known artificial EMsource, which is usually used in CSEM methods,will also be helpful.

The assumption that the source of the SES islocated not in the hypocentral area but near the sitemay prefer to support the hypothesis of the localmechanisms for the generation of the SES such as

Želectrokinetic effect Dobrovolsky et al., 1989;.Bernard, 1992 , but the possibility that a conducting

channel leads the electromagnetic energy generated

Ž . Ž .Fig. 9. a All the mb USGS, PDE )5.1 earthquakes from April30, 1988 to September 15, 1990 whose epicenters are within 300

Ž .km of IOA. b The same diagram as in Fig. 7 except for thepossible SES on which the issuances of the earthquake predictiontelegrams of VAN were based. The signals with alphabets corre-

Ž .spond to the earthquakes with the same alphabets in a . Theregression line showing the preferred direction of the MT electric

Ž .field Fig. 2b is also added to the diagram.


in the hypocentral area to the vicinity of the record-Ž .ing site Varotsos et al., 1993a; Utada, 1993 still

cannot be ruled out. In either mechanism, however,it seems difficult to explain the following relation-ship between the SES field patterns and the corre-sponding earthquakes at the present stage. As shownin Fig. 9, the orientations of the electric field for theSES, A, C and EorF are much deviated from thepreferred direction of the MT electric field, althoughthe corresponding predicted earthquakes A, C, E andF occurred in rather distant areas from IOA. Inaddition, there is no systematic relationship betweenepicentral distances and the SES field orientations,and also between epicentral azimuths and the SESfield orientations. According to Varotsos and Lazari-

Ž .dou 1991 , in addition to the selectivity rule, theratio of NS and EW component of SES can also beused for the prediction of the epicenter of impendingearthquakes. This suggests that a specific hypocen-tral area is connected to a specific location andconfiguration of the ‘direct’ SES’s source in thevicinity of the recording site. In order to explainthem and determine the underlying physical pro-cesses generating the SES, the nature of the SESfield pattern, its connection with the correspondingepicentral area, and the mechanism of the respectiveearthquake have to be more thoroughly described. Amonitoring of the EM signals with wider frequencyrange in wider area is necessary. In addition, amulti-disciplinary continuous observations of electri-cal conductivity, micro-earthquake activity, strain,ground water level and ground water chemical com-ponent in the vicinity of the site will also be neces-sary.

5. Conclusion

In order to examine the validity of the earthquakeprediction technique developed in Greece by VANŽ .Varotsos, Alexopoulos and Nomicos and elucidatethe underlying physical processes, it is essential tounderstand the physical properties of the Seismic

Ž .Electric Signals SES on which their method isbased. In this study, we have tried to determine thelocation of the SES’s current source. For this pur-

Ž .pose, we performed the magnetotelluric MT record-Ž .ing of the ULF band 3–200 s in the vicinity of

VAN’s sensitive station, IOA.

By comparing the MT electric field of ionosphericor magnetospheric origin, electric field of lightningorigin and SES, the following facts were revealed.Ž .1 MT electric field varies completely in ENE–WSW direction, probably reflecting the significantcontrast in electrical conductivity between land andsea andror between conductive valley sediments andresistive mountain body, trending in NNW–SSE di-

Ž .rection. 2 The lightning generated electric fieldorientation is somewhat scattered but still holds theMT preferred direction. As its amplitude is larger,the deviation from the MT preferred direction is alsolarger, probably due to its different source configura-tion andror no linear saturation of the recording

Ž .system. 3 On the other hand, the SES electric fieldorientation is much more scattered than that of light-ning origin. These observations may indicate that thecurrent sources of some SES are located not in theepicentral area but in the neighboring area of thestation. It is still unclear how the SES, associatedwith distant earthquakes, are emitted from the sourcesnear the IOA station.

Acknowledgements

The authors are deeply indebted to John Makrisand Prof. Panayiotis Varotsos for their most gener-ous and uninterrupted assistance for the field surveyin the vicinity of the Ioannina station and letting theauthors refer their valuable continuous strip-chartrecords. They also wish to thank one anonymousreferee, Randy Mackie, Malcom Johnston, HisayoshiShimizu and Prof. Seiya Uyeda for their constructivereviews and valuable comments to improve this pa-per. This work has been partially supported by aGrant-in-aid for International Scientific ResearchProgram No. 06044085 from Ministry of Education,Science and Culture.

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