Research Article Field Detection of Microcracks to Define

Hindawi Publishing CorporationInternational Journal of GeophysicsVolume 2013 Article ID 651823 18 pageshttpdxdoiorg1011552013651823

Research ArticleField Detection of Microcracks to Define the NucleationStage of Earthquake Occurrence

Y Fujinawa1 Y Noda2 K Takahashi3 M Kobayashi4 K Takamatsu4 and J Natsumeda4

1 Risk Control Association Kanda Hirakawa Chiyoda-ku Tokyo 101-0027 Japan2 Tierra Tecnica Ltd Musashimurayama Tokyo Japan3 Communication Research Laboratories Koganei Tokyo Japan4OKI Engineering Co Ltd Nerima-ku Tokyo Japan

Correspondence should be addressed to Y Fujinawa hjfujinawaclcilasnet

Received 4 July 2013 Revised 21 September 2013 Accepted 28 October 2013

Academic Editor John P Makris

Copyright copy 2013 Y Fujinawa et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Main shocks of natural earthquakes are known to be accompanied by preshocks which evolve following the modified Ohmorirsquos lawin average over many samples Individual preshock activity however is far less systematic for predictive purposes On the otherhand the microcracks in laboratory rock experiments are always preceded to final rupture And previous investigations of fieldacoustic emissions showed that the activity increases prominently before and after the main shock But there is no detection ofany phenomena to identify the nucleation stage Here we show that a special underground electric field measurement could detectmicrocracks Pulse-like variations were classified into three groups (A B C) by frequency The B-type is suggested to define thenucleation period activity increases sharply following the modified Omorirsquos law before the main shock and there is no activityafterward The B-type is subgrouped into three types possibly corresponding to crack-rupture modes The variations are supposedto be induced by crack occurrence through electrokinetic effects in the elastic-porous mediumThe detection distance is suggestedto be several orders larger than that of the acoustic emission due to the effective smallness of dissipation rate and the waveformcan be used to infer the rupture mode

1 Introduction

The identification of anomalous phenomena in each stage ofseismic cycle is a prerequisite for understanding physical pro-cesses to be applied to earthquake forecasting (eg Mogi [1]Rikitake [2] and Scholz [3]) For the short-term predictionnumerous observations by seismic geodetic hydrologic andelectromagnetic approaches have been tried to investigatevarious phenomena to find precursory phenomena and tobuild physical and statistical models [1 3]

Among all the preshock has been considered to be themost direct and plausible phenomenon for identifying theearthquake nucleation stage Previous investigations of pre-shock activity of natural earthquakes or microcracks activityin laboratory experiments (eg [1 3] Jones and Molnar [4]Ohnaka [5] andMaeda [6]) have provided essential evidenceas accelerating activityThose evidences have been utilized tobuild the physical and statistical models of nucleation process

[1 5]Thesemodels have been further developed for instancesuperimposed undulation of foreshock activity was utilizedto infer dynamical critical phenomena to estimatemore exactfinal rupture time (eg Sornette and Sammis [7] Kapiris et al[8] Eftaxias et al [9] andmany papers referred in the report)

As for natural earthquakes however the evidence is gen-erally limited to average images There is large extent ofvariability in particular cases of earthquake with the result ofno plausible clue to be applied for prediction of individualearthquake (eg [3]) The most crucial problem is too smallnumber of preshocks to fit the characteristic formula ofthe evolution to infer the nucleation stage For instanceonly four and three preshocks were observed in case ofthe Hyogoken Earthquake (M72 January 17 1995) and theTohoku Earthquake (M90 March 11 2011) respectively Thesituation has not been improved even by the most extensivemicroseismic network (Hi-net) of Japan

2 International Journal of Geophysics

GPS clock

VLF

VLF

3-comp magnetic signals

2-comp horizontalelectric signals

V

V

V

1mDC-07Hz

ULF (001ndash07Hz)

Time

Central

station

Event recorder

Vertical electric signal

Borehole antenna

Dipole antenna

Magneticsensor

VLF (1ndash9kHz)

40sim100mΦ

300sim1200m

Figure 1 Schematic diagram of multiple component electromagnetic field observations [25 28] of the full option The electric field ismeasured by a pair of horizontal dipoles of 8m in length with a span of 20sim50m and using a vertical monopole of 600ndash1200m surrounded bywire with a radius about 10ndash20m buried at a depth of about 1mThemagnetic field is measured by three induction-type magnetometers (BF-6 EMI) installed perpendicularly to each other Triggered data in three electrical and magnetic fields are transmitted to a central station oncea nightThe present observation at Hasaki was conducted by the simplest model using only borehole sensor with continuous data acquisition

On the other hand the rock fracture experiment canalways observe the characteristic rapid increase of acousticemission activity in the nucleation period [1 3 5 10ndash14]Especially the accelerated increase of microcracks is shownto follow the modified Ohmori Law Detailed investigationon activity in terms of spatiotemporal distribution and focalmechanism of microcracks revealed physical process of rockfracture The investigation extends to the seismoelectro-magnetic phenomena (eg Hayakawa and Fujinawa [15]Hayakawa [16] and Eftaxias and Potirakis [17])

The acoustic emission technique has been widely usedin the geotechnical engineering field to examine geologicalmaterials since the late 1930s (eg Drnevich and Gray [18])The technique has been used for monitoring the mine andstones by using dense seismic networkwith spacing of severaltens of meters Concerning natural earthquakes howeverthere are only several pioneering studies to investigate fieldacoustic emission activities (egMorgunov et al [19] Hattori[20] andMolchanov et al [21])Those researches showed thatthe acoustic emission intensity is high about half-day bothbefore and after the earthquake occurrence The index of theactivities are limited to the strength and could not distinguishthe difference between the activity before the main shockand that after the shock without providing any useful clue todefine the nucleation stage

Electromagnetic approaches have been providing manyinteresting clues to shed light on preparatory process ofearthquakes (eg Park et al [22]) As for the nucleationprocess of natural earthquake Eftaxias has been investigating

in detail by referring laboratory and earthquake generationmodel by assuming that the VLF emissions are induced bythe microcracks [9 17]

In the present paper we present an electromagnetic fieldobservation ofmicrocracks by using high quality instrumentsproviding new clues to detect particular features to identifythe nucleation stage

2 Methods

21 Sensor We used electric sensors to observe microcracksinstead of ordinary acoustic emission transducer The sensorin the field is a special antenna made of a vertical casingpipe that is installed into a borehole of 200 to 1800m deepThe boreholes are not for electric field measurement butfor intrinsic purposes as measurements of seismic motionground water level and chemical contents ground deforma-tion or pumping of natural gas or waters for drinking and hotspring belonging to several organizations and private persons(Fujinawa et al [23ndash25]) The antenna is one of monopoletype immersed underground (Figure 1) of conductive crustsimilar to the towing tail cable antenna for submarinesrsquounderwater communication

The electrical circuit is composed of the preamplifierfiltering and data logging Additionally a simple countersurge circuit is inserted between sensors and preamplifiersto withstand electric surge induced by lighting dischargeThe recording frequencies are DC band (0ndash07Hz) ULF(0001ndash07Hz) and ELFVLF (1 kHzndash9 kHz)The data of VLF

International Journal of Geophysics 3

Figure 2 Locations of the electromagnetic field observation sites (e) in central Japan utilizing borehole antenna 150ndash1200m long as theprincipal sensor Those sites have been constructed during 1989sim1996 Every site measures the vertical component of the electric fieldAdditionally mutually perpendicular short dipoles of 20sim60m long were used for measurement of horizontal electric component at fivesites (Hasaki Chikura Koufu Nagaoka and Sagara) by 2002 And the induction type magnetometers were used at two sites (Nagaoka andSagara) by 2002 [25 28]

band is logged by an event trigger system focused to locatethe VLF pulse-like signal for early stage of investigationLater envelope of electric strength is recorded to monitor theactivity of the field Lower frequency component is recordedcontinuously with the sampling interval of 2 seconds Thepresent observation used continuous recording

22 Prior Observation Prior observation has confirmed thatthe system has high robustness against both meteorologicaland urban noise enabling us to get large signals related tothe crustal activities (Fujinawa et al [23ndash26] Takahashi et al[27]) Exceptions are the large noises induced by nearbylighting discharges and smaller ldquonoisesrdquo by severe geomag-netic disturbances compared with the normal state Afterconfirming the performance of the borehole antenna theobservation was conducted only by borehole antenna

The observation network has been built and operatedsince 1989 and the sites situate at several kinds of geologicalconditions nearly uniformly in central Japan (Figure 2)Several kinds of anomalies were picked up and investigatedfrom the points of view of finding and evaluating candidates

of precursor in relation with earthquakes volcanic eruptionand ground slides The ULF band anomalies have been mostextensively investigated because they were detected at almostcases of nearby volcanic eruption at Izu-Oshima andMiyake-Jima and seismic swarms as Hodaka in a geothermal regionnear volcanos (Fujinawa et al [25])

TheULFband characteristic waveformwas first identifiedby the temporally rapid recording at the time of volcaniceruption activities in 1990 at Izu-Oshima (Figure 3 Fuji-nawa et al [23]) The signals have never been detected in anormal state but observed in almost all volcanic eruptionactivities and seismic swarms occurring near the observationsites the earthquake swarm around Mt Hodaka in 1998(Takahashi et al [28]) and Mt Nikko-Shirane in 1999 andvolcanic eruption at theMiyake Island in 2000The phenom-ena have been suggested to be induced by an electrokineticeffect (eg [25 26] Nourbehect [29] Fitterman [30] andIshido and Muzutani [31] The ULF variation is called aldquogeyser-like ULF signals (GUV)rdquo because of similarity of thetemporal change of electric strengthwithwater height changeof the geyser We imagined that the whole of the pulse-like


(a)

0

20

40

60

80

100

120

0 10 403020

Time t (s)

(mV

)

(b)

Figure 3 (a) Records of electric field anomalies on 11 October 1990 related to the small volcanic activities in the Izu-Oshima Island by theborehole antenna [25 28] Slow speed recording shows numerous pulse-like signals (left part of (a) before 642 JST) Rapid recording by 60times (right part of (a) after 642 JST) shows the particular waveform similar to the geyser height evolution (b) Amodel of confined pressurechange is proposed to explain the electric field changes based on the electrokinetic effect

variations detected by the slow speed recording are of thistype We could not analyze higher-frequency phenomena indetail because of the systemrsquos slow recording speed

23 New Detector and Observation The present observa-tion was conducted using detectors having larger dynamicrange The experiment was originally to investigating thecoseismic signals with propagating velocity much larger thanthe seismic wave (Fujinawa et al [32] and references inthe literature) We installed a new detection system on 3March 2011 at Hasaki (Figure 2 now Kamisu) which is about300 km south of the epicenter of the Tohoku Earthquake(Figure 4 modified from Suzuki et al [33]) The boreholebelongs to the National Research Institute for Earth Scienceand Disaster Prevention (NIED) The frequency bands are ofDC (0ndash145Hz) and AC (145Hzndash9 kHz) AC recording bandis amplified by gain of 100 And the sampling rate is 45 kHzbefore March 11 18 kHz from 24 March 2011

24 Data Analysis To construct diurnal monitoring of DCand AC components we averaged in the interval of 1 secandor resampled by 150Hz from the original data sam-pled 45 kHz (by 11th March or 18 kHz after 24th March)Figure 5(a) illustrates the result of the averaging every secondfor one day data on March 25 Figure 5(b) is another recordfor diurnal monitoring using data through both resamplingby 150Hz and averaging of 1 sec Difference is in apparentbackground noise levels of 0003mV and 0040mV for theprocedure without and with 150Hz sampling respectivelyThe larger noise level for 150Hz resampling suggests theprocedure without 150Hz resampling is more suitable tomonitor the activity of the events It is noted that theaveraging procedure reduce the pulse strength with theresult of difference of pulse strength in the monitoring andindividual events After finding there are pulse-like events wepicked upwhole events of strength larger than predeterminedthreshold of 2mv from original raw data There are no other

procedures to depictmeaningful signals except those inducedby lightning surge in this method of measurement havinghigh robustness

3 Results

Figure 6(a) is the daily DC monitoring record on March 312011 corresponding to the calm state Except the backgroundnoises there were only diurnal variations due to the effect ofearth and ocean tide (eg Lai et al [34]) On the other handthe record onMarch 25 (Figure 6(b)) includes large pulse-likevariations Figure 6(c) is ACdaily record onMarch 4 showingnormal state for background noises of about 003mV seen asa belt except short time period of activity between 1700 and1900 (ldquoActivityrdquo in c)) Contrary to this normal recordMarch9 (Figure 6(d)) had numerous pulses

Whole pulse-like variations with strength greater than2mV about 40 times the background noise level of 0025mVwere picked up from the raw data There were a total of225 events The successive large pulses corresponding tonearby lightning have not been detected Most of the eventsare isolated in the sampled interval of 100ms except casessuperimposed small events as described in the discussionsection The largest event is of the coseismic variation cor-responding to the main shock reported previously [32] Theevent is taken up later in the discussion section Waveformsof the whole events were enlarged for the investigation oftheir characteristics and origin Those events are found tobe classified into three groups depending on their signalduration The typical waveforms in each group are shown inFigure 7 Whole pulse-like signals belong to one of the threetypes indicating that there are no extraneous noises

The type of an event (Figure 7(a)) was simply the so-called ULF geyser-like variation of the sort that had beenrecognized earlier (GUV) [23 25 26] by us as is shownin Figure 3 The ULF variations have been investigated alsoby many authors from the point of view for prediction of


HasakiHasaki

Figure 4 Geographical relationship between the observation siteHasaki (X) and the rupture zone 500 km times 200 km (dashed line)Calculated displacement distribution () of the Tohoku Earthquake(995333 magnitude 90) Also shown are the two largest preshocks whichoccurred two days before (e M73) and one day before (e M68) theearthquake (revised from Suzuki et al [33])

earthquake and volcanic eruptions by using electromagneticmeasurement on the surface (eg [8 9 21ndash23] Nagao et al[35] Revil and Saracco [36] and Han [37]) But there arestill no consensus on the characteristics of waveform andthe generation mechanism though there are large extent ofsimilarity in the waveforms and pulse width and uniformityof amplitudes

Types B and C events were firstly identified in thisobservation of increased dynamic range in our methodTheyhave a much smaller duration time from 10ms to several tensofms for type B and 03ndash05ms for type C comparedwith thatof A-type of several minutes

Type B pulses are subdivided into three types dependingon their waveform (Figures 7(b)ndash7(d)) Main phase of thetype B-1 (Figure 7(b)) is similar to type A in its abrupt riseand maintaining polarity similar to the relaxation waveformbut is different in decaying stepwise It is noted that almost allevents of B-1 contain higher frequency components

Similar electric variation without stepped decays hadbeen previously reported in association with acoustic emis-sion in rock deformation experiments [10ndash14] The electricfield variation associated with the acoustic emission wasdetected in the time period of stable sliding in the nucleation

period For the dry granite the variation has high similaritywith B-1 type in the points that rapid growth followedby exponential decay with dominant positive polarity Thedifferences to the present case are in different time constantsof 50120583 second and in compound rate of dominant phase oflower frequency with higher frequency components In caseof stick-slipmovement they found the variation similar to thewave packet type of B-2 without P phase Moore and Glaser[38] reported similar self-potential spikes with less sharp riseof electric field change in hydraulic fracturing experiment

The simple relaxation type electric variation was alsodeduced in the analytical estimation by Fenoglio et al [39]concerning magnetic anomalies associated with the LomaPrieta Earthquake1990 (Fraser-Smith et al [40]) In thatmodel confinedwater is assumed to eject as the Poiseulle flowduring crack opening inducing electric field changes throughthe electrokinetic effect

Type B-2 (Figure 7(c)) is a wave packet similar to theseismic wave and acoustic emission waveform But thedominant frequency is 1-2 kHz several orders smaller Inthe rock fracture experiment using wet granite Yoshida andOgawa [13] detected high frequency variations of dominantfrequency of 03MHz having similar waveforms as B-2

Type B-3 (Figure 7(d)) is a superposition of types B-1and B-2 with various rates of compounding The waveformcompounded by relaxation type variation can be seen alsoin the results of Yoshida and Ogawa [13] The startingtime difference of the two components is short enough tosuggest that they are due to the same dynamic motionThe source mechanism of the microcracks was determinedusing the moment tensor method by Sellers et al [41] andTimms et al [42] They revealed that many cases have anisotropic component corresponding to tensile opening orcompressive closure The mixed waveform B-2 is thought tobe caused by a crack of this composite type of mechanismThe catalog of all B-type variations is shown in the appendix

Type C (Figure 7(e)) is a wave packet with a carrierfrequency of about 5 kHz about ten times larger than that oftype B-2 and the shortest duration of some 05ms Amplitudereaches some 30mV several times larger than that of A andB type This type does not seem to be subdivided as B typeunder the limit of present sampling interval

In the crack theory [3] it is known that the displacementfield of a crack has three modes tensile mode (I) andshear modes of two types in-plane shear mode II and anti-plane shear mode III The space and time distribution andfocal mechanism of microcracks were investigated in rockexperiments [38ndash42] Lei et al [43] And it was found thattensile-type cracks occur rather uniformly in space and timewhereas shear-type cracks tend to concentrate in the area ofunstable movement Based on previous and present findingswe can suggest that type A and type B-1 correspond to tensilecracks types B-2 and C to shear modes and type B-3 to thecompounded mode

Figure 8 shows the time evolution of the number of pulsesper hour of types A (green) B (red) and C (blue) Theobservation of this session started 1300 March 3 2011 till1300 March 30 There are several time periods of recordinginterruption (horizontal grey bold line) owing tomalfunction


(mV

)

Hour

000

minus001

minus002

minus003

minus004

minus0050 3 6 9 12 15 18 21 24

A20110325 18kHzAVG1Sdat

(a) Simple average (1 sec)

00

10

(mV

)

minus001

A20110325 AVG1Hz 18kHzAlldat

Hour0 3 6 9 12 15 18 21 24

(b) 150Hz sampling and average (1 sec)

Figure 5 Comparison of two kinds of monitoring data (a) Raw data on 25thMarch 2011 sampled by 18 kHz and averaged for 1 second Smalldrift of about 0003mV is due to the limit of circuit performance (b) The same raw data through resampling by 150Hz and averaged for 1second

DCMarch 31 2011

386

388

390

392

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

(a)

386

388

390

392March 25 2011

DC

(mV

)0 3 6 9 12 15 18 21 24

Hour (JST)

(b)

March 04 2011AC ActivityActivity

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

minus010

minus005

005

010

000

(c)

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

March 09 2011

minus010

minus005

005

010

000

(d)

Figure 6 Monitoring records of normal state and active state (a) DC channel record on March 31 2011 corresponding to the normal statewithout any short period fluctuation of large strength except for a diurnal variation due to the earth and ocean tideTherewere no conspicuousenvironmental noises to make it difficult to detect anomalous signals (b) DC channel record onMarch 25 with pulse-like fluctuations around100 and 2300 The signals are very similar to those found in the analog record at several cases of crustal activities [25 26] The strength ofthe signal is about twice as large as the earth tide (c) The AC channel record on March 4 corresponding to the normal state except duringthe time period 1700ndash1900 During the calm state there were no large variations except for the atmospherics and environmental noisescomprising a horizontal belt-like part (d) AC channel records on March 9 corresponding to the most active day The amplitude of the pulsehas a distribution and the positive polarity dominates compared to the negative polarity because of the pulse-like signals are of B type havingB-1 and B-3 types of unbalanced polarity The peak value is diminished because of averaging procedure for dairy monitoring

of the instrument or electric power shutoff after the mainshock by 1300 March 24 Days from 12 to 23 (verticaldotted lines) are compressed in this figure In the followingstatement on time evolution of pulse-like events should beunderstood under the limit of this period of observationfailure Before the Tohoku Earthquake (ldquoEQ311rdquo in thisfigure) only B type events occurred except for only one pulseof the type A After the earthquake on the other hand typesA and C events occurred without type B events

From these results we can understand the dominanceof the positive polarity in diurnal monitoring record onMarch 9 (Figure 6(d)) On this day there are only B-typevariations as seen in Figure 8 The B type consists of threetypes B-1 B-2 and B-3The B-1 and B-3 types are dominated

by positive polarity as seen in Figure 7 On the other hand B-2 is not dominated by neither positive nor negative polarityThe compressed records result in the apparent domination ofpositive polarity

As for the B-type variations there are only several eventsper day on March 3 and 4 The number of events startedto increase on the 7th had a prominent peak on the 9tha pronounced lull on the 10th and recovered considerablyon the morning of 11th until the earthquake occurred Thelull does not mean there are no activity of microcracks Thesmaller the threshold is the more the number of events areThe feature is in agreement with the evolution of space-timedistribution of microcracks of rock fracture experiment [41ndash43]


Hour (JST)330 400 340 350

386

388

390

392 DCMarch 30 2011

(mV

)

(a) DC A-type

(mV

)

Second (JST)5309425 5309430

ACMarch 6 2011

minus6

minus4

minus2

0

2

4

6

001 s

(b) AC B-1 type

7086920 7086925 7086930

March 6 2011

Second (JST)

(mV

)

AC

minus6

minus4

minus2

0

2

4

6

001 s

(c) AC B-2 type

5333895 5333900 5333905

ACMarch 6 2011

Second (JST)(m

V)

minus10

minus6

minus2

2

6

10

001 s

(d) AC B-3 type

Second (JST)21232278 21232280 21232282

March 25 2011AC

(mV

)

minus30

minus20

minus10

0

10

20

30

0001 s

(e) AC C-type

Figure 7 Waveform of the pulse-like particular signals Every event has large strength compared with noise level with the result of nonedata filtering (a) DC band (type A) Type A has a duration of several tens of seconds to several tens of minutes and a height of about 2-3mVThe form was first identified by the temporally rapid recording at the time of volcanic eruption activities in 1990 at Izu-Oshima [23 25]Afterward we detected similar signals at almost cases of nearby volcanic eruption and seismic swarms [25] (b) AC band (type B-1) TypeB-1 waveform of the pulse-like signals in the AC band The form is very similar to type A except that it is of smaller duration (several tens ofms) and shows several steps in the process of relaxation There are events both with positive and negative polarities But the majority eventsare positive (c) AC band (type B-2) Type B-2 waveform of the pulse-like signals in the AC band similar with a wave-packet There is nocoda phase in contrast to seismic waves A similar pattern was detected in the case of wetted granite specimens with a dominant frequencyof 500 kHz [12 13] The S-P time was about 30ms suggesting an epicentral distance of 270m The absence or very slight appearance of a Pphase indicates an epicentral distance larger than a few km (d) AC band (type B-3) Type B-3 waveform in the AC band with a waveformcompounded by type B-1 and type B-2 There are different degree of compounding (e) AC band (type C) Type C waveform in the AC bandThe waveform is the same as type B-2 but frequency is about 5 kHz and amplitude is very large of 20ndash30mV some 10 times larger than thoseof types A and B types

The activity of the type B phenomena evolved similarlyto those of the acoustic emissions just before the rupture inthe rock experiment [3 13] and to those of the foreshocksbefore main shocks of natural earthquakes (eg [4 6 7]Varnes [44])The cumulative number of the foreshock periodevolves following the modified Ohmori law The character-istic evolution has been taken as one of the most directand clear evidence to identify the nucleation stage in thepreparatory processThe accelerated increase of strain causes

the particular form of time dependence intrinsic for rupturein the heterogeneous crust [7 8]

Fitting the modified Ohmori law (119905119891 minus 119905)minus119899 to the

cumulative number 119873(119905) (a black curve in Figure 8(b)) ofcracks at arbitrary time 119905 (hour) from the final rupture time119905119891 results in the index number to the exponent 119899 = 30 andto 119905119891 = 109 The present index number 30 is the largestcompared with 17 for the foreshock [44] and 05 for themicrocracks in the laboratory experiment [11] The assumed


8

4

0

8

4

0

12

8

4

0

12

EQ311

Even

t occ

urre

nce

rate

(Nd

h)Ev

ent o

ccur

renc

e ra

te(N

dh)

Even

t occ

urre

nce

rate

(Nd

h)

201133 38 312 324 328 41

March 2011

180

150

120

90

60

30

0

(N(t))

Cum

ulat

ive

num

ber

Date (JST)

201133 38 312 324 328 41Date (JST)

201133 38 312 324 328 41Date (JST)

N(t)

(a) DC A-type

(b) AC B-type

(c) AC C-type

Figure 8 The Tohoku Earthquake magnitude 90 occurred on March 11 2011 There are several time periods (grey horizontal line) withoutdata due to logger problems before the main shock and regional electric power shutoff (a) Time evolution of number of pulse-like signalper hour of type A (green) The unit is shown on the left hand side Almost all type A variations occurred after the main shock (b) Timeevolution of type B (red) including B-1 B-2 and B-3 The type B variation is suggested to be particularly useful for imminent predictionThebold continuous curve attached by119873(119905) shows the cumulative number of B-type variations The unit of the cumulative number is shown inthe right hand side (c) Time evolution of type C (blue)This type occurred only after the main shock when the after-effect stage of the seismiccycle

rupture time 119905119891 = 109 agrees satisfactorily with the actualvalue of 116 The characteristic evolution suggests that themicrocracks activity is related to the nucleation process of themain shock of the Tohoku Earthquake

The three types B-1 B-2 and B-3 occurred in nearlysteady percent 33 46 and 21 for thewhole periodHoweverthe compound type (B-3) decreased its role in the moreactive period (from 9 to 11 March) with the result ofincrease of activity of tensile type (B-1) and shear type (B-2) (Figure 9) In particular the shear type became dominantfrom the midnight of the 9th as approaching main ruptureThe tendency is also reported in the rock experiment [41ndash43]The index number to the exponent of inverse power law 119899 forwhole B type is 30 Individual fitting of types B-1 B-2 andB-3 to the inverse power law in the period from March 3 till1100 March 8 results in 119899 = 27 30 and 22 respectivelyThe index numbers to the exponent 119899 do not differ largelyin agreement to the near steady percent of each type in

0

15

30

45

60

75

90

B-1

B-2

B-3

EQ311

Cum

ulat

ive n

umbe

r (N(t))

Date (JST)201133 38 313

Figure 9 Evolution of the cumulative numbers 1198731(119905) 1198732(119905) and1198733(119905) for B-1 B-2 and B-3 respectively B-2 type tend to dominateas approaching to final rupture But there are no big changes inpartial importance in the activity


March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376

P wave arrival timeS wave arrival time

Figure 10 Coseismic electric field variation for the Tohoku earthquake (311) at Hasaki (lower sheet) and the strong motion seismograph(UD component) at nearest K-net observation site CHB005 (upper sheet (1) EW (2) NS (3) UD) The vertical line ldquoorgrdquo ldquoPrdquo and ldquoSrdquo areorigin time of 311 P wave arrival time and S wave arrival time at CHB respectivelyThere is very faint electric phase corresponding P arrivaltime and none phase corresponding to S arrival The first large negative peak coincides with the peak acceleration and the second prominentpeak did not correspond to any phase

comparison with previous result of 05 for rock experimentand 17 for foreshocks

Occurrence of higher frequency events of C type and lowfrequency A type after the main shock can be interpretedaccording to the after effect stage of the seismic cycle Itis supposed that there is a strong adjustment of stress andstrain in the rupture area inducing normal closure and shearslips (eg Yoshioka and Scholz [45]) causing high-frequencyacoustic emissions [11] similar to the C-type microcracksAround the joints on the other hand larger-scale confinedwater movements are assumed to occur similarly to casesof seismic swarms and volcanic activities inducing A-typeelectric variation of large duration of the order of minutes[25]

4 Discussion

41 Correspondence to Foreshocks Main Shock andAftershock Period

411 Foreshock Period The main shock occurred on March11 some 50 km south of the foreshock at almost the sameregion as predicted and was made public several years

before There occurred two foreshocks of major class oneof magnitude 73 (March 9) and other of 68 (March 10)The microcracks might be imagined to be associated withthe foreshocks that occurred two days before the Tohokuearthquake when the activity of microcracks was largestThere were no coseismic electric variations corresponding tothese foreshocks themselves as detected previously in 2002[32] And the detection distance of the characteristic electricvariation was estimated to be at most 100 kilometers [25 32]Additionally the epicentral distances of these f foreshocksfrom the observation site Hasaki are about 300 kmnortheast(Figure 4 Suzuki et al [33]) So that we suppose that the B-type variations are not related to the foreshocks

412 Main Shock It has been revealed that the great ruptureconsist of three subevents main asperity and subsequenttwo events [33] first in the northern part second succeededsouthward and third propagated from southwest to north(eg [33])

Anomalous electric signals corresponding to main shockare shown in the lower sheet of Figure 10 And the strongmotion records of K-NET (CHB005) near Hasaki arein the upper sheet There are coseismic variations but


the prominent peak appeared at around the peak accelera-tion and the second peak with opposite polarity appearedabout one minutes later But there are no significant electricvariations in correlation with the origin time and P S wavearrival time (ldquoorgrdquo ldquoPrdquo and ldquoSrdquo in the figure resp) as reportedin the previous reports [32] It may be caused by complexrupture comprised of three stages [33] We note that the timescale of the electric variation related to main shock is of afew minutes compared several tens ms of B and several msof C-type cracks On the other hand the maximum electricstrength corresponding to the main shock is 5mV nearly thesame as those of microcracks

The fault displacement shows a simple distribution [33]without any trace to separate rupture blocks We can thinkthat the whole fault including the southwestern region(Figure 4) ruptured simultaneously The time evolution ofthe B-type activity suggests intimate relation to the mainshock through Omorirsquos law (Figure 8) On the basis of theseevidences we can suppose that the microcracks of B type arerelated to the nucleation process of the main shock whichinduced the fault movement amounting several centimetersnear the observation site

413 Aftershock Period Aftershock occurred in an extensivearea in and around the main rupture of 311 During 3weeks after the main shock there occurred as many as 427aftershocks with a magnitude larger than 50 in and aroundthe rupture zone of 311 In this period there occurred A-and C-type variations of highest frequency of some 5 kHz(Figure 8) After the main shock there were no events oftype B and none of coseismic variations association withaftershocks The increased strength of AE related to thenatural earthquakes after the main shock [19ndash21] might becaused by this kind of emission

42 Radio Wave Emission Electromagnetic emission obser-vation inVLF bands has beenwidely conducted to investigatethe nucleation process It is generally reported that there areconsiderable correlation between occurrence time of earth-quakes and those of emission the strength of electromagneticemission increase before andor after earthquake (eg [2021] Fujinawa et al [46] Parrot et al [47]) Some reportssuggest that MHz emission occur several weeks before andthe VLF emission occurs only in the nucleation period [89 17] in contrary to the appearance of VLF type variation(C type) in this report We do not have data to discuss theMHz event because of the detection frequency that is less thanabout 10 kHz As for VLF emission we need supplementarydata such as location information [46 47] to know whetherwe are treating the same phenomena or not

43 Detection Distance The AE monitoring for rock burst isusually conducted with the spacing of several tens of meter(eg Hasegawa et al [48]) suggesting low value of Q factorof about 3 They found there are two kinds of AEs lowerfrequency ones of several hundred Hz and several kHz formonitoring rock burst for tunnel worksThese two dominantfrequency bands seem to correspond to those of B type and of

C type of our observation But we need more investigation atpresent to determine if there are any intrinsic reasons or not

The detection distance of seismoelectromagnetic signals(SES) by the borehole antenna is suspected to be some threeordersmuch larger than that of the acoustic emissionmethod[25 32] Here we do not concern the radio wave in theatmosphere they can propagate surrounding the earth Thelong detection distance of the electric signal detected fromthe origin time [32] was assumed to be due to the highresolving power of the observation system andor to the smallattenuation rate of the seismoelectric variation itself Anotherpossibility is the conversion of the seismic wave to electricmode at the conductivity contrast (eg [31 36] Gershenzonand Bambakidis [49]) The converted waves can propagatelong distance because of power law dissipation comparedwith the exponential dissipation The effective propagationspeed on the order of 100 kms or possibly electromagneticspeed in the earth is much larger than that of the diffusionspreading for the realistic conductivity of 01ndash0001 Smminus1 Wecan expect that the microcracks can be detected at a largerdistance by the electromagnetic measurement of high qualitythan by the ordinary technique using acoustic emission

44 Generation Mechanism Fujinawa et al [32] reported theremote detection of the Earthrsquos electric field variations fromthe start of fault rupturing at about 100 km from the faultregion using borehole antenna Investigation of generationmechanism of the seismoelectromagnetic signals (SES) (eg[36 49]) in the field indicates that the dominant mechanismis the electrokinetic effect compared with the piezomagneticpiezoelectric nonclassical piezoelectric and induction effects[32] A simple model of electric variation propagation inporous media using the basic Maxwell equations and elec-trickinetic interaction effects [25 26] show that detectivelength reaches to several tens of kilometers

The characteristic electric field variation induced bycracks through electrokinetic mechanism can be more sys-tematically discussed on the basis of formulation of Pride [50]and Haartsen and Pride [51] It has been shown by analyticalapproaches and numerical simulations that there are fourkinds of wave slow Pwave and fast P wave (ordinary P wave)S wave (ordinary S wave) and electromagnetic wave (EM)The theory is extended to no-Newtonian fluid (Araji et al[52]) and numerous application have been conducted on theinteraction of seismic wave and electric field in the porousmedia (eg [52] Schoemaker et al [53]) Han [37] triedto interpret the ULF geomagnetic variation induced by theslow slip by using formula of Pride [50] and fault model ofGao and Hu [54] The A-type variation after the main shockcan be interpreted as the electric field variation induced byslow movement of confined fluid though normal closure oropening in the process of rearrangement of strain or stress

45 Phases There are some events in B types which haveclear primary phase preceding main phase as illustrated inFigure 11 The primary wave (P) has very small amplitude ofsome one-tenth to the secondary phase (S) and majority ofevents have no trace of primary waveThe feature is similar to


the coseismic variation with the ordinary seismic wave [32]The tendency is the same for the whole cases of detectionsin the field But the feature is contrary to the simulationresults (eg [51 54] Ren et al [55]) The electric strengthinduced by P-wave is much larger than that by S-wave Wecan suppose that the tendency may change according tospecimens of rocks [54 55] or distribution of contrasts ofsediment Electromagnetic mode (EM) has been confirmedin the laboratory and simulation experiments [52 53] In thepresent observation there are occasionally small forerunnersas illustrated in Figure 11 The arrival time difference E-S corresponds to the electric wave propagation velocity ofsome 900 times S wave velocity But the EM phase is toovague to confirm the EM mode The slow P wave has verysmall velocity of several ms [52 53] and has too slightstrength hardly recorded in the present ability of recordingin agreement with the numerical simulation results that theindependently traveling EM The wave has strength threeordersmaller than those by seismic waves [54 56]

The S-P time of events A20110308 04h49m03s (Fig-ure 11) A20110308 23h55m23s A20110309 00h09m14s andA20110309 01h51m25s (Table 1 in the appendix) are 20 2517 and 14ms corresponding to 160 200 133 and 109m ofepicentral distances respectively But almost all events haveno or slight appearance of P phase It is supposed that S-P times are larger than 50ms corresponding to epicentraldistance larger than a few km

46 Complex Events Just before the main shock occurrencethere appeared some events superposed by numerous smallevents as illustrated in Figure 12The relative occurrence timeis considerably different with the different phase of the mainevents It suggests that small event increased to occur inthe neighborhood of larger events Another phenomenon isthat there appeared long chain of small events We suspectthat microcracks activity has changed at the last stage ofnucleation phase corresponding to intermittent criticalitycausing fluctuation of cumulative curve fitted to theOhmorirsquoslawThese complex and long chain events occurred only on 2days before the main shock without appearance on 10th 11thMarch that is in the lull period of microcracks activity Thephenomena may be related with the intermittent criticality[6]

47 Intermittent Criticality There appeared undulation ofmicrocracks activity after the most active period around 9thMarch Similar phenomena have been reported in laboratoryexperiments and field observation (eg [7ndash9 23] Huang et al[55]) The undulation has been suggested to reflect theintermittent criticality indicating entering another phase ofnucleation to interpret the superposed undulation of theseismic activity as the case of the Loma Prieta Earthquake [7]Another example concerns rapid change of magnetotelluricimpedance (Honkura et al [57]) some half hour before theIzmit earthquake (2000) in Turkey Eftaxias and his group[8 9] used their VLF radio wave emissions to investigatenucleation stage using their observation evidence of disap-pearance of the emission just before themain shockThey also

S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat

minus6minus4minus2 T0 (EM)

Figure 11 Sample waveform of events having the primary phase(ldquoPrdquo in the figure) before the main phase (ldquoSrdquo in the figure) The EMmode is suspected at around the origin time 1198790 (EM) consideringvery large speed ofmore than one hundred times that of fast P phase

(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat

Figure 12 A sample waveform of the event of B-2 superimposedwith many small events suggesting connection of groups of smallscale cracks This type appeared only on the most active day ofmicrocrack that is on 9th March (Figure 8)

tried to explain the IC based on fracture dynamics criticalphenomena and percolation theory

There is a disagreement in the VLF activity between someof previous results and present cases It is reported that thereis no VLF after the main shock in the report of [8 9] Onthe contrary we detect the C-type variation in frequencybands of VLF We suppose that present events are suggestedto be different from to the so-called VLF emission as thosereported in [46 47] and [7ndash9] We need supplementary datato compare different observational results For instance thelocation of the VLF wave rarely agrees with the epicentralregion [46 47]

Here we present the case that the activity is dominatedby the B types in the final stage superimposed numerouscomplex microcracks which suggests appearance of differentregime of the nucleation process but without drastic phasetransition

48 Asperity We suggest that the microcracks occurred inthe southeastern peripheral of the great rupture of length500 km in relation to the nucleation stage of themain ruptureWe can imagine that the accelerated strain accumulation isnot restricted to the so-called asperity region at around theepicenter of foreshock or main shock (Figure 4) Significantaccumulation is suggested to occur at the whole area of futurerupture with the result of microcrack activity though thereis difference in the intensity or type of cracks Extensivemonitoring of the microcracks can reveal these points toprovide a clue to answer this question


Table 1 Catalog B-type Pulse 2011-03-03sim2011-03-11

Number Second Hour Type Strength (mV) Duration (ms) Comment3-March

1 7408915 2058 B-2 minus35 40 P phase2 7699465 2139 B-3 minus55 30 P phase3 7775256 2160 B-1 2 25

4-March4 6375870 1771 B-3 115 305 6452210 1792 B-2 5 406 6548530 1819 B-2 minus115 107 6593720 1832 B-2 12 108 8114830 2254 B-2 minus30 10 P phase

6-MarchTill 1400 (no-data)

8 505711 1405 B-1 2 309 5309425 1475 B-1 38 4010 533368 1482 B-1 160 4011 6417930 1783 B-1 33 40 Clear P phase12 6599555 1833 B-2 65 1413 7086925 1969 B-2 35 2014 7590215 2108 B-2 95 2515 8320630 2311 B-3 minus22 4016 8423755 2340 B-2 83 25 Subevents

7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45

8-March23 1734305 482 B-2 80 35 P S phase24 2123245 590 B-3 124 45 P S phase25 2151230 598 B-1 41 37 HF At 1st step26 2609985 725 B-2 21 427 2687150 746 B-3 57 3728 3138805 872 B-1 70 45 Subevent in coda29 3330720 925 B-1 41 60 P S phase30 3362430 934 B-1 85 6031 3916385 1088 B-1 51 4332 3972780 1104 B-1 4 4033 4229730 1175 B-2 2 734 4445800 1235 B-2 2 1535 7588130 2108 B-2 22 836 7982130 2217 B-3 58 3037 8009035 2225 B-3 minus38 4538 8020315 2228 B-2 62 2039 8211700 2281 B-3 115 5040 8223460 2284 B-1 3 4041 8261735 2295 B-3 58 4042 8390435 2331 B-2 27 1043 8390440 2331 B-2 5 10


Table 1 Continued

Number Second Hour Type Strength (mV) Duration (ms) Comment44 8530720 237 B-2 41 1045 8588595 2386 B-2 minus117 2546 8612275 2392 B-3 minus28 30 P S phase47 8673830 2409 B-2 22 1048 8695340 2415 B-2 42 1549 8737920 2427 B-2 21 1050 8759000 2433 B-2 60 1051 8902735 2473 B-1 5 45

9-March52 249370 069 B-1 25 4753 381785 106 B-2 37 33 High freqcom at 1st step 15 kHz54 410935 114 B-3 83 4355 532710 148 B-3 6 4456 537460 149 B-1 22 3257 622180 173 B-3 21 4058 627240 174 B-3 35 50 Complex59 645495 179 B-3 54 6060 668475 186 B-3 89 30 P S phase61 709150 197 B-3 183 40 P S phase62 850115 236 B-1 3 3063 1140410 317 B-1 35 35 P S phase64 1204080 334 B-1 35 40 P S phase65 1218525 338 B-1 80 2066 1298365 361 B-1 75 50 P S phase67 1406705 391 B-2 63 30 P S phase68 1420680 395 B-1 5 40 P S phase69 1689185 469 B-3 113 30 P S phase70 1689200 469 B-2 41 1071 1911305 531 B-2 45 10 P S phase72 1919805 533 B-1 3 35 P S phase73 2117170 588 B-1 2 3074 2165755 602 B-1 35 4075 2213665 615 B-1 4 3576 2647925 736 B-1 35 3577 2783215 773 B-1 22 4078 2919110 811 B-1 3 3079 3082420 856 B-2 22 2080 3151155 875 B-2 22 1081 3364120 934 B-2 43 1082 3364935 935 B-1 2 3583 3532325 981 B-1 4 4584 3718160 1033 B-2 minus21 1085 3919376 1089 B-2 4 1086 4267460 138 B-2 4 4087 5406855 1502 B-2 2 1088 5513570 1532 B-1 113 4089 6615610 1838 B-2 minus51 1090 6677525 1855 B-2 51 3091 6736535 1843 B-1 4 4092 6757465 1877 B-2 3 27 Complex93 6765180 1879 B-2 28 10


Table 1 Continued

Number Second Hour Type Strength (mV) Duration (ms) Comment94 6776685 1882 B-1 37 3395 6799975 1889 B-1 3 30 Most complex96 6806310 1891 B-3 108 4597 6821165 1895 B-2 5 15 Most complex98 688016 1911 B-3 119 6099 6921145 1923 B-1 35 35100 6976960 1938 B-3 minus40 35101 7057330 196 B-1 31 35102 7116205 1977 B-2 29 28 Complex103 7149680 1986 B-1 41 50 Subevent104 7169470 1992 B-2 35 10105 7193950 1998 B-2 28 10106 7257375 2016 B-3 minus42 25 Complex107 7275465 2021 B-3 minus160 40108 7338265 2038 B-2 4 10109 7340905 2039 B-2 22 8110 7361915 2045 B-1 3 40 Complex111 7389335 2053 B-2 39 10112 7491870 2081 B-3 92 35113 7575080 2104 B-1 minus40 30 Subevent114 7635305 2121 B-2 97 10 Subevent115 7673230 2131 B-2 39 10 Subevent116 7795825 2166 B-2 43 15117 7870960 2186 B-3 105 40118 7890815 2192 B-2 minus76 10 Typical B-2 type119 7925825 2202 B-3 2 33 P subevent120 8004245 2223 B-2 21 9 P subevent121 8245120 229 B-2 4 10122 8263230 2295 B-3 65 30 P subevent123 8313870 2309 B-2 minus101 10124 8454140 2348 B-3 110 30125 8479530 2355 B-3 65 35126 8484135 2357 B-1 4 30 Subevent127 8534190 2371 B-1 44 34128 8535270 2371 B-2 6 10129 8537455 2372 B-1 3 45130 8767700 2435 B-1 5 38 P phase131 8883715 2468 B-2 31 10

10-March132 380015 106 B-2 41 20 P phase133 426210 118 B-2 22 22 P phase134 586960 163 B-2 37 15 Subevent135 702015 195 B-2 25 15136 787035 219 B-2 2 10137 811680 225 B-2 2 10138 1715675 477 B-3 120 40139 2411535 670 B-1 5 50140 2434845 676 B-1 55 37141 7560920 2100 B-2 105 35 P phase142 8405655 2335 B-2 2 15


Table 1 Continued


143 1327685 369 B-1 35 45144 1340030 372 B-1 2 35145 155065 431 B-3 42 45 P S phase146 1832585 509 B-2 22 20 P S phase147 2180145 606 B-2 4 20 P S phase148 2239530 622 B-2 70 10149 2250365 625 B-2 80 10150 2262850 629 B-2 3 10151 2289170 636 B-2 minus70 15152 2343295 651 B-2 55 10 P-S 40ms153 2348585 652 B-2 minus110 35154 2542175 706 B-2 4 15 P S phase155 2565820 713 B-1 125 35156 2728735 758 B-1 2 35157 2731950 759 B-2 minus85 10158 2738500 761 B-1 2 35 P S phase159 2863915 796 B-1 32 36160 342957 953 B-2 26 20161 3456455 960 B-2 41 10 P S phase162 3680460 1022 B-2 25 10163 4529080 1258 B-1 2 35 P S phase

5 Conclusion

We showed that the electromagnetic method can detectmicrocracks preceding natural earthquakes by a specialunderground antenna as an alternative to acoustic emissionmeasurementThenucleation stage of a crustal rupture can beidentified by monitoring the electric field variations focusingon particular waveforms in the selected frequency rangesThese pulse-like phenomena are suggested to be plausibleprecursors for an imminent prediction

Earthquakes are understood to be fracture of crustalrocks or stick slip of neighboring blocks Preshocks occurgenerally in the rupture preparatory stage and the activity hasaccelerated phase preceding main rupture But that featureappears when averaged for many earthquakes Preshockactivity of individual earthquake is less regular to be used forconfident predictive purpose

Microcracks are always detected in the nucleation stageof accelerated deformation in almost all laboratory experi-ments There are several efforts to investigate microcracksrelated to natural earthquakes by means of acoustic emissionmeasurements But they have not succeeded to find particularphenomena to identify the nucleation stage though theincrease of acoustic emission intensity has been detected justhalf a day before and after main shocks

We have been observing electromagnetic field changes forinvestigating the field characteristics related to earthquakeactivity and seismic wave in order for earthquake predic-tion and for application for early warning by using specialunderground antenna The measurement method has been

proved to be robust to the natural and man-made noises byobservational investigation since 1989 Here we show that theinstrument could detect microcracks appearing in the nucle-ation stage of the Tohoku Earthquake Whole large pulse-likevariations were grouped into three (A B andC) by frequencyand waveforms and found to characterize different phasesof the earthquake occurrence the preparation main shockand after shock processes Those variations are grouped bytime constants type A with duration of several to severalten minutes just the same as the pulse-like signals detectedpreviously at the time of volcanic eruption and seismicswarms B of several hundred Hz and C of several kHz TheA type was identified in the early stage of observation at thetime of volcanic eruption in 1992 and has been suggested tobe induced by seismic swarms and volcanic eruption TheB type variations are subgrouped into three waveform typeB-1 of similar to the GUV except in the stepped decay inthe relaxation phase after sharp rise type B-2 of wave packetsimilar to acoustic emission and type B-3 of superpositionof B-1 and B-2 Different types of electric waveforms ofthe B type are suggested to correspond to crack rupturemodes of tensile and shear and to be generated by confinedwater movement through the electrokinetic effects on theground of previous investigations on laboratory experimentsThe earthquake preparation period is characterized by theappearance of characteristic microcracks beginning periodof the nucleation period is characterized by rare occurrenceof A types nucleation period is by B types and particularevolution of the subgrouped typesThe second kind of cracks(B type) is shown to occur only in the nucleation period


activity increases sharply before the earthquake followingthe modified Ohmorirsquos law A and C occur at the stressarrangement period after themain shock Fieldmonitoring ofmicrocracks activity by EMmethod is shown to be practicallypossible to investigate the nucleation stage providing a break-through for the short term prediction method

Appendix

Waveform Catalog of B-Type Variations

We picked up 224 events under the condition of larger eventthan three times of back ground level All of the eventsare grouped in one of three groups without any undefinedones originating from urban and natural noises It is veryunique point of e-field observation by the borehole antennaAmong the three types the B type is the most interestingfrom the point of view of identification of the nucleationstage of natural earthquake We present a catalog of wholeB-type events consisting of 163 events Almost all eventsare compounded by B-1 and B-2 But events containingdominantly the tensile mode are grouped into B-1 and theshear modes are grouped into B-2 The grouping is done notquantitatively but qualitatively The results of grouping existin Table 1 The Table is constructed as follows

(1) column ldquoNumberrdquo sequential number of events of thetype B

(2) column ldquoSecondrdquo occurrence time (JST) in second ofthe event

(3) column ldquoHourrdquo occurrence time (JST) in hour(4) column ldquoTyperdquo sub-group type of the B event(5) column ldquoStrengthrdquo peak height of the pulse in mili-

volt(6) column ldquoDurationrdquo width of pulse in milisecond(7) column ldquoCommentrdquo several features of waveforms

appearance of P phase association of subevent com-plex type and so forth

Acknowledgments

This work was supported by JST Grant AS2121347A to OKIEngineering Co Ltd The authors thank T Matsumoto andY Okada (NIED) for support to use the borehole and FFreund andWCLai KHattori andQHHuang for valuablediscussions and comments Detailed and valuable commentsby Editor Professor J P Makris and reviewer K Eftaxiasare greatly appreciated The authors used data of K-NET atCHB005

References

[1] K Mogi Earthquake Prediction Academic Press Tokyo Japan1985

[2] T Rikitake Earthquake Forecasting andWarning Tokyo Centerfor Academic Publications D Reidel Publishing Tokyo Japan1982

[3] C H ScholzThe Mechanics of Earthquake and Faulting Cam-bridge University Press Cambridge Mass USA 2nd edition2002

[4] L M Jones and P Molnar ldquoSome characteristics of foreshocksand their possible relationship to earthquake prediction andpremonitory slip on faultsrdquo Journal of Geophysical Research vol84 no 7 pp 3596ndash3608 1979

[5] M Ohnaka ldquoA shear failure strength law of rock in the brittle-plastic transition regimerdquo Geophysical Research Letters vol 22no 1 pp 25ndash28 1995

[6] K Maeda ldquoTime distribution of immediate foreshocks ob-tained by a stacking methodrdquo Pure and Applied Geophysics vol155 no 2ndash4 pp 381ndash394 1999

[7] D Sornette and C Sammis ldquoComplex critical exponents fromrenormalization group theory of Earthquakes implications forEarthquake predictionsrdquo Journal de Physique I vol 5 no 5 pp607ndash619 1995

[8] P Kapiris K Nomicos G Antonopoulos et al ldquoDistinguishedseismological and electromagnetic features of the impendingglobal failure did the 791999 M59 Athens earthquake comewith awarningrdquo Earth Planets and Space vol 57 no 3 pp 215ndash230 2005

[9] K Eftaxias S M Potirakisb and T Chelidzec ldquoOn the puz-zlingfeature of the silence of precursory electromagnetic emis-sionsrdquo httparxivorgabs12115151

[10] X Lei K Kusunose M V M S Rao O Nishizawa and TSatoh ldquoQuasi-static fault growth and cracking in homogeneousbrittle rock under triaxial compression using acoustic emissionmonitoringrdquo Journal of Geophysical Research B vol 105 no 3pp 6127ndash6139 2000

[11] S Yoshida P Manjgaladze D Zilpimiani M Ohnaka and MNakatani ldquoElectromagnetic emissions associatedwith frictionalsliding of rockrdquo in Electromagnetic Phenomena Related toEarthquake Prediction M Hayakawa and Y Fujinawa Eds pp307ndash322 Terrapub Tokyo Japan 1994

[12] S Yoshida O C Clint and P R Sammonds ldquoElectric potentialchanges prior to shear fracture in dry and saturated rocksrdquoGeophysical Research Letters vol 25 no 10 pp 1577ndash1580 1998

[13] S Yoshida and T Ogawa ldquoElectromagnetic emissions from dryand wet granite associated with acoustic emissionsrdquo Journal ofGeophysical Research B vol 109 no 9 Article ID B09204 2004

[14] X Lei K Masuda O Nishizawa et al ldquoDetailed analysis ofacoustic emission activity during catastrophic fracture of faultsin rockrdquo Journal of Structural Geology vol 26 no 2 pp 247ndash258 2004

[15] M Hayakawa and Y Fujinawa Eds Electromagnetic Phenom-ena Related to Earthquake Prediction Terra Scientific Publish-ing Company Tokyo Japan 1994

[16] M Hayakawa Ed Atmospheric and Ionospheric Electromag-netic Phenomena Associated with Earthquake Terra ScientificPublishing Company Tokyo Japan 1999

[17] K Eftaxias and S M Potirakis ldquoCurrent challenges for pre-seismic electromagnetic emissions shedding light from micro-scale plastic flow granular packings phase transitions and self-affinity notion of fracture processrdquo httparxivorgftparxivpapers130113011045pdf

[18] V PDrnevich andR E GrayAcoustic Emissions inGeotechnicalEngineering Practice American Society for Testing and Materi-als (ASTM) Philadelphia Pa USA 1981

[19] V A Morgunov M N Lubashevsky V Z Fubrizius and ZE Fubrizius ldquoGeoacoustic precursor of Spitak earthquakerdquoVolcanology and Seismology vol 4 pp 104ndash107 1991


[20] K Hattori ldquoResults of acoustic emission observation at Mat-sushiro station Japanrdquo Final Report International FrontierResearch Group on Earthquakes (RIKEN IFREQ) GroupDirector Seiya Uyeda 2003

[21] O Molchanov A Schekotov M Solovieva et al ldquoNear-seismiceffects inULFfields and seismo-acoustic emission statistics andexplanationrdquo Natural Hazards and Earth System Science vol 5no 1 pp 1ndash10 2005

[22] S K ParkM J S Johnston T RMadden F DMorgan andHF Morrison ldquoElectromagnetic precursors to earthquakes in theulf band a review of observations and mechanismsrdquo Reviews ofGeophysics vol 31 no 2 pp 117ndash132 1993

[23] Y Fujinawa T Kumagai and K Takahashi ldquoA study of anoma-lous underground electric field variations associated with avolcanic eruptionrdquo Geophysical Research Letters vol 19 no 1pp 9ndash12 1992

[24] Y Fujinawa and K Takahashi ldquoElectromagnetic radiationsassociated with major earthquakesrdquo Physics of the Earth andPlanetary Interiors vol 105 no 3-4 pp 249ndash259 1998

[25] Y Fujinawa K Takahashi T Matsumoto et al ldquoElectricfield variations related with seismic swarmsrdquo Bulletin of theEarthquake Research Institute vol 76 pp 391ndash415 2001

[26] Y Fujinawa T Matsumoto and K Takahashi ldquoModeling con-fined pressure changes inducing anomalous electromagneticfields related with earthquakesrdquo Journal of Applied Geophysicsvol 49 no 1-2 pp 101ndash110 2002

[27] Y Fujinawa and K Takahashi ldquoEmission of electromagneticradiation preceding the Ito seismic swarm of 1989rdquo Nature vol347 no 6291 pp 376ndash378 1990

[28] K Takahashi Y Fujinawa T Matsumoto et al ldquoAn anomalouselectric field variation with the seismic swarm(1)- undergroundelectric field observation at Hodaka station (1995ndash1999)rdquo Tech-nical Notes 204 National Research Institute for Earth Scienceand Disaster Prevention 2000 httpdil-opacbosaigojppub-licationnied tech notepdfKJ-01 204pdf

[29] B Nourbehect Irreversible thermodynamic effects in inhomo-geneous media and their applications in certain geoelectricproblems [PhD thesis] Massachusetts Institute of TechnologyCambridge Mass USA 1963

[30] V Fitterman ldquoElectrokinetic and magnetic anomalies associ-ated with dilatant regions in a layered Earthrdquo Journal of Geo-physical Research vol 83 pp 5923ndash5928 1978

[31] T Ishido and J Muzutani ldquoExperimental and theoretical basisof electrokinetic phenomena in rock-water systems and itsapplications to geophysicsrdquo Journal of Geophysical Research vol86 no 3 pp 1763ndash1775 1981

[32] Y Fujinawa K Takahashi Y Noda H Iitaka and S YazakildquoRemote detection of the electric field change induced at theseismic wave front the start of fault rupturingrdquo InternationalJournal of Geophysics vol 2011 Article ID 752193 11 pages 2011

[33] W Suzuki S Aoi H Sekiguchi and T Kunugi ldquoRupture pro-cess of the 2011 Tohoku-Oki mega-thrust earthquake (M90)inverted from strong-motion datardquo Geophysical Research Let-ters vol 38 no 7 pp 1944ndash8007 2011

[34] W-C Lai K-C Hsu C-L Shiehu et al ldquoEvaluation of theeffects of ground shaking and static volumetric strain changeon earthquake-related groundwater level changes in TaiwanrdquoEarth Planets and Space vol 62 no 4 pp 391ndash400 2010

[35] T Nagao Y Orihara T Yamaguchi et al ldquoCo-seismic geoelec-tric potential changes observed in Japanrdquo Geophysical ResearchLetters vol 27 no 10 pp 1535ndash1538 2000

[36] A Revil and G Saracco ldquoThe volcano-electric effectrdquo Journal ofGeophysical Research vol 108 pp 2251ndash2270 2003

[37] P Han Investigation of ULF seismo-magnetic phenomena inKanto Japan during 2000ndash2010 [PhD thesis] Chiba University2012

[38] J RMoore and S D Glaser ldquoSelf-potential observations duringhydraulic fracturingrdquo Journal of Geophysical Research vol 112Article ID B02204 2007

[39] M A Fenoglio M J S Johnston and J D Byerlee ldquoMagneticand electric fields associated with changes in high pore pressurein fault zones application to the Loma Prieta ULF emissionsrdquoJournal of Geophysical Research vol 100 no 7 pp 12951ndash129581995

[40] A C Fraser-Smith A Bernardi P R McGill M E Ladd RA Helliwell and O G Villard Jr ldquoLow-frequency magneticfieldmeasurements near the epicenter of theMs 71 Loma Prietaearthquakerdquo Geophysical Research Letters vol 17 no 9 pp1465ndash1468 1990

[41] E J Sellers M O Kataka and L M Linzer ldquoSource parametersof acoustic emission events and scaling with mining-inducedseismicityrdquo Journal of Geophysical Research B vol 108 no 9 pp1ndash16 2003

[42] N E Timms D Healy J M Reyes-Montes D S Collins D JPrior andR P Young ldquoEffects of crystallographic anisotropy onfracture development and acoustic emission in quartzrdquo Journalof Geophysical Research vol 115 Article ID B07202 2010


[44] D J Varnes ldquoPredicting earthquakes by analyzing acceleratingprecursory seismic activityrdquo Pure and Applied Geophysics vol130 no 4 pp 661ndash686 1989

[45] N Yoshioka and C H Scholz ldquoElastic properties of contactingsurfaces under normal and shear loads 2 Comparison of theorywith experimentrdquo Journal of Geophysical Research B vol 94 no12 pp 17691ndash17700 1989

[46] Y Fujinawa K Takahashi T Matsumoto and N KawakamildquoExperiments to locate sources of earthquake-related VLFelectromagnetic signalsrdquo Proceedings of the Japan Academy Bvol 73 no 3 pp 33ndash38 1997

[47] M Parrot D Benoist J J Berthelier et al ldquoThe magnetic fieldexperiment IMSC and its data processing onboard DEMETERscientific objectives description and first resultsrdquo Planetary andSpace Science vol 54 no 5 pp 441ndash455 2006

[48] O Hasegawa Y Hosokawa T Mori A Takeichi and HFujii ldquoApplication of AE monitoring for rock burst and rocknoise phenomenon under tunnel constructionrdquo Japan CivilEngineering Society 2008

[49] N Gershenzon and G Bambakidis ldquoModeling of seismo-electromagnetic phenomenardquo Russian Journal of Earth Sciencesvol 3 no 4 pp 247ndash275 2001

[50] S Pride ldquoGoverning equations for the coupled electromagnet-ics and acoustics of porous mediardquo Physical Review B vol 50no 21 pp 15678ndash15696 1994

[51] M W Haartsen and S R Pride ldquoElectroseismic waves frompoint sources in layeredmediardquo Journal of Geophysical ResearchB vol 102 no 11 pp 24745ndash24769 1997

[52] A H Araji A Revil A Jardani B J Minsley andM KaraoulisldquoImagingwith cross-hole seismoelectric tomographyrdquoGeophys-ical Journal International vol 188 no 3 pp 1285ndash1302 2012


[53] F C Schoemaker N Grobbe M D Schakel S A L de RidderE C Slob and D M J Smeulders ldquoExperimental validationof the electrokinetic theory and development of seismoelectricinterferometry by cross-correlationrdquo International Journal ofGeophysics vol 2012 Article ID 514242 23 pages 2012

[54] Y Gao and H Hu ldquoSeismoelectromagnetic waves radiated by adouble couple source in a saturated porous mediumrdquo Geophys-ical Journal International vol 181 no 2 pp 873ndash896 2010

[55] Y Huang H Saleur and D Sornette ldquoReexamination of logperiodicity observed in the seismic precursors of the 1989 LomaPrieta earthquakerdquo Journal of Geophysical Research B vol 105no 12 pp 28111ndash28123 2000

[56] H X Ren X F Chen and Q H Huang ldquoNumerical simulationof coseismic electromagnetic fields associated with seismicwaves due to finite faulting in porous mediardquo GeophysicalJournal International vol 188 no 3 pp 925ndash944 2012

[57] Y Honkura M Matsushima S Barıs et al ldquoRapid changes inthe electrical state of the 1999 Izmit earthquake rupture zonerdquoNature Communications vol 4 article 2116 2013

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of


EarthquakesJournal of


Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining


Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of


Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering


GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of


OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in


MineralogyInternational Journal of


MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Geological ResearchJournal of


Geology Advances in


GPS clock

VLF

VLF

3-comp magnetic signals

2-comp horizontalelectric signals

V

V

V

1mDC-07Hz

ULF (001ndash07Hz)

Time

Central

station

Event recorder

Vertical electric signal

Borehole antenna

Dipole antenna

Magneticsensor

VLF (1ndash9kHz)

40sim100mΦ

300sim1200m

Figure 1 Schematic diagram of multiple component electromagnetic field observations [25 28] of the full option The electric field ismeasured by a pair of horizontal dipoles of 8m in length with a span of 20sim50m and using a vertical monopole of 600ndash1200m surrounded bywire with a radius about 10ndash20m buried at a depth of about 1mThemagnetic field is measured by three induction-type magnetometers (BF-6 EMI) installed perpendicularly to each other Triggered data in three electrical and magnetic fields are transmitted to a central station oncea nightThe present observation at Hasaki was conducted by the simplest model using only borehole sensor with continuous data acquisition

On the other hand the rock fracture experiment canalways observe the characteristic rapid increase of acousticemission activity in the nucleation period [1 3 5 10ndash14]Especially the accelerated increase of microcracks is shownto follow the modified Ohmori Law Detailed investigationon activity in terms of spatiotemporal distribution and focalmechanism of microcracks revealed physical process of rockfracture The investigation extends to the seismoelectro-magnetic phenomena (eg Hayakawa and Fujinawa [15]Hayakawa [16] and Eftaxias and Potirakis [17])

The acoustic emission technique has been widely usedin the geotechnical engineering field to examine geologicalmaterials since the late 1930s (eg Drnevich and Gray [18])The technique has been used for monitoring the mine andstones by using dense seismic networkwith spacing of severaltens of meters Concerning natural earthquakes howeverthere are only several pioneering studies to investigate fieldacoustic emission activities (egMorgunov et al [19] Hattori[20] andMolchanov et al [21])Those researches showed thatthe acoustic emission intensity is high about half-day bothbefore and after the earthquake occurrence The index of theactivities are limited to the strength and could not distinguishthe difference between the activity before the main shockand that after the shock without providing any useful clue todefine the nucleation stage

Electromagnetic approaches have been providing manyinteresting clues to shed light on preparatory process ofearthquakes (eg Park et al [22]) As for the nucleationprocess of natural earthquake Eftaxias has been investigating

in detail by referring laboratory and earthquake generationmodel by assuming that the VLF emissions are induced bythe microcracks [9 17]

In the present paper we present an electromagnetic fieldobservation ofmicrocracks by using high quality instrumentsproviding new clues to detect particular features to identifythe nucleation stage

2 Methods

21 Sensor We used electric sensors to observe microcracksinstead of ordinary acoustic emission transducer The sensorin the field is a special antenna made of a vertical casingpipe that is installed into a borehole of 200 to 1800m deepThe boreholes are not for electric field measurement butfor intrinsic purposes as measurements of seismic motionground water level and chemical contents ground deforma-tion or pumping of natural gas or waters for drinking and hotspring belonging to several organizations and private persons(Fujinawa et al [23ndash25]) The antenna is one of monopoletype immersed underground (Figure 1) of conductive crustsimilar to the towing tail cable antenna for submarinesrsquounderwater communication

The electrical circuit is composed of the preamplifierfiltering and data logging Additionally a simple countersurge circuit is inserted between sensors and preamplifiersto withstand electric surge induced by lighting dischargeThe recording frequencies are DC band (0ndash07Hz) ULF(0001ndash07Hz) and ELFVLF (1 kHzndash9 kHz)The data of VLF









(a)

0

20

40

60

80

100

120

0 10 403020

Time t (s)

(mV

)

(b)






3 Results





HasakiHasaki














(mV

)

Hour

000

minus001

minus002

minus003

minus004

minus0050 3 6 9 12 15 18 21 24



00

10

(mV

)

minus001


Hour0 3 6 9 12 15 18 21 24



DCMarch 31 2011

386

388

390

392

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

(a)

386

388

390

392March 25 2011

DC

(mV

)0 3 6 9 12 15 18 21 24

Hour (JST)

(b)


(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

minus010

minus005

005

010

000

(c)

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

March 09 2011

minus010

minus005

005

010

000

(d)







Hour (JST)330 400 340 350

386

388

390

392 DCMarch 30 2011

(mV

)

(a) DC A-type

(mV

)

Second (JST)5309425 5309430

ACMarch 6 2011

minus6

minus4

minus2

0

2

4

6

001 s

(b) AC B-1 type

7086920 7086925 7086930

March 6 2011

Second (JST)

(mV

)

AC

minus6

minus4

minus2

0

2

4

6

001 s

(c) AC B-2 type

5333895 5333900 5333905

ACMarch 6 2011

Second (JST)(m

V)

minus10

minus6

minus2

2

6

10

001 s

(d) AC B-3 type

Second (JST)21232278 21232280 21232282

March 25 2011AC

(mV

)

minus30

minus20

minus10

0

10

20

30

0001 s

(e) AC C-type







8

4

0

8

4

0

12

8

4

0

12

EQ311

Even

t occ

urre

nce

rate

(Nd

h)Ev

ent o

ccur

renc

e ra

te(N

dh)

Even

t occ

urre

nce

rate

(Nd

h)

201133 38 312 324 328 41

March 2011

180

150

120

90

60

30

0

(N(t))

Cum

ulat

ive

num

ber

Date (JST)

201133 38 312 324 328 41Date (JST)

201133 38 312 324 328 41Date (JST)

N(t)

(a) DC A-type

(b) AC B-type

(c) AC C-type




0

15

30

45

60

75

90

B-1

B-2

B-3

EQ311

Cum

ulat

ive n

umbe

r (N(t))

Date (JST)201133 38 313



March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376





4 Discussion






















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in









(a)

0

20

40

60

80

100

120

0 10 403020

Time t (s)

(mV

)

(b)






3 Results





HasakiHasaki














(mV

)

Hour

000

minus001

minus002

minus003

minus004

minus0050 3 6 9 12 15 18 21 24



00

10

(mV

)

minus001


Hour0 3 6 9 12 15 18 21 24



DCMarch 31 2011

386

388

390

392

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

(a)

386

388

390

392March 25 2011

DC

(mV

)0 3 6 9 12 15 18 21 24

Hour (JST)

(b)


(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

minus010

minus005

005

010

000

(c)

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

March 09 2011

minus010

minus005

005

010

000

(d)







Hour (JST)330 400 340 350

386

388

390

392 DCMarch 30 2011

(mV

)

(a) DC A-type

(mV

)

Second (JST)5309425 5309430

ACMarch 6 2011

minus6

minus4

minus2

0

2

4

6

001 s

(b) AC B-1 type

7086920 7086925 7086930

March 6 2011

Second (JST)

(mV

)

AC

minus6

minus4

minus2

0

2

4

6

001 s

(c) AC B-2 type

5333895 5333900 5333905

ACMarch 6 2011

Second (JST)(m

V)

minus10

minus6

minus2

2

6

10

001 s

(d) AC B-3 type

Second (JST)21232278 21232280 21232282

March 25 2011AC

(mV

)

minus30

minus20

minus10

0

10

20

30

0001 s

(e) AC C-type







8

4

0

8

4

0

12

8

4

0

12

EQ311

Even

t occ

urre

nce

rate

(Nd

h)Ev

ent o

ccur

renc

e ra

te(N

dh)

Even

t occ

urre

nce

rate

(Nd

h)

201133 38 312 324 328 41

March 2011

180

150

120

90

60

30

0

(N(t))

Cum

ulat

ive

num

ber

Date (JST)

201133 38 312 324 328 41Date (JST)

201133 38 312 324 328 41Date (JST)

N(t)

(a) DC A-type

(b) AC B-type

(c) AC C-type




0

15

30

45

60

75

90

B-1

B-2

B-3

EQ311

Cum

ulat

ive n

umbe

r (N(t))

Date (JST)201133 38 313



March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376





4 Discussion






















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


(a)

0

20

40

60

80

100

120

0 10 403020

Time t (s)

(mV

)

(b)






3 Results





HasakiHasaki














(mV

)

Hour

000

minus001

minus002

minus003

minus004

minus0050 3 6 9 12 15 18 21 24



00

10

(mV

)

minus001


Hour0 3 6 9 12 15 18 21 24



DCMarch 31 2011

386

388

390

392

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

(a)

386

388

390

392March 25 2011

DC

(mV

)0 3 6 9 12 15 18 21 24

Hour (JST)

(b)


(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

minus010

minus005

005

010

000

(c)

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

March 09 2011

minus010

minus005

005

010

000

(d)







Hour (JST)330 400 340 350

386

388

390

392 DCMarch 30 2011

(mV

)

(a) DC A-type

(mV

)

Second (JST)5309425 5309430

ACMarch 6 2011

minus6

minus4

minus2

0

2

4

6

001 s

(b) AC B-1 type

7086920 7086925 7086930

March 6 2011

Second (JST)

(mV

)

AC

minus6

minus4

minus2

0

2

4

6

001 s

(c) AC B-2 type

5333895 5333900 5333905

ACMarch 6 2011

Second (JST)(m

V)

minus10

minus6

minus2

2

6

10

001 s

(d) AC B-3 type

Second (JST)21232278 21232280 21232282

March 25 2011AC

(mV

)

minus30

minus20

minus10

0

10

20

30

0001 s

(e) AC C-type







8

4

0

8

4

0

12

8

4

0

12

EQ311

Even

t occ

urre

nce

rate

(Nd

h)Ev

ent o

ccur

renc

e ra

te(N

dh)

Even

t occ

urre

nce

rate

(Nd

h)

201133 38 312 324 328 41

March 2011

180

150

120

90

60

30

0

(N(t))

Cum

ulat

ive

num

ber

Date (JST)

201133 38 312 324 328 41Date (JST)

201133 38 312 324 328 41Date (JST)

N(t)

(a) DC A-type

(b) AC B-type

(c) AC C-type




0

15

30

45

60

75

90

B-1

B-2

B-3

EQ311

Cum

ulat

ive n

umbe

r (N(t))

Date (JST)201133 38 313



March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376





4 Discussion






















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


HasakiHasaki














(mV

)

Hour

000

minus001

minus002

minus003

minus004

minus0050 3 6 9 12 15 18 21 24



00

10

(mV

)

minus001


Hour0 3 6 9 12 15 18 21 24



DCMarch 31 2011

386

388

390

392

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

(a)

386

388

390

392March 25 2011

DC

(mV

)0 3 6 9 12 15 18 21 24

Hour (JST)

(b)


(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

minus010

minus005

005

010

000

(c)

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

March 09 2011

minus010

minus005

005

010

000

(d)







Hour (JST)330 400 340 350

386

388

390

392 DCMarch 30 2011

(mV

)

(a) DC A-type

(mV

)

Second (JST)5309425 5309430

ACMarch 6 2011

minus6

minus4

minus2

0

2

4

6

001 s

(b) AC B-1 type

7086920 7086925 7086930

March 6 2011

Second (JST)

(mV

)

AC

minus6

minus4

minus2

0

2

4

6

001 s

(c) AC B-2 type

5333895 5333900 5333905

ACMarch 6 2011

Second (JST)(m

V)

minus10

minus6

minus2

2

6

10

001 s

(d) AC B-3 type

Second (JST)21232278 21232280 21232282

March 25 2011AC

(mV

)

minus30

minus20

minus10

0

10

20

30

0001 s

(e) AC C-type







8

4

0

8

4

0

12

8

4

0

12

EQ311

Even

t occ

urre

nce

rate

(Nd

h)Ev

ent o

ccur

renc

e ra

te(N

dh)

Even

t occ

urre

nce

rate

(Nd

h)

201133 38 312 324 328 41

March 2011

180

150

120

90

60

30

0

(N(t))

Cum

ulat

ive

num

ber

Date (JST)

201133 38 312 324 328 41Date (JST)

201133 38 312 324 328 41Date (JST)

N(t)

(a) DC A-type

(b) AC B-type

(c) AC C-type




0

15

30

45

60

75

90

B-1

B-2

B-3

EQ311

Cum

ulat

ive n

umbe

r (N(t))

Date (JST)201133 38 313



March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376





4 Discussion






















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


(mV

)

Hour

000

minus001

minus002

minus003

minus004

minus0050 3 6 9 12 15 18 21 24



00

10

(mV

)

minus001


Hour0 3 6 9 12 15 18 21 24



DCMarch 31 2011

386

388

390

392

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

(a)

386

388

390

392March 25 2011

DC

(mV

)0 3 6 9 12 15 18 21 24

Hour (JST)

(b)


(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

minus010

minus005

005

010

000

(c)

(mV

)

0 3 6 9 12 15 18 21 24Hour (JST)

March 09 2011

minus010

minus005

005

010

000

(d)







Hour (JST)330 400 340 350

386

388

390

392 DCMarch 30 2011

(mV

)

(a) DC A-type

(mV

)

Second (JST)5309425 5309430

ACMarch 6 2011

minus6

minus4

minus2

0

2

4

6

001 s

(b) AC B-1 type

7086920 7086925 7086930

March 6 2011

Second (JST)

(mV

)

AC

minus6

minus4

minus2

0

2

4

6

001 s

(c) AC B-2 type

5333895 5333900 5333905

ACMarch 6 2011

Second (JST)(m

V)

minus10

minus6

minus2

2

6

10

001 s

(d) AC B-3 type

Second (JST)21232278 21232280 21232282

March 25 2011AC

(mV

)

minus30

minus20

minus10

0

10

20

30

0001 s

(e) AC C-type







8

4

0

8

4

0

12

8

4

0

12

EQ311

Even

t occ

urre

nce

rate

(Nd

h)Ev

ent o

ccur

renc

e ra

te(N

dh)

Even

t occ

urre

nce

rate

(Nd

h)

201133 38 312 324 328 41

March 2011

180

150

120

90

60

30

0

(N(t))

Cum

ulat

ive

num

ber

Date (JST)

201133 38 312 324 328 41Date (JST)

201133 38 312 324 328 41Date (JST)

N(t)

(a) DC A-type

(b) AC B-type

(c) AC C-type




0

15

30

45

60

75

90

B-1

B-2

B-3

EQ311

Cum

ulat

ive n

umbe

r (N(t))

Date (JST)201133 38 313



March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376





4 Discussion






















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


Hour (JST)330 400 340 350

386

388

390

392 DCMarch 30 2011

(mV

)

(a) DC A-type

(mV

)

Second (JST)5309425 5309430

ACMarch 6 2011

minus6

minus4

minus2

0

2

4

6

001 s

(b) AC B-1 type

7086920 7086925 7086930

March 6 2011

Second (JST)

(mV

)

AC

minus6

minus4

minus2

0

2

4

6

001 s

(c) AC B-2 type

5333895 5333900 5333905

ACMarch 6 2011

Second (JST)(m

V)

minus10

minus6

minus2

2

6

10

001 s

(d) AC B-3 type

Second (JST)21232278 21232280 21232282

March 25 2011AC

(mV

)

minus30

minus20

minus10

0

10

20

30

0001 s

(e) AC C-type







8

4

0

8

4

0

12

8

4

0

12

EQ311

Even

t occ

urre

nce

rate

(Nd

h)Ev

ent o

ccur

renc

e ra

te(N

dh)

Even

t occ

urre

nce

rate

(Nd

h)

201133 38 312 324 328 41

March 2011

180

150

120

90

60

30

0

(N(t))

Cum

ulat

ive

num

ber

Date (JST)

201133 38 312 324 328 41Date (JST)

201133 38 312 324 328 41Date (JST)

N(t)

(a) DC A-type

(b) AC B-type

(c) AC C-type




0

15

30

45

60

75

90

B-1

B-2

B-3

EQ311

Cum

ulat

ive n

umbe

r (N(t))

Date (JST)201133 38 313



March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376





4 Discussion






















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


8

4

0

8

4

0

12

8

4

0

12

EQ311

Even

t occ

urre

nce

rate

(Nd

h)Ev

ent o

ccur

renc

e ra

te(N

dh)

Even

t occ

urre

nce

rate

(Nd

h)

201133 38 312 324 328 41

March 2011

180

150

120

90

60

30

0

(N(t))

Cum

ulat

ive

num

ber

Date (JST)

201133 38 312 324 328 41Date (JST)

201133 38 312 324 328 41Date (JST)

N(t)

(a) DC A-type

(b) AC B-type

(c) AC C-type




0

15

30

45

60

75

90

B-1

B-2

B-3

EQ311

Cum

ulat

ive n

umbe

r (N(t))

Date (JST)201133 38 313



March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376





4 Discussion






















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


March 11 2011 144721 (s)

0 20 40 60 80 100 120 140 160 180 200 220 240 260

17422666

CHB005 (1)

17611349CHB005 (2)

5408633CHB005 (3)

385

390

395

400

(mV

)

1445 46 47 48 49 50 51 52 53 54 55(min)

org P S

minus17422666

minus17611349

minus5408633

5gal

5gal

9gal

9gal

1gal

1gal

456 s + 1446181 = 144737795 s + 1446181 = 1447376





4 Discussion






















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in

















S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in






S

P

20

46

(mV

)

1734300 1734305 1734310(s)

A20110308 4500Hzdat



(mV)

6880160 6880165 6880170(s)

minus8

minus4

0

4

8

12A20110309 4500Hzdat













7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in








7-March16 1695085 471 B-2 2 1517 2010365 558 B-2 33 1218 6448180 1791 B-2 21 1319 6857995 1905 B-1 22 3720 6914405 1921 B-3 42 4021 6955945 1932 B-2 21 922 7917740 2199 B-3 79 45



Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


Table 1 Continued




Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


Table 1 Continued




Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


Table 1 Continued



5 Conclusion








Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in



Appendix








Acknowledgments


References





































































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in


















































Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in
















Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in










Volume 2014

Mining


Journal of



Geophysics







Journal of




Advances in











Geology Advances in

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