Radon Monitoring Programs in the South Iceland Seismic Zone1977
Transcription
Radon Monitoring Programs in the South Iceland Seismic Zone1977
RadonMonitoring Programsin the SouthIcelandSeismic Zone1977-20O6 nill Einarsson,nnII Theod'rsson,Asb Rft Hjaflar&ifrir, Gisli Jdnssonand G Ai6nI. Guij6nsson ScienceIrstilule, Uni!ersily of lceland.Reykjavik, Iceland E-nuil I [email protected] ; [email protected] ABSTRACT Studi"r of tudon concentration in $'ater fran seothernal v'ells have been on.lucted $'hh interruption it1the South l.elanl SeisniL Zone since 1977 sp ed b! r1rcng indkations thot ndon changes occ ft?d prior to eaflhquakes. Three time seriet of radon chanc?s arc availeble: t ) An t 8 i'ean series of neaswements usnryu labo r intensivenethod (Lucas eU) on.lircrcte sanples. B) A ?rcatD inptt)ed method, usins a liq id scintiLLation(LS) detectoa used sinca 1999, whare 200 nl wakr santples --ere sent ttric? a week to ow labontorr C) Frcn a ?\|lr designed sreenl, A|to-Radon, that nakes conniuous neasurcnents oJ' adon in gcatlEnnol punzpinq stations. Tine serier AJot ha period 1977-1993 rcvealed nh\ earthquaker. Lotedrudon anonalies. They werc represented by both positive and nesatiw ?rcufioils Jron ttu nean wlues, and occvred nostb prior to the 'eisnlic events. Radon anomalies verc observe.l belore 30 ol the 9u si8,tificunt ?vnts satisj|hg selectio,t ctit ria. OJ all obsened anotutlies 35% werc rclated ta seisnicit\,80q.ofwllich wercpositbe. Samplinefot time senesR begana tear bclorc lte tse desnudive earth.ruakesoJ June 2000 (M,,6.5) occufte.l in the mi.l. e ol ou sanvlitus iattrorlL larye \iariatiobs ir ndon wrc obse^'e.l thttl werc etrelated owr the whole setsmicdllyactiw.one and were appaftntly related to tlv seismic evetts. P.eae*mtu &anses I 5 htottlL' before he events consisted oI a ilow decrcase oJ radon, i tettupted br positive spikzs. Co- teisnlic stepsa(conpaned the earthq@kes. The rcdon values decrcase.l at all stations, nost likely rclated to tlv co seis,nn change in grcund wuter prcssure obsen'e.l o\'.r th? $,holearea. Post'seismicret m to bft-seisnic levels about 3 months aLt?rthe earth(luakesi\ ptubabb also linked trith the prcssurc chahge itl the Pk\&rLnlr r)f1tpttlttntuti.nlt Bninionaiht: S?sbn.t Aea.hohkulpruLrnr! lor Edrth4uukts ) n.r' ,ierflrins ',' a,t, Auio"Radon. hr.r been d?.r.tapedfot .ontitlrousl\ uorlk)rint tulor in EtoutLltrutcr oi site. It is a si,rpte anrt tobu\t tiLtui.l nirlill.tti,t.otuttit\ srttcn. Th? rafl ts ol6 h.ounln,! p(rio.t dr( \(at oh.? .t\,,r 21h.urs h urS;,lS tttu'svg?toa|ertat co,t4rtt?t:Fours\stus areprcs.nttr ,i otJ.4tn11r,nhitl rhe s.isni( zone. at 0O: faullin I, INTRODUCTION Va.mus s(udicshrve suggestedthar a changein rhe concenrralionof radon in groundwareris an carlhLluareprccurso( secc.g. revicws by Haukssonlj l and King I2l. and recenrstudiesby Roeloffs l l l r D d T r i q L r ee r a l . l 4 l . E v e n r h o u g hn u n r e r o ues x l r n t t e so f p r e n r o n i t o rE y don anomatieh s ave bccn r(lcn(i1iedand (lescribedin rhe lireraLrrcstrtjsricd analysjsofrhe retaLi(DstriD bcrwee, rt]don i r r J . , , 1 1 . , t , . . l, ,k. r, ,b c ( r J i f r i ( . u lbt e l r u ' eo l l h r l r c t o f t , , n g, i T e . e , i e . i . , . u r . " , r , ) 1 . , i \ . arers. rs suitlble nnnriloring systemshavc not beennvailabte.Afrer t8 years cf tnoniloring rador, in grouDdw rer ilr Icchnd, using r labourinrensivernerhod(Lucascell), agreltly improved;ethod. uslngrllquidscinrillarion(LS)dcrccror.wasdevetopedarrheSciencctnsliruteintggg.where200 rnl $rter sanrples\\'.rc senr twice a week io ou. tabor{kry for radon measuremcntin:r simDlc autorlrxric\rnrple chrng€r'.ln ordertoget I continuousr"do, record anit rcducewo.kine rime a new ' r . f l n . A . I n R . r u ^ r )q. J . .J ( r e l o p .d I n r c o n r i n u o J m " e r " u r c m e nn( t r . d o n i n g e o r t r emr a t p u m p i n g statiolrs.Irr Lhispapcr we docunrenrand give an oveNiew of rheseprojecrs,borh rhe measurinir lech,)ryucand the apparcntrelationshit betwecn errrhq[akes and radoDcxcursiors. 2. THE SOT]THICET,AND SI'ISMIC ZONE AND THE EVENTS O!- 2OOO Thc Soufi ]cchnd SejsmicZonc is a lransfornLlypeplalebourdrlf, a branchof the mid-Alhnlic ptate boundaryrhar crosseslceland 15. 6j. The zone has bren defined by Jesrrucuonarers oi hrsrorrcar e:llhquakes,llolocene su|facerupruresand insrrumenraltyderenninedeoicenfes.I( is orienredE-W n n d i s l 0 l 5 k m w i d c .D e s l r u c t i o an r e a so f i n { : l i v r d u a t e J r r h q u JJkncJ\ s u r f J l ef J U i r r n ( F i g . t l q h o w . howc\cr, rh|r crch cvcnr is associrredwith tulking on N-S srriking planes,perpendiculrr10dre marn zonc lhe orer-xli leti lalerd rransR)flrnx)Lionatongttre (rre thus appears1c)be rccommoda(eoDy rght-hteral ixulting on many parnltct,rrnsverse hulrs aDdcounterctockwiserorrrioJrof thr bbcts bclrecn Ll'.m, bookshclflauhnrg" [71. Itanhquakcsin Soulh Icelandtend b occur in rnaio. sequencesin which mosLot rhe z_one rs rllaclcd. Thescscquences lasi fron a fbw drys to abour3 ycan. Erch sequenceiypicalty beginswith l m.gnllude 7 evctrI in the eftrcm part of rhe zone, fotlowed by sma er events fa(he. west. The sequenc.soccLrr,tl inreAals thal rrnge between45 ard j tZ years [71.and ir has be€n argted rhrr a coDpleresrrdinrele,rse ofrhe whole zoneis accomplishedin about 1.10yedis [8]. tn l9E5 rhelong trnrc snre tbe lasl scquenceled ro a long-rerm forecasl[9], rcfined in t993 [10]. oi a mrjor eanhquake scquencewjthrn the nexrdccldes.This forecasiwas tutfi ed in June2000 when 1wo magnirude6.5 cldrt\ r)cculcd ir the cenaal part of the zone. The seqrcncebeganon June I7 at l5:40 wirh a magnjlude6.5 evenr in rhe easkrn Darrol rhe ^ n c ( l i i g 1 ) .T h i s i r n m e d i a r e lryr i g g e r exd f l u n . yo t r c r r \ r r y J t o n S t c J \ r r q 0 L r n - t , , n g s r r ; r c h , , f t h e pldteboundrry Loihc wesr (e.9. | 1ll and tlll) lhe trrgesrcve.ls of wlrich occumedon N S faults.A Fig. 1 TI lo l0 k 3. RA THE1 T Obse varyin They a Rtdo, Monitorine Ptugra$ in the SouthlcebnA Seisniczore .... secondrnainshockof aboutthesamenagnirudeoccurredabout20 km wesrof thefirst oneon June21 at 00r5i. The senseof laulting was right-lateral strike slip conforming to rhe model of.,bookshelf faulting" for the SouthlcelandSeismicZone. 64'N 20'w Fig. I The locationof aheradon sampliDgstationswithin the SouthIcelard S€bntc Zon€shom with lriargles.Tbin Un6 showHolocencsurlsc€fEcturcg mapp€db€foE 2{XtO, tbe two thick linessbowthr sourcefaulb of th€ two e thquakssof Jue 17 dd 21, 2(n0, nxp€ctivety,s delineatedby afiershocks.Tbe senseof faultiDswasright-laleratstrik€-slipon both fsults. The main zonesolrupture wereabour15km long andfiacturedrhecrustfrom rhesurfacedown to l0 km dep& [3, 14]. Large hydmlogicalchangeswere observedin a wide areasu{ounding the seismicallyactivezone.Pressurechangesh boreholesfollowed a regularpattemconformingwith crustal stresschanges[15]. Pressuredecreasedin areasro rhe NE and SW of the epicentersbut increasedin the qua&antsto lhe NW andSE.Thesechangeswere large,but were reversedin a few 3. RADONSTUDIESIN SOUTHICELAND THET977.1993 TIME SERIES The first radonprojectin lcelandwasinitialedby Egill Hauksson,thenat Lamont DoherryEanh Obseflatory,in cooperalionwirb Universiry of lceland.The radonmonitoring netwoik in the SISZ consistedof up to 9 stations.Samplesof geothennalwaterwer€colected ftom ddl holeseveryfew weeksandsenlto the laboralor) for rddonanalysisby scinlillationcounring.The resutting.imesenes varyin lengthliom 3 lo 16years.Many eafthquake-relared radonanomalieswereidentified[ 1, 16.17]. They are representedby both positive and negativeexcursionsfrom the mean values,and occur mosdyprior to theseismicevents.A representative sampleis shownin Fig. 2. For a statisricatanalysis Pknrednus of IIE ktcmathnalRruinstomiu ststianon Geochenial P/e.ulets Jor Edrthquakes K a b e r h o l t l1 9 7 9 1200 r 1000 f; ooo & 600 400 200 0 J F M A M J J A S O N D Sampletime-series of radon at th€ strtion Kaldarholt for thc y€sr 1979. The only signincant earthquake for thh yer ws a nrgnidrde 3 eyenr oD September5. lt ws pBeded by. cl4r radon &onily denned hy 3 6ample.. The sMe evot w6 pE.ded by radon anomtlis in 6 timc seriasat 4 stations. Fig.2 wereselected of thennomirliesandcomparisonwith the se;smicitytime series,significanlearthquakes accordingto the criterir M=2.'1logD-0.43.mdM=2 o) where M is the magnitude and D is the distanceto a radon monitoring station. Thus 98 independent seismiceventswere selected.They were in the magn;tuderrnge 2 - 5.8. The main rcsults are as L Radonlnomalies were observedbefore 30 olthe significantevents. 2. l5o/, ofnll obserled anomalieswere relatedro seismicity- 3. 80% of the anomrlies observed before eanhqurkes were positive. 4 lf a positiv€,rnomalyis detectedat one statior. the prob)bility of a significanl earthquake i, 38q.. occurringatuerwards 5. Somesamplingsircswere found lo be more sensitivethanothers.Thesensitivityappearsk) dependon local geologicalconditions. 6. A few .ddon .momalies appearedto be relaled to eruptive activity of the neiShbouring Hekla Fig.3 Rado Scienc€lnstitu THE 1999 - TIME SERIES A new radonprogramwasinilia(edin 1999usinga new inslrumenr.h is basedon arolelliquid scintill ion technique,describedbelow 18, 19, 201.Samplingftom geothermalwells in the Soutb Icer:rndSeisnricZone begana year beforethe denrucliveeanhquakesotJune 2000 occurred.Wrer L Pre's days Ralon MMitoihg PtoBrMs it rhc South l.eiand Seisnic Zone .... -:- - :-_---_----_----tf r \ri ;.tt - - ;-l ,// ru.r R.rdon dnn€{eri€s of lhe s.atioDsS€rod!, Laugaland and Itr€mniskeid for the y€ar 2000. The stltion locatiolls arc given in Fi& 1. Ih€ premonitory radon low iB s@ in February-Mecb premotrl.ory spikes in February-April, the co-seismic drop in Jone, and the post-seismic Nvery in Sept€mber The bhck line is a fiv&point t:|ming lieEge. Verticd lins trdicate the 'txcess magniude" of €arthquak€s {t thc r€spective ltadon, i.e. the true ma8nitude ir €xc€ssof the th.€hold defined by fomuh O). samplesw€re laken about twice a week fiom geothemal hol€s at seven sites (Fig. 1) and sent to the ScienceInstilute for analysis. The June 2000 earthquakes originated in the middle of our sampling Fis. 3 A representative radon tilne series is shown in Fig. 3. The radon variations folm a distinct patlem thar €an be relaled to the earthquakes [2t]. The following features can be verified: l. h€-seisnic decrcaseof radon. Anomaloudv low values were rneasuredin dle Denod 101-16? davs beforc dle earthouakes. Ptrrrrdnu\.l!lr:.rt(ndonatLln*orni!St$n'nonGc 8 l. i L tunnai Pft(\ o r tl i i E d r t h r t r d k ? \ prior to Lhcearthquakcs' Pr.-seisnlicrncrcrse SPikcsxPleu in lhe tiNe serics'10 l'14 drys This is nrost Co seisnric step The t ldon vduc! decrcasert lhe iimc of the first earlhquakc lhe whole ove' likelt rclrtc(l ro the co seismicchrnge in ground i'rtef prcssureobservcd problbly P(\l seisnic .ctunr lo Dtf'seisntic Ievels abotrt3 months rlier the c'Lrrhqurkes rlso linked with lire pressutechrnge in th€ geothcrnr'rlsystems' OF ICEL4NI) RADON M\:ASURF,MEN'I'S AT SCIENCE INSTITUTE OF UNIVERSITY Itrdo|risnreasutedllscicncelnstilulcNilhlwollexible.clonclyle]ated'varirlionso|(heliqt'd operalcdin ouf labornlory seinrillr( ion tc.hr iquc.( I ) witir rD ^uLl)matr.samplech'rngs lbf I 2 s'r'rples a rsc c q u a r r v a d o nc o n l i N r o t r s llvz 3 l B o l h s ' t " s L c m L 2 2 lr. i d r 2 ) s y s r c m (sA u t o - R r d o nl)h r t m o n i r o r u\cthcslnre wrte' Ihcv on iDwatcrrDdin rif.butwe halc conccntraled i"it,,,,,","' -**'1"g,*r,,tr amplllld' "i."-nic unit (3x I0x20 cm'), whi'h hasxslabilizedhi-qhvollagennit rn ',,i;,.'p'"-r*tlr",l which is trsdl PC compurer: with a , .,,ri,.],,","" Lp'r* r*rgr,t anrlvsef and run;t for cornfirrnicathg lnd collcctirrgdrlr' for slaiing the mcdsurenrcnls n r o d eD s c d ' l h e s e n s i t i \ i t y. n d s t a b i l i t yo f t h e s y s t er ! i s b a s e ' lo n r h e s p e c i f i cc o u n t i n g pulse x delayed for '''Po unitlooks rlphr pulses Aftc'each pulse.rhe electronic dehllalcounrir! of I s t i o n r t h ebe'n w i f t i n a r r i v e l 6 m s ) u , , ,i i i ^ , . l * e l * r . q o Z . t t h e a i p h a p u l s e s o f ' ' ' P o0 r x l i f e 0 pulseo||ts|no!hc|Duclidc'],.Pb'Thee|cctronic[nitcounlsthesedc|xyedPu|ses|n.rscpxrrt. to about 3 pulscs pef hour ctrnnel ]'lis s|].ciflc couttnrg lrodc reducesthe r:idon btckS'orrlrd pulrs are $unted' rnd it all where nrode coLrtd.d Io i(I)o fulses pcr hour in noflntl counrioS ' rPo countralc of lhrce trmes a scintillxror' L 0 ml n.,io. tr," .y".u ii*t ""icmal di$rrrb.uccs Wnh I nBq/L ttl waler and li'i .',"'i,,"i i",,,,' ." "r ,he bncksround ( in 6 h perods) corrcspondsro about i B q / n r r i nr i r . measurirgradon in wllcr in the lxboratory' I00 or 200 n samplesre I.4D!LB1!4/!yr!r!!! For _{) o l l ' r r r c ' r r l l r J r r n t n e Jl r o n r l h i w ' r e r l u l \ r r r lo l \ ' r l l t r r r J | r \.urrr s)stcnr for rboul 5 rinutcs us'ng an inrnerrl oil brsctl. re usetblc) bv circulrting 'rir in a closed liquid scitrtillalioncountins svstcn)hr' O!,r!& !f,Sll!!! Auto Rudo! is a simple ,Ind robusi "C) rrom a well is walcr (75 100 gcorheflnxl The conrinurLrslynrorrirorirrgradon in sroundwLrter' rrrnLrdrsrnererrthr bnrh regrll:tres conducterlto Ltrerlerectrirrnrir via n l5 m long clPpet rubel2 mm L ' f h ( r h c l l o $( r b o u l1 0 m l / n r i n ) a n d c o o l s l h e w l t e r t ' r r t r u u t l l r \ r J L t r l h e n r u n s L h r o u g n J i r c ' n Thc rad(r ni.||uscs il"g ,lr":,"," u'1" -;1. *rrich is inside fie uppcrnros!part of the dctcotor^unil Aconcenrur'orl nrlvial). in a20 based, miner at oil ml. ih!;ush rhewir otlube xnd into rhesciuriltrlof(10 unr' fte Al equilib hours l0 in about is estlblished "qt,,f,LIl"", f..,t".', *^t"r rDd the scintilh((n h c ounLin! o f 6 r e s u l t s T h c i D t h e w l t e r ' t h r n r l t ] d r c , n c c n t r a r r oi ns 5 0 L i r D ehsi g h e ri n l h e s c i n l i l l a L f f corrrpurcr' a ccnllll to r essage prriod rrc scrrlofct elery 14 houts by rn SMS 4. CONCI,USIONSAND DISCUSSION lhrt crthqurkes 'rrc pre'flleo Thc lor g tcrn (ime serics()f radonlioln Soulh lcellnd stronglysLrggcsl 1helime series Thcsc {erles hy rrLtonrnomrlies. Nurrrefouserxnrplesof sLrchrcla(ionshipexist rn SouthIc 5. ACI PRI]PA E. Ptls REFE L f z t .1. I .1. l 5. I I i I I Rado, ltt,tit.in'k Ptusrahs in ttu Sodl l..taud Seieki. Zone.... sufer fromlow tine resolution,however.Many of theanomaliesarconly documenredby onesamplc dd nrdDy potentia! anom.]lies wereprcbablymiss€dbecause of thelow sarnpl;ng frequency. Several earthqu*eswere precededby anomaliesrecordedar more than oDesrarionand by more thd one sample. Theprogramthatstarted'n 1999hadhighersamplingfrequency andwa-sbegunjustin time tor lhe largeearthqual€sequence in 2000,by far rhe mosl signincanrevenl ior 88 yearsin rhis eanhqurkc pronezotre.Largechangesin radonwerc .ecordedwith high correlarionber eenrhe rado.lilneseries overlheentireare:rin thctimeperiodI 999,200L Starions ftat arerensof kilomerrcs apartshowverysimild nucruaiions in theconcentration of radoni2ll. Considering dresholthalf-life ofradoothesefluctuationscan hardlybe ascribedto materialtranspodbetweenthe starions.A connnon originof thefluctuations muslbe assu|ned. Two significant eventsoccurredin thecrustof Icelandduringthistinre,theHeklaemptionofFebruary26 March8 i24l andthec^.thquake South sequence ofJune.It mustbe considered very likely rhattheseevenrswererhecausalagentsfor the mdonflnciuarions. This conclus'onis firrtherstrengthened by thetemporalcorelarionof rhe poslea(hquake radonrccov€rywith lhe pore-pressure recoveryandits poro-elastic response observed byJ6nsson ct al. [25j.We believetheseobseryations areofcrucialimportance fo. lheunderstanding offte physicalmechnnism of premonitory radonnnorn.ies.The new progranof semi-coniinuors radonmonitoringprornises to bringdataofhigherqualityandbetterlime rcsolurionto castlight on therelationship betweenenrthqual(es, crustaldefomration andradonprecursors. 5. ACKNOWLT;DGEMENTS Theradonprogrumshavebeensupported by gtuts from manyagencies. includingthe tcelmdrc Res€drch Council. the Seismisproject, and the EuropeanUnion under the projectsPRENLAB and PREPARED. NunEfouspersonshaveparticipacdin theradonpljects andmeasurements. In particulal welike roneniior GndbjitrgArad6ttjr,Egill Hauksson.Johncoddard,SveinbjttmBjitmsson.Sigurdur E. Prilsson. Sigurj6rJiinsson. andthe altendants of tlrcsamplingsitesin SouthIceland. REFERENCES l. 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