Reprinted from Earthquake Prediction-An Inr€marional Revi€wMaurice Ewins Series 4

CopyriSht @ 1981 by the American Geophysical Union.

SEISMICIIY IAIfIRN IN THI SOUIH ICIIAND SEISI'fIC ZOTlf'

Pdtl Einar6son, Sveinbjdrn aJ6rnsson, and Cittia roulser,

Science rnstitute, University of Iceland, ReykJavik.

Fa8nu S 'c l6nsson and Th6rMn 5xaf tad6L ' i . .

Icelsdic Meteoroloaical oflice, Reykj avik.

Abstract. The South Iceland Sei6nic Zobe i6an E-W tfenitina zone of itestructive, hisioricearthquekes, ihat t8.kes up trasfom notion be-treen the subnarire Reykjanes lidge and theEastern Volcanic ZoDe in Southern Icetand.ItaJor earthquale sequerces affectilg lost of the'fo rm long seisnic zone recur at intervals !6ne'ins betveen L5 and ]12 yeals. Tte 6equence6often beein vith a lalge event (M> f) in theeastern part, of the zole folloved !y sinllar orsliehtly snalle! events falther rest. Sinsle,daaeing shocks oceu nore frequentlyr ed aleusually located at the ends ol the zone. Ins ! : L e o f l h e . l e a . C - l n a r i A m c n o f L h p e p i . e n -ters, no suface evidelce cd be fouit fof analor E-W striking fault. En echeton arrays offadts anil fissues de foud, ]rorever, inili-.atina right-1ate.&1 st?ike-sLip atolg lortherlystfiking fa!1t6. Destruction zones of individ-uat earthqu6.l<e6 a16o iend to be elongated in theN-S direction. Th€ distribution of recent nicreea.thquakes @d the suface faulting duin8 ldstlarAe earihqua.hes seeD to iriticate thai the seiericity is a66ociated vith brittle defornatio. ofa 10-20 kn ride zone located above a X-W trerd-ing zone of aseisnic defomation in the tovercnst or upper mltle.

No naJor eartbquake sequence has occuredsince 1896, ard the vhote zorc ba6 bee! veryquiet fo! the last 50 year6 of instruelta1 o!-se!@tion. Ju.lsine flon pasi history thele is an isn l robao i I i l J o f a l sse ear lhqLe lp occr r inein the Souih lceldd Seisnic Zone {ithin thenext fev decait€s. A nodest effo.t of earthquakeprediction ba6 beeb initi6ted, inctuilirg ladonnonitoring of eight geothernat welts ed theoleratio. of volaetric straiMeters at sevcn

lntroductton

the mjority of ilestructive esrihquekes inIceland sirce its setttenent iD ihe ninth cen-try A.D. o.currcd {ithin the South lceled

seisnic Zone. In ihis !6per re revies the his-t o " i c a l " e c o r d o " s e i s n i c s c t i v ' t J r r r r l i s _ o r - ,

and desclibe its plesent siate and type of fauit-ing. lle then give a short accodt of currebieffo.ts of edthquake lredtctton, Fron the hi6-torical record and the length of the l)fesentquiet perioit ve conctude that a najo! earth-qu tuc sFouFr .p :s i { - J Lo o cLr jn 'h is zonev .Lh in Lh- rF \ t fFv oe .aoe5. . -h - spqdcn.e i -expected to begiD vith an event of nagnitudeabout 7 in tne eastern part of the zone *tthsubsequent events of sonewhat snalter naglitude

teciontc 6etting

rhe South lceland Seishic Zone is a pdt ofL le D 'd -At 'anL ' p la lF bou da ly ' rs r c rossesIcetand. the loundey foltovs lrhe crest of theReyklanes Ridge southvest of Icelanit dd thecre6t ol the (olbeinsey Biitee to the north,within lceland the boundsry iE displeced to iheeast by i{o featules that rese4ble transromtaults i! hany !e6!ects, one in south Icelad,the other nes the north coasi. lhe irasfomf a l l r c h a l a c t e r i s t i c s e r e t h e L i e h s e i s n . i L J .occurrence ot lalee earthq@te6, the geonetricreisitonshi! to the spleading a*es, and the typean i t sense o l fau t t ins ( r i s . t J . So th zones , onthe other hed, tack the clear tologlaphtc ex-plession characteristic oi fGctae zobes on tbeocean floor, and in neither zone is the trans-lom notion ta.ken up !y a single, mJor faflt.The diverains plate boudalles in lceland aresidtarly conptex. In South lcel&d, for ex-4p1e, 6lreailina a!I)ear6 to occur in irc laral-Iet !iI'l zones, the Vestern and the Easteln Vol-can ic Zones (F is . 1 and 2) . on the Reyk jeesPeninsda the p1&ie bouduy is obfique, ud thevector of lelative plate notion plobably has aconlonent ol separation as {e11 as a stlike-sti!conlonert (xrein et a1., 19?3, 19??). rhe pen-insula iE an aea of hiah seisnic as velt as vol-canic activity, rhich is not usual fo! other

EIMRSSON ET AL. 141

!is. L, Epicenters anat iocal nechanlshs ofealthq!€.kes i! the Icetand reeion. Epicentersale fron the PDE lists of USCCST late! NOI-A andU . C S . ' o r t h e i i m c o - r i o o l 0 6 2 - . o ? 8 , o p e n c i _ -ctes denote epicenters detemined rtth fevetthen 10 P-rave readinss or eliceniels of ealth-quakes s@1ler than nb=1r.5 Dots ale events ofnb=!,5 sd latee!' lo.ated tith r0 or nole read-ihes . Large do ts a re events o f nb=5,0 ahdlareer. rhe foca.l necianisns are shoE schenat-icalLy as tover henisplere equal-srea lroJec-tions, conpressionat quadruts htack. I1re vo1-canic rift zon€s or Iceland ale shoh.

active portions of the plaie bouilary in lcelald.Near 21"!l the boundary iliviales into the l{estenVorcanic Zone, vhele the seisnicity is onty noit-erate, and the South Iceland Sei6nic Zone vhe.e@Icantc activity is resly absent. The SouthIceland Seisdic Zone blidges the sa! betreen thetvo votcuic rift zones in Soutn lcelad, andia.kes u! the trensforn noiion between tne Reykl&nes Fidse and the lasiern Volcanic Zone.Stefmsson (1967) suegested tnat, the larse esth-quakes in souih Iceland vere associated vithshear novements, vbi.h vas later sullDrted bythe distritution of nicroearthquakes (vard et€1 . ! 1969) and t {o rocs t nechar i6n so lu t ion6(I,{ard, 1971). tsr.l et at. (1969) and vard (1971)used the tern t'Reyklses Fractue Zone" for thestrote zone fron the vestern tip ol the ReykjdesPenin6ula to the Xastern Volcuic Zone. It hessince h€en customry to diviite this zoDe accoril-ing to t€ctoric characteristi.s into the ReykJa-nes Penilsu.La and the South Iceland Seisnic Zone(T lyssvason, 19?3, l inersson, 19?9J , 'h icb ea-tends eestvards f.on ibe Jdctton rith tbe l,est-e r r vo tcan ic zone near Hena i l l (F is . 2J .

Volcanisn i! the lastern Volcanic Zone culLar-ues south of its juctio. vith the South IcetsndS e i s n i c l o n - , b u l r l a o e s n o r s c c m l o b c a s s o - : -ated vith substatiat riitine. Featdes nolnal-

142 ITMRSSON !T Ar.

1y associated vith the axla.l lift zoae6, suchas fissure sralhE t nomal reults eil lons erup-t : v e f : s s , ! e s , E r F I s c k i n S r s d l h p v o l c e i cproducts ale chenicafly different fron thetholeiites of tbe diaL zones (J€-kotsson 1972,1979, saenEdsson, 1978) .

' {o ! th o^ Lhe Lrssrorn zonp .e -eDL . i f t .nsstructues occu in both the Eastern end the{estern volcuic zones indicatins thai clusiaI6p-ee. l rns Laxes p .ace jn bo th zones . l lea t -c loved structuat ilqta Eugge6t ihet nost of thecrustel ruteria] ln thls legion ls ploduced bythe lrestem Zone dd that the Easteln Zone mybe a relatively recent featue (PrilDaaon andSaenundsson, 19?4J. In tbe posiglaciat tine bothzoles have lroduced equal Modts of laea( J e k o b 6 s o n , ' 9 l 2 ) , a r o i r h i s L o r : e a l L : n e o n l Jihe Eastern zone has been volceicalty acttve,I'his nay he taken to indicate thai a MJo! chanAein the configrration of the plate boubdaries inSouthern Iceland is occding. l1]e locus orplste diversence seens to te shii'bins tuon theWester! to the Eastern volcdic Zone. This in-terlreiatioD inpties that the South lceland Seis-nic Zone is a youa featue, even on the ttnescale of tens of thousands ot years,

The h is to r ica l leco ld

written accouts of edthquekes exist fof nosiof the iine since IcetMd {as settleil in theninth century A.D., although not conienlFraryb e f o r e t h e l a l e r v F l l l h c e n L u J . " - l l e o u a l i l }aDd relialility of t,hese accodi6 is quite vari-abte. In Eone ceses the vriter hinsell vitless-ed the earthqualie, ir other ca6e6 he lived i! adittelehi part o? ihe coatry. Sone of the re-ports vere riiten a fev hundted years afte! theevent. Most oi the relorts are very snoft andusl)ecific, especially lor the iirst centufies,Ihorvs-1dur &orodd6en coUected all relortsavailable to hin in his books o! Iceladic earth-qu&kes and volcanoes (TholoddseD rA99, 1905,r u ) r . s s u d ustudy of the nitten docsents and assenbled atable of euthquakes stron8 enoDAh to cause theco l lapse o l houses ( in Tryssvason e t a I . , 1958) ,considereit by nin to initicate shaking oi inten-sity VIII o! noie. FEther studies by SveinbJ6mBla , rns6on (1975, 19Td) have revea led mo_F i rco ' -retion on the destluciion zones of inilividualeerthquakes (see !ie. 2), and reports ot aneerthquake in 1828 {e!e foud to be ineolrect.

latle t lisis destructlve esthquake€ in theSouih lceldd Seishtc Zone aDd slrors vhere dd-aee dd feulttng occulred. It is Ialeely basedoD fhorarinssonts tabler but only ihe ilistlictsalong the min elicentral belt are shom. Iheevent of 1828 is onitted, d eveni id 1389 isadded. The latter event is nentioneit in oneaccomt of questionable retiability.

Annats ed reports before 1f0O are considererlto be iiconl)lete {ith reslect to destructivee e r l h q u a k e s . I i s ' o l e v o - o y , i n p e . r i . u l a r ,

Lt

Fig, 2. Map ol the seisntc zones of Sw-Icelard, shdirA sone of the iectonic feaiueE of ihereykJanes PeDinsula anil the South Icetand zone. rhe destruction zone6 of !!€ euthquakes oir?32-31r. 1?811. 1896 dd 1912 6!e shom. {ithin these zones nole the 50t of houses at eachfarn vere totatty ruined. Corresponding intelsity is ItM VIII - Ix.

that ho ealthqE.|es arc nentioned ih the fif-teenth century, for vhich relotis of othe! kindsof natual disasters are also conspicuou6ly lack-ins (Trysevason e t a r . ,1958) . rhere is noleason to lelieve that this ras an dusuatly

The events of 1184 and 1896 stud out in Ta!1eI because ot ihe sideslread destruction theycauseil. Ih boih cases the d@se {as caused bya selies of ealthquakes and lot by a sinsie slrock.The iirst end talsest shock of 1?Slr occured onAUSU" l l r , ud caEcd d&sSe in 6n erLens iv "zone centered on the distlicts skeid ed Holt.lvo days later another shockr gonevhat snallerthar the first one, caused d@ge rcstly in rt6i.In 1896 the first ealihquke sttuck in the eve-ning of AuaEt 26 ed caused itestructioD ln azone centeteit on ihe Land ilistlict. Anothersbock, llobably about the s&e size, occurreitthe follovtng Dorni.a. The bighest intensitiesapled to have been in the see aleas a6 duiDs

the first shock sd the aeaee zone {as exterdedsligltty to the nofth. O! SepteDber 5 dmageros calsed ta!!he! rest !y fhat aplears to havebeen a double ealthquake. The destructlon zonesde clearly dtvidedt one i6 celtereit on Skeld abdHolt, the othe! on r16i and Ouus. relt relorts(Ihoroddsen, 1899) dd de6cliption6 of the seis-nogrm fron Strassbus (Rudo1ph, 19031 sullortthe sue. Fi"ther dMage vas causeil by shoclson Septenber 6 in Olfus ed on Septenber 10 in

Ihe manitude of the South lcelard earthquakeshes been a natte! ot 6one dislute. The only in-stluentatty detemined nasnituile i6 that of the1912 event. It is assigned a sulface vave Eag-nitude of 7 by Kdrnik (1969), Tlyssvason (19?3)useil the ladius of itestructiot to eEtinate thenaenitude of other eadhquhes, and concludeitthet tbe Ialgest event of 1?84 p.obably exceeded

'msnitude 7.5. Sinirarly he foud a nagdituileot 7-71"/2ta! the larsest eve.t of 1896. other

I

E1NARSSON ET AL. !43

lable 1. De5inctto! ed rau-LttnA arulne tdAe eerrhquak€B 1, Southeln lcelaDd.

- 9

9 d 9 ; ;

1161r1 1 8 21 2 1 1\29t !1 3 0 01 3 0 81 3 3 91 3 7 01 3 8 9

1 5 4 51 5 8 1

16r I l762\1 6 3 01 6 3 3r6571 6 ' 1 r1 7 o 6

r726t732l7 3lr1 ? l 9r 7 5 21 7 6 61? 8111? 89r8291 8 9 6

I X' t 1 ?' t 1 ? t

,!

?

?'lx

7 7 I

xx

x

€ +

x?

A n n o l s 6 t a t e t t : a r h o u 6 e s f e l t t n1 ! r s i n f e r ! e d f r o h e n n a 1 6 t , h a tC a s u ! l t l e s i n d i c a r e t h a t h o u s e s

t h i s d i s t ! i c t ,n o u s e s f e 1 l i n t h e d i . t ! i c t .c o l t a p 6 e d , h o r t t i k e l y t n t h e s e

severel tbibts ljo be edphasined about Fig. 2:1. fte deE!tuction zoneE are dru8ed ln an

F'V trendtas zoDe.2. !6ch desiactlo! zon€ ts eloaAateit, ,ith

the tone sl8 uEEtty oliented i,ransae!5etyto ihe min deisEic zone,

3. If the ee6 of the destluctiob zone is 6ieasae of the lagnitude of the ealthquake,the larAedt event i6 ihe one of t?8tr. l4osrothe eveDis ale equst io o! sDalte! rndthe 1912 esthqlake,

i t i E t ! t c t s .0 S u r t a c e f a u l i i n g n e n t i o n e d o .

T h e i t i s i r l c r s a r e s h o v n o ! a

pe inA t l re des t tuc l ion zones in r iA . 2 one nayf t ld ,TrygAv66on 's eEt lba tes a 1 i l r ie b ish .b le rqsson i t919J es t iba ted a lEan i lude o f ? .1ror the lalsest r?8! ealtlquake.

Afte! 1700 the quatjly of the edthqlale se_::y::

"1..1":_, od ir lecoee6 possible uo tuplhc_er |en t o f thp desL l l cL ion zones . The zobes

1.e . v jLh in Lhe j r losddres Dore rhan 5o t o f a l iour fd lna6 a t each fa ld ve lF , ! tnF i t . Ihere ere

144 IINARSSON Et AI..

L

l+. The Ialsest eatthqusl€s tend to occu in iheeasieln helf of the seisntc zo.e.

The leee, h i . lo r : . ea- thquakes can b6 tenLa-tively srouped into three cateaorles accolilinsto locEirion 4d tine sequence.

CaLeqow l : EsrLhquakes a t Lhe eas le .n end o lr r ' i - i6 i iFo : " r r i c ts RssAlver l i r 6hd Lsnd) ' ne8rthe luctton {ith tbe Easteln Volcetc Zotre. lhe

esthqua.kes ol 1912, 1829, 17261 1581 beloq to

this category. one ual. also vant to cout heleearihous,l<es associated vith ihe etuptlors in theviciniiy of Fekta rc1ceo in 1913! t8l8' 1?25and 1551a. EarthqEkes ililecttv associated vith

eruptions of lekla itself geleraltv tend io be

sna1le! (Th6ra!1n66on, 1967). Tle lalaesi ealilFqus,ke a6sociated vith the begimln8 of tne H€kta

eruDl ion 'n l9 l r7 vas o f mgEiLude 5 l l1 r e i l d idnoL cause 6r r aaage ( ' r lger . "s "n . -978b) . Ear t tFqus . ies d r ine thF erup t io r6 o f 970 a ta .C80 sFre

cate4on l l : EsrLhqGkes aL ,ne {esL 'm en i l"r- i f f i i i fatst ' i " r" olrus dd rt6j) , near rheluciion vith the testeln Volcantc Zone' such as

ihe events or l t1g, 1766, r7r2, \7t+9, r '106,16l,1597, !5\6, 1370. one n6nr also iant to coutna! . r te ear thquakes o f oc t . 9 . 1915 (M '6) anqApr i I r , l9cs ( I "F5 . ) ) , eh ich caused on .y n inordmEc in o rxs ( r ryeeEson, 1978s , loTo) .i6 notevolthy that lhe eairbquates of 1?66 and159? occurred itullng erulttons of Hekla 6t iheother end of the seisEic zone. Sinitarlv duingthe 19117 qekla eluption d edthquake sv8n(mxinm nasnitude [ 1/L) occurled in O1f!6(tryesvsson, 1978b). rn slite or the snau nag-nituite of the events, they caEed sone d@ge tohoEes and rere assoclated vith srface isrniinganit cheges i! hot s!!1n95. The earthqua,ke6 of

l?89 {e r - aeconpMied bJ sp ! r_ . ao le r i f t in8 o fthe fie.sill fault s*am in the \fesiern volcdic

Ca leaon TTT: Sequences o ' es rhq 'a ie6 l laL. . * -6 ; i * * r o r rhe zone. The seqJeJ ,eestarts rith a tarae eethqlake it the eastempart of the zone fotlded hy lalge, but stight-ly smlle! events faiihe! sest. The6e laieievents ale not afte!6locks ln a strici sense,6ince tley occu oui6ide of the area affected hv

the first larse event. Ihe sequences ol 1896'1'a8lr and t?32-3lr betone to tbis categorv' possl_

bly atso the earthquekes i! 1530-33 and 1389-1391. The dsation of the reJor sequence mv bequite variable, 2 days in 1f8!, 2 eeeks in 1896dd 2 yess in l?32-3!. The ealthquakes ot 1294!e!e acconpalied by taulting dd deage in &area ex le r o ina f roE the d su lc t ske io to Fars6 . -velti!. sinilally, the eerthqueke in 1339 ap-

Iess tohsvc a l fec ted Lhc le fge a-ea be teee l - l6 i

dd Rangenelli!, lhese large rocrcseisnic aessnay indtcate that nole than one ealthqueke oc-cEred in each case' and that these evenis be-long to catesoly IIL

This catesorization, vhich inctudes nost earih-quake6 for vhich enough infomation i6 avaitable,hss sone inpo.tdt inplications in telB orearthquake forecasiing.

-

1. MaJo! earthqua.ke sequetces, afrecting nosiof the set6nic zone (catesor:. IIII, occuat the avelage late of once !e! century.the irteivals betteen sequetrces ale 1f2r 50'99 , 239, 5A dd h , yea lg . oae shou. ld no te 'horeve!, that the 239 yeds lntervat inctudesthe lifteenth centda, for {hlch the hlstor-ical recold is knom to be inconlIete vithlesrEct to oiher kinds of events. lhe ave!-ase raie founit is thelefore nole likelv tobe loo 1d Lhs Loo h igh . E lc lud i ra Lne 239yea!6 lniervaf ihe lonaest one i6 112 vears.Wtth af] the staiistical tiEitatiols of 7s4!fes' one nalr colcluite thet there is Eorethd 80t protailllty of a naJor earthqualesequelce occulIng within 112 years of the_ a € L o n e , 1 , e , b e " o r e L h e Y e a ' 2 0 0 8

2. A far8e ealthqEke in the eastem hau oftbe seisblc zone ls like1y io be follotreil bvlarse earthquakes in the {esten !a!t a fevdays to a fev yess later.

3. A fe* (2-4) earthquakes uay occu! et eithe!edil ot the seisnic zone (categoaie6 I mdIIJ teteeen t*o successive haJo! sequences.

l r . Pr to - to a mJor 6equence p leoon i to ry e f fec tsof tone md intemedtaie duration ale o!1yerpected io occu tn the eastern !a!t of tlezone. In the vesteln part of the zone shor'!-tem effects are only to be expected after alalge event has alre6{v occuted in the east-

lecent activitY

Continuous Eeisuic recording be8u in Icelandin 192, (Tryssrrason, 1979). The f1.5t stationds in ley&lavik dd hail tro horizontal instru-nents {ith s nagnification of about 100. T.reinsirwntation vas ir.lloveil in 1951, vhen a ver_tical sholt-lelioil Splengnether lendu,Iu with amsnilication or up to !O0O va6 ailileil rhe oldhorizontal instluelts {e!e inst8.lled in ,tlru-eyri in North Icelsntt in l95li anit in Vt\ in SouthIcelmd in 1955. A short-pelioit station ta6 aitd-ed in sida (sID) in sE-Icelanil in t9t6 ed inlgilsstadir (Ecr) in 1967. A \{lrssN siation hssbee! in oDelation in alrdeyli (I!KU) stnce 1954.Nore of these 6tations ra6 located rtthin 1ro knof ihe Sorth Iceland SeisEic Zone. Since 197!there h6s bee! a steat incresse ln the nmber ofshort-period seisnogralh stations ilistlilutedthlouehout the active zones of Icelud. fllesLaL ions sh(m .n !1e . 3 vF"P eE+ab l ished tn 19?2( !v), r97! (Ax), 1976 (sL), 1977 (rR) ard 19?8(E!). n1e 6tation ST has been in inieroittentoperatton since 1976.

Elicenilal locations f,o! the last 5 years areshom tn !ig. 3. the tsgest ealthquake daingthis period reached naenitude !, but earthqukeses smlt as laenituae 1.5 are inctuded in the@p. The nost lronineni colcentlation ol se16-hicity occus in the Eenetll aea rea! the vest-eln eitge of the m!. this activitv, described

.in son; detail by roltae! dd Einalsson (1980),

i6 related to ihe hish teepelatule Aeothemal

EINARSSON ET A!. 145

Lv& -\4,I

I

/,1(t- ^ ^ a !

1' '/' r'^{* "il;*rt'';'

r fi ----.4

area and the tectonic coq)lexities nea! theIlenaitl centrat volc&o. Its connection wii'n the

south lcetard seisni. zone is uclea!. Farther

east the seisnicliy ts very diffuse and onlv

b lo€dt j dp l ineaL" - the se sh ic ?o ' P The _p i -

eeniers ale clearly not associated tlth a sinelefault. It is notevorlhy that latge Ildts of the

seisnic zone ale very quiet,earthquake Level, A cluster of elicenters lsfoud nea! Arnes in ihe central I)art of lrhe zone.Most of these eaents, ineludine the largest eteliin thi6 na!, occurreil in several snall sequencesin t9?8. Tne distribution of elicenters {ithin

the cluste. vasuely suggesis a X-S siluctue.The seishicity oi the south Iceland selsnic

Zone since 1930 is s@rized 1n a tine-space

ltot in ris. )r. The loteri.g of the detectionthleEhotd vith tine is quite apldent in thtsfigde. It is ctose to nagnituite 3 t/2 ror rl'e

tt!6t decedes but approaches ragnitude 2 for thelast years. In spite ot this, the nain fealrDres

of the seisnicity are denolstraled. In aene!€-1tne seisniciiy i6 ve.y 1ov. Onlv 5 events baetvreacb eegnitude 5. xsdhquakes tlat occured tnJuiy 196? are the only ones in tlre zone for vhicba local nechanisn solution is available. Ihesoluiion i.6 loorly eonstrained (l,rard, 19?1) butshovs s t r tke-s r ip feu l l ins (see F ig . 1 ) . Therelaiively liah nackeroud activiiy or the areaa - o u n d H e n g j l l s L a n d s o u . ' n t h i s ' i s u e 4 s i n

2lu 20wFis. 3. Epicehlels or ear-chquakes in sw-rceland duing 1911F?9' seisdc 6tati'ns de mrked tiur

iri"".r"".' n""a" {dashed 1i;es). 1ake3 (-aat!) ara rivers (-'1) are sh<m for oiientation

Fig. 3. A .en6!kab1e feature of the actiritv inthe central palt of the zone is its tendencv toeone in busts. Such brsts occured in 19?0 end1978, ror exsple. the activity is not finitedto one spot. Earthquakes occu ove! a vide eeavitnin eoch lust of activity. the soace vol_Me6 of the ealihquakes ale too sna1l fo! thento ilirectly influelce the occarence of eachother. It seens nore tikely that the events are

tlissereil by a rcgtonal strai! !u13e that affects

a lafse palt of the 6eisnic zone. Sone of theevents of 1978 {ete lreceded bv a detected 'adon

anonaly (Haulsson dd Coditard, 1981).

Fa!1ting

lviitence of recelt isdting is lowd in nmer-ous localities in the seishic zone. Historicataccouts nention suface bres.haee as6ociated vith

a fev eeithque,kes, but the descriptions a.e gen--_a , lJ sho- l ano in on v ve ly ' v "ases a_ ' lhey

6pecilic enoush to allov identification 'f the

larnts tn the iield. l"tost accounts p!io! io

19oo are coue.ied in Ttoroddsenis (1899) book'

{he!e faultine i6 reniloneil in as6ociaiion tith

the ear thquakes o f 129! , 1308, 1339, 1391, 1630,

178! ud 1896. I. aditition, fairlv detaitedde6cription6 of faxlting ue a@ilable 1n nets-p s l F r r e p o r t s o J t h e F a . r h q r e k ' o l 1 9 1 2 .

Bedrock outcrops are relalivefv scelce in the

146 IINAISSON !T AL,

1.t

1930

1 9 5 0

19 70

--

- : - = - F

2 tow 200w

Fis. lr. Tine-slace diagrs ot seisnlc sctivltv in the South Icelana Seisnic Zoie in the tine

in;erval t93o-8-0. rhe aire of occulence of the eartbqubhe t6 siven ss 6 iunction of loneitude

ol rlc elicenter. lhe length or the !e glae6 tbe nagnitude of the eveDt'

South Icelsnd Set6bic Zone. I4osi of the &ea

is covered by thtck €oi1, glaci€,1 noraines, aa

lavss and gtaciofluviatile batelial. The appe6r

ance of the faults ilelends stronglv on the natule

oi tle su.face natelial *he!e they are exposed'

Ihey are nost easity iitentifietl {he!e t-3 m of

soit ljes on top of lava, thich ts a co@on con_

dilion in ihe vesteln pa!! of the zone' in the

districts IL6i abd skeia. Ihe iaulis be'one ob-

sculed vhele the toose tuielial is very thick as

in the noraites in the disttic! Eolt' in ihe cen_

tret part of tbe zone. Farthe! east' i! the ilis-

irict lanil, nost ol the feDtts are exPosed in

soil!-covered tava. Ih one place e fault cuts

through the snooth srface of a ls,troehoe 1ata.

la c ! 'L - o f lhe i " {6 ! i sDr" a lpesrance. Lhere

sle e le{ featue6 chalacteristic ol nost of the

taults, ihat na.ke theh lileltiliable in the field'

Ca6es oi doubt adiiitedlv alise' especiatlv shele

the fauli traces nelge titb brooks, cattle treck6

ed old hriiue paths. flor 6truct8e6 in the

lavas can uLso !e deceltive, rhe nost !&ninent

featEes ot the fadts ue tensior e6shes, thai

se arranaeil i! en echelo! slavs (ris. t). the

sashes often aplea! as fissues at tbe suface.

In nany instaices the taf 13,ye! hes noi luptu-ed lut has sagged itdn to fotu 4 etongated ite-plesston. Tndtvidual gashes mv be tens of cen-

tineten to tens of meters tong. lhe en echelon

latiem cd !e seen on Dev scales, ranginA floDreters to a kiloneter, even alons the 6ue fault

ed supelinposed oD each othe!. The 1912 eafth-qu te , fo ! pxd l le ras accoqd ied bv fau l t ina,r."" ..ver"l fau.lt sesnenls of a req hhdFd

reteis to over a kilooeter in lensth. All the

s€dents exce lL one a .e d -Maed er eche lon in

a io "Lr , r re 'a : 'e zo ' " ( r l s . 6 ) . lach ses ten l :s

coEposed of 6horte! en echelon segEent6t 6'ne of

rhich also consitt of e! echelo! aadhes dom to

s Ecele of netels. In other ca6es the en echelon

lsttean i6 relativelv uilod a6 is shom nea!

ihe c.nter or rle. 5, lrolsblv lefleciing ui-. fon conilitions nea! the 6uface' in this case

veaetatetl 1at. Tbese taults bav be i,he faults

EINARSSON ET AL. 147

Iig. 5. Sdface fa!-ltine iluiig tko eerthquakes.one occured on Aug6t 26 or 27, 1895, the otle!feL l t las p .obab ly ac t ive in 'he earLhquakp o .

fo .nco i r rhe car rhqua-ke or '630 , c : red by T lo , -o i ldsen (1899) .

rne fault ilaces ale tn mny cases @ked !ysna1l muds oi soil and broken rock. The noundsaIe usuauy located letreen the tils ot tvo enechelon tension gashes, end have appelent1y beenprsr -d u ! dur ing the fau l t no le re i l . thc Lop orsone of the rcud6 has been cut into stlces sub-!e !a l l c l to hp d j recr ion o f the .e ,ghbo l tnsgashes. In a f4 aleas, vhele the ten6lon gasb_es are obscueit due io to.al conditions, the rowof bouds ls the !!ina!y ildicator of the laurtt race in the f ie1d .

The age oi ttre fautts is ucedainr *ith a f{p / . -p r ' ions , Ihe 19 t2 fau . l rs -a be ider . i - : . ias v - - - as rhe "sur ls in rhe r *d " i * t i ; ' ; ; ; -ciated rith the edthquakes of the 1896 sequence.tadts also novert in the Skeid dlstiict? plobabtydulihs tbe event o! Sept, tth, 1896, but theseare nor hard to dtstineulsn fron the !@erousofde! teults in the sde area. ?he fautts !re-smed to be f.on the 1630 ealthquake sele neD-tioned earller. Intelestingty, these seen to beas vel1 lreserved as the ones fomeil hole lna250 yers late!. Ii is ot gleat inpo.tance todat,e the faults r bui this nay prove to be diffi-cult, Tephrochronofoetr is plobebty ihe hosrproElsina tool for this !D!o6e. O,e/nay hop€to obtain a seisdc hisrory fo! a good palt of

148 ETNARSSoN !1 AL.

lir€lostglaciat tire, dld tbus be alIe to dete!_rtbe rore aeliably the recullence tlees of esin_qua,hes. tr inportant question is st6or vbethe!each fau.lt noves nole rhe once. So lar no ev1_dence has leen foud to ildicate repeated nove-@!t on the 5de iautt.

Atl fau1t6 that hsve been nalped so fa" areshoh in Fig. 6, as fa! as rhe scate of lne nana l l@6. More faJLs w i t t uooubted ly be a i ldeoto thi6 m! tate! si.ce on\l parts of tne zonehave been syste@ticalty 6ealched.

A11 the en ecnelon dlays identified so fa!ae ollented in a northerly dllection. 6 f*s1letLtly tqad6 the {IIE. Indivtdual iensrongeshes have a \rE trend, soneti@s quite valiablebetveen N and N60oE. ttle en echelon airanaeehtis nes l l y a lsays t , the sMe ser . " . *a rn i t .a r . .right-lateral disllacenent elong an uderfyinaf3u1t . verL ic8 l d ispacdents s le very rde lysee!, except locaUy vidhin an afay of tensionaasnes vhere blocks hare treen tili:ed- dohthromor lwhed up in the ta!.1t aoEds ite6clibed above.In one case the .e!6e of hotion along the iaulrcan be detemineit directly lrontue. A va11 built of tut ed stones crossesthe irace ol ihe fautt active in the eadhouakeso ' Iusus l z6-a? , 1896 ned +re fsn rsa s ;o t r tbthe district l,and. Ihe rall Is dis!1eed ?5-80cn righttat€rally rhere it croEses the fault.

Tiie stnctue and the appealdce of the f!ac-tules desclibeil irere is different lron th6t ofthe iissure6 dd faults of ibe volcelc lift zoDasof Iceland. There the fmctses de srouped irto6vams thathave tlends sinila! to thet of tbeflaci,ues theE6el@s. En ecbelon srrangenentscu be fouil rtthin the €vams, but the sense isnot atwan's the sde sd olten depends on thestrike of tbe faxtt rith leslect to the trend ofthe E*am. lscb sran has tlre stluctue of aarabelr and veriical dlsptacenentlis6urea are comon, t1re fra.tures ale acrlvedrine rilting eptsodes silila! to rhe culentevent8 in the (lefla !au1t svam in N!-Icetand.ed - .c d isp lsceben ls \ave been s^om !o be iDthe fon of {ldebins of the tissures and subsi-dence of the graben floo! (Gerke et ar., 19?8,B j6rDsson e t a l . , 1979, ln j l le . and R i t te r 1980,S iaurdsson, 1980) .

Faults of the Soulrh Iceland Sei6nic Zore veleprevlous\. itescribed by T.. EilsrssoD 11967, 1968)eit Tryesvason (1973) {ho both in!e!!r;t ueE aslishi-lateral sirike-s1t! faults. Tr, Einds6on,hoFeve!, only looked at individua.l sesnents,vhich have a nore nolth-easterly trend than theoveratl fractEe lattern, an.l generatized hislnterpletation to the {hote of the Icerudic rift,zoles, rhere indiriilual fissres have sidla!

rhe fault6 of the South Iceteit SetsDic zoneshom ia lig. 6 reveat a lattern {hich resentle6thet of the earthqua&e destruction zones in Fig.2: flle fau1t6 delineate ab !-H trendine zone,vheleas €ach indiriitual fault drsy ha6 6 N-StreDd. rhe edtbquakes seen to be associat,ed

Heslvatn Hek la

2fw 20"lis. 6. A s1@ary m! ol tbe Souih Icelanit Seisolc Zone, sho{ine the suface fauli breaks,seiBnic statlons. 6tfaimete. Iocatiols, radon sepling 6ites and the nmes or the naJorearthqu6ke iUstricts. livels and lakes are shom fo! orieDtatlon. Iaults in the vestern

!,art ot the zone sre from an unpultished nap by llelgt Torfason.

{ith brittle defomation of a 10-20 kn qrde zoneoveilyira en E-W tlending zone of aseisnic deto!-nation in t.he tove! clust or ulper hantle. Thedisllacenent along the 2one is left-lateta1truslorE notion to accomodate the slleading inthe Easterb Volcsic Zone, but the britile crustreslonds by right-laterat fa!-1ting elone the .on-jusate p1ales- rhe reaso. fo! tbis behaviour isnot clear, but it night be related to the appar-ent yolth of this zone, It is possible thai therinire dislrlacenent aloDe the zone is not yets u c c i " : - , o b - e d l h - r i o l e c . u s r a n d p " o d u ea naJor rhroughgoins fault.

Culent ltediction lesealch

Receni ad€nces in earthqua,ke llediction havehoiivated elfort to collect data that niaht berseful ir i\rtue predictio! vork in Soutl lcel&d.f , a - r . q ' o h F l r e i l i c L ' 0 1 h a s h : e h s i 9 i r i ' a n c e i nthis nain farbing area of lceland. AtthouAl thetoss of lite h6s not been gleat in past ea!!h-qusles, lbey have caused extenslve dsage arilecononic 1oss. A large ealthquhe lourd doubt-

tess lave a 6erious ihpact on the nodetn comui-ty not in this area.

The losslbility lentionea above, that the seis-licity niaht be lefated to silai! lulses of re_gional extent, Sives hole that tbe earthquales are

lreceded by !rccdsory effects detectab:le ov.�arconsiderabLe areas. Effects sueh as fo.eshock6and a!!oma-l dinal behaviour, Siven nuch atten-t ion ln Ch ina {Ra le ie l e t s l . ,1977, wa l lace andT€ns, 1980), are protJably not of nuch value hereNo sbnornal foleshock activity is relolted priorto any of the larae histolic earihquakes, andstolies ol abnomal aDirdl behaviou lrior toearihquake€ are !a!e.

Conditions fo! geolhysical nonitolina tolk aleCs i r l J la \o r3b le i r South Ice land. ThP 8"cE sffst, ed all larts of tt are easily ece6sable!y roads. Bedlock outclols c@ be fosil in nost

!eis, and a ldee nsber ot abatrdoned dlil1hole6 is available.

Itle cslent lredtction plosru is centereilarouil ihe nonitoting ot seisnicity, lailon andstrain. rhe irstrsents 6nd sml)tln8 6!e nostlvlakcn cere o ' by oca l peo l Ie . Lhus fo r r j rg a

'INARSSON ET A!. 149

lirk bet{een ihe scientists dd the 1oc41 po!!.14-

Seisntcity. In addition to the 6 seisEic sia-iiors shom in !ig. 3 thele ue 9 5tatlo!6 i@e-diately ouislde of the @p dea. I.ith the ex-ception of the station 1n ReykJavlk, all the sta-tions have a veiticel, sholt-1)eliod sei6aonete!.Ihe signals are recordeit rith len and ink onpape!, The vtslble recordina is m inPortetieaiure of a nonitoling syBten in a votcdic area.Radio tine is useil on aII the station6, thus:linilating clock colfections. the sei of 10-ratable eerthqukes 1s cotrllete alove naenitude?, lut events dom to at tea6t @gnituile t areletecteil.

Fadop. Disclete s@!li!a of geothemal fluidrof rsilon detemitation tss initiated et 8 5iie6in soutb Icelanil in 1978. A de6cripiiion of thisr.oarm is siven by l{a!.ks6on od coddud (1981).iMples ere taken every 1-2 reek6 ard setrt toleykJavik fo! anatysis, rhe plesent sdpling:ites are shom u lig. 6. Setelal dobaliesrave been o!6e!ved pdor to ealthqu6kes, dd iheiortheimost statio! (rlirdi!) alpeus to be rcstrensitive to radon ehanges. A continuous radonneter has nd been installed at that station.lhis lroJeet is !u in cooperation betreen thej.ience Institute and Ieont-Doherty GeologicalDser€toly, lhe very lronisi.e resrrlts of thisrork hale plovided stinu-Lus ror "urihe. predic-

strain. sacks-Eveltson votuet!ic 6traimeters,eiE-liJtarrea in 1979 at ? sites (Fie. 5). artetr settline pelioil of about a ronth alt iDsl;ru-)ent6 sh*ed strain bu+d-u! at a nore or less:o!€tant !aie. Tnis proJect is ru i! coopera-.ion letreen the Iceledic Meteorological Oflicedd the Cahesie Institution i! washington.

other uojects. Ehe ls.ke se5tvatn (Iis. 6) is.scd a6 e reference leve1 ror the @n\lgrins of' l t chaDses in Lhe cent la .L !e ! t o f thase i6e ico!e. Sevela1 bench$dks have leen instaued.roud the 1a.ke in cfiffs dire.tly above theate!. Chanses in lelative lake levef can beeasuled dlleetly rith a luler. rhe leasue@entsave to be mite on celn days. A tilt ?es6lnitoni 10-o can be obtaiDed in ihtsiay, {aiet 1evelauses are also operated doud the lake ty EaillalhssoE and Roaer BilhM of lmnt-Doherty Geol-gicel obsenatory lo! the 6a@ papose.Severel eeodiheie! lines h4ve been neasuleil.

he intent is io reneasule these tines and e*cendhe geod ihe ter ner lo rk ov- r th - eho le zone i ,

Acknosledsenent s . Nm€rous initividuals have.ntribuied to the lesearch de6c.ibed iD thisa!e!, desiane.t instrrcbts, opelated nonitolingletions, nel)ped fautts, analyzed seisnic lecords1.. Future vo.k in South Iceland deleDds crit-:elLy on continuing coope.ation vith these:o!Le, Dr. EeLsi Tort4son alloved ihe usl ofis upuhlished m! of faLlts in the vestein paltf t ie se isn ic zone.

'O EINARSSON ET AL,

The coDst.uction aDd operatlon of sone of the'€eisEic staiiols yas linanced by the NationalEner6| Autbolity of Icelald dd by NATo Resealch

Sisddu {tolallnsson, Eysteia Trysesason,dd two sno4lmus reyle{els Gde heflful susses-tioos fo! the inlrov@ert oi the @u6c!iPt.

!ete!eaces

Bl6ms6on, A . , C , JohnBe! , S . S iaads6on, C, Thor -bel8sson and !. Tryggvsson, Riltins of tbep laLe bouoa, .J :n nor lh Ice lsnd 1975-19?8, Iceo lhys . ReE. . 8 l+ , 3029-3038, 1979.

e.t;;;;;;3;r"i-r'quakes in rcerMd iro rce-lardic vith tnslish abstlact), N6ttrirufraed-

fry]jl, !.:", 110-133 , 19?5 .8.16 !n6son. s . . ln r la rce Ea.Lno l€ te in Sou lh l ce-

l3 !9 . Repo. t o_ 3 {o .k inS s toLp to th ' c iv i -D e f e n s e o f l c e l E n d ( i n l c e . a n d i c l , 5 r r p p . ,ReykJavlk, 19?8.

Elnd6son, P6U, Seisnicity ed euthquahe focalnecheises along ihe lid-Aitetic plate boud-ary betreeb Icetand ed the Azores,rectonorhystc6, t2, I27-I53, t9(9.

Eiadsso!, Tr., nre Icelanilic tlactue systen sdthe infellea causal stless fiel,it, rn: Icelaniland llid-oce& FioEes (eil. s. s,lotnssorJ;-E;;sc i . I s ldd ica , Pun l .38 , 128-1 !1 , 1951.

f,indsson, 1r., Sulnsine aldge6 a5 e effect ols L r e s s f i " 1 o s , , . C € o p h r s . R e 6 . . 1 3 . 1 ) 6 t - " 5 1 5 ,1968.

Foulge!, G., ed P. lilslsson, Fecent ealthqEkesin the ilengllt-EeUisheldi uea in Srfi-Ice14il'J . ceo lbys . , !1 , 171-1 f5 , 1980.

Cefke, I(., D. M611e!, B. Ritter, oeodatlschelagenessugen zu BestiMug horizontaler

. Klu6tenbesegwgen in No!dosi-Is13!at,wi6senschaftticbe,&beiten de! Leh!6tnb1e firCeodasie. PhotosMetrie ua Kartocraphie aader Technischer Upiyersit6t Hmover, N!. 43,pp. 23-33, 1978.

HatksEon, E., dil J.C. ooildald, ladon eethqudkeprccaso! studies in lceland, subEitted ioJ , Geo lbys . des , , ryoa ,

Je,hobsso,, S., Chedstly dd dlstlitution pattemo" P-ceD bas tu . i c rocks i . I ce taod, L l tho6.

2, 365-386, 1972.J8to!6son, S,, Petlolory of Recent tasalts of ihe

ea6te!! mlcaDic zone or lce1sd, Acta Xat.Is ted ica 26 , 103! ! . , 1979.

K6uik, V., Seisnicity of the Esolean Ar!q,

( l - i r , . . ! P6 l E inMsson 6nd M. w lss , Mic_o-earthquakes on the nid-Atlantic !1ate boud-uy on the ReykJoes Peninsula in lcetand, Ilceoprys . Re6. , 11 , ! . t08 l+- t099, 19? i .

K le in , I . v . , P . l ina isson e i t M. {yss , rheReykJees Penins!.la, Iceled, earthquake stamof Se l teu le r l9T t a rd i t6 !e .Lon ic s ian : f - -

, !3, 85t-888, 1977.M611e! , D . , A . R i t te ! , Ceodet ic Deasudents i !

the ! i " t zone or NE- Ice lsnd, J . C-oo l r . /q , . ! I .r10-1r9, 1980.

i

Pd,1n6so!, c., and K. Saenuilsson. Icetand inrel5tioD to the Mid-Ailaltic Flalse, AlnuafB e v . E a , h P l e e L . S . i . r 2 . 2 5 - ) 0 . 1 9 7 1 ,

Ra le igh , B . , C . Be@et t , ! . C ! . ig , T . teks ,P. Mo lnd , A . Nr t J . Savaee, C. Scho lz , R .Iane!, F. I'/u, PledictioD of the FaichenaedLhquke, Eos , lYans , An. Ceo lhvs . Un ion .tg, 236-272,-@7--

Rudolph, E., selshonetrische Beolachrusen (1889-1896), BeltraEe zu oeophvslk, :, 9I-1?o, 1903.

saerudgso!! (., Fi5sure 6aams and centlat vol-ceoes of tbe neovolceic zones of Iceland,ceol. J. slecial Is6ue Io. 10, lr15-!32, 19t8.

Sleurd66on, 0., Suface defotution of the Kraftafi6sue sr€n in t*o ltfttla evenis. J.ce9phys . , L r , t>L-159, l98o :

Stefddaso,, n., Sme llotteG of seisEologic6l6tudles o! tbe Mid-Atldiic Rldge, Inr lceleasd tEd-ocee Ridaes (.a. s. l;orns"or)l-E6i-sc i . Ig land ica , pubr . 38 , 80-89 , 195t .

Stef6nssoo, F., Catsstlophtc earthquakes tn ]ce-rarir, Tec Lonolrry6 ic s ! 5), 273-218. t9t9-' I ' hoEr lhssoo,

S isudr , '1 ' !e e ! !D t jo rs op Bek lair r: supricar trnes. rrffii-iiliiii]aL7-1941J. I , p . 1 - t?0 , Soc . Sc : , t s tand ice ,196?.

IhorodoseE, fh . , Larse ear tborees in Ice tsd( i n T c e l 8 n i l i c ) , T h F . i c e t a n d c L i r . S o ! . ,copenhaee! 269!!., 1899 &d 1905.

Iholoddsen, lt!., ple oeschichte der islerdlschen

l!]]!e!g. Me. fioyal Acad. sclen.es. Demtrk,Natbal Sci . Div , , Se- ie6 8, ty . 158! ! . ,colenhaaen, 1925,

?ryeeaason, Eystelnn, Seisdicity, edthqual<esFams &il plate boudeie6 in tle Iceland-esion, Bul t . se jso_. soc. Ao. . 63. 1 lz?-r3lr8. 19tf-- .--

TryssaasoE, ! . , c€r lhoua les in Ice tMd lq to - rg( i i I ce1and i c J , Sc lebce lns l j tu r e r Un lve ls 'ayor l ce t&d, ne lo lL P 'H-78-21, 92p! . , t9 -8a .

r ryagvason- E , r .Ear tho !a ! ,es ! ce tahd t9 l ro - l r9( in Ice ldd ic ) ! Sc lehce hEt i iu te r Un ivers i tyor lce16!<1, Relo"t nE-78-22, 5tpp., r9?8b.

: r ryeeaasot r , ! . , EdLhquskes Ib T .e jsd 1S5O-59( tn Tcet .n< l i c ) ! Sc ience TnsL iLuLe, Un iErs i yof lcerald, Repolt Ru-?9-05, 9app., a9I9,

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a i i i c ) , J . fna inee! ' s Soc . Ice tandt 13 . t -9 ,t s 5 b .

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{dd, P.!., Nev iltellletation of the geo1o6, ofIce16bd. Geol. soc. trA!. 8u11., 9?:, 299r-3or2,J.977.

\ ' iud , P . ! . , S . P61@son, ed C.L . Drake, l { i c !o -edtbquakes ald the rid-Aijldtlc ltdse in lce-1ad! J . ceorhv6. Res . , tu , 565-6d, , \969.

EINATSSON EI AI. 15