264 Physics of the Earth and Planetary Interiors, 44 (1986) 264—273ElsevierSciencePublishersB.V., Amsterdam— Printedin TheNetherlands

Seismicactivity in North Aegeanregionasmiddle-termprecursor

of Calabrianearthquakes

EnzoMantovani,DarioAlbarelloandMarcoMucciarelliDipartimento di Scienze della Terra — Universita’ di Siena Via Banchi Di Sotto 57, 53100 Siena (Italy)

(ReceivedFebruary24, 1986; revisionacceptedApril 22. 1986)

Mantovani, E., Albarello, D. and Mucciareffi, M., 1986. Seismicactivity in North Aegeanregion as middle-termprecursorof Calabrianearthquakes.Phys.Earth Planet. Inter., 44: 264—273.

A comparativeanalysisof seismicitypatternsin theItalian andBalkanregionsiboth relatedto thedynamicsof theAdriatic Plate) has revealed a noticeablecorrelation betweenthe~tin~iepatternsof seismic energy releasein theCalabrianArc (southernItaly) and in the northernmostAegeanregion. The systematicoccurrenceof damagingearthquakesin the CalabrianArc within a few years.after themostintenseseismicperiodsin theNorth Aegeanzonemaybeusefulfor middle-termearthquakeprediction.

1. Introduction ___

The seismicity in Italy is spreadout overmost Pannonian _____

of the country and the time spacedistribution of basin

earthquakesdoesnot seemto show anyparticular ______

regularityuseful for the planning of observationsrelated to middle and short term prediction of ~1

earthquakesTo overcomethis difficulty, it ap- Tyrrh~ian ~ ~i,,pears~necessaryto look for otherkindsof evidence basin ~ ~

by following more deterministic approachesOf _____ ...

fundamentalimportance,in this regard, is the _____ . . .. ~

knowledgeof ‘structural and tectonic featuresin ~L-. 0”the Mediterraneanregionand of its possiblegeo -____ ___________

dynamicevolution. On this basis, one can study A .• - - c Athe time-spacedistribution of earthquakestrying - R I -

to obtain information on the connectionsbetween i ~ 2[~] 3~ 4~ 5~

ongoingtectonicprocessesand,seismicity.The Italian peninsularoughly correspondsto 6..à~ 74~ 8uuu+ ~

the deformed western margin of the Adriatic formedmargin. Symbol 5 identified theorogemcbelts derivedplatform (Fig. 1) which is shifting northeastward, from the deformation of internal units and of the Adrican

margin (Parottoand Praturlon,1981). (6) Compressionalfea-tures and subductionzones.(7) Main transcurrentfault sys-

Fig. 1. Tectonicsketchof the centralMediterraneanregion. tems. (8) Dominantdirectionsof motionof theMediterraneanHatchedareasbelongto the African domain: (1) remaining microplateswith respectto Eurasia.(9) Horizontaldirectionofpartsof the thinned northern margin; (2) foreland. Dotted compressionalstressalong main collision zones. (For refer-areasindicate the Europeandomain: (3) foreland; (4) de- encesseeMantovaniet al., 1985.)

0031-9201/86/$03.50 0 1986 ElsevierSciencePublishersB.V.

265

with a counterclockwiserotation,within the com- it cannotbe given the samesignificanceas morepressional regime between Africa and Eurasia recentdata.(Vandenberg,1979; Mantovani, 1982; Mueller, Theneedto operatewith a homogeneoussetof1982; Mantovarnet a!., 1985). Consideringthis datamadeus choosebetweenthe useof magm-tectonicframework,we investigatedthe possibility tude valuesor intensity values for all the earth-that the seismicity in the Italian region is in- quakesconsidered.Unfortunately,both solutionsfluenced by movementsof the Adriatic block have some degreeof inherent uncertainty.Wewhich, in their turn, are related to the seismic finally opted for magnitudesassumingthat suchenergyreleaseoccurring along the compressional dataaremore directly connectedwith the energymargin betweenthisblock and the Balkan region releasedby earthquakesthat occur in different(Mantovani,1985). zonesandat different depths.We usedlocalmag-

With this working hypothesis in mind, we nitudeswhere availableand macroseismicmagni-analysedthe time-spacedistribution of seismicity tudes in other cases.Only crustal earthquakesin the circum-Adriatic regions, looking for some havebeentakeninto account.form of relationship between seismicity in the

Italian and Balkan areas.This kind of approachhasalreadyproducedsignificantfindingsconcern- 3. Dataanalysising otherzonesof the world (see,e.g.,Kanamori,1972; Mogi, 1973; Shimazaki,1976, 1978; Seno, Thesearchfor arelationshipbetweentheItalian1979; Ri.kitake, 1983) and some theoreticalbasis andBalkan seismiczoneswascarriedout throughfor the interpretationof this type of evidencehas an analysisof the similaritiesbetweenthe energybeenprovidedby severalauthors(see,e.g.,Ander- releasepatternsin areaslocatedalong the oppo-son, 1975; Yamashina,1979; Lehner et a!., 1981; sitemarginsof theAdriaticblock. To facilitatetheRice andJi-cheng,1983; Li andKisslinger, 1985). analysiswe divided the circumadriaticareainto a

numberof subregions(Fig. 2), on the basisof themain tectonicfeaturesandseismicitydistributionin theseareas(McKenzie, 1978; Miljush, 1978;

2. Data Deweyand Sengor,1979;LePichonandAngelier,

1979; Channell et al., 1979; Burchfiel, 1980; Sul-The data used for this study, as far as the starova et al., 1980; Funiciello et al., 1981;

Italian areais concerned,were :taken from the Mantovani and Bosehi, 1983; Horvath, 1984;Italian catalogueof earthquakes(ENEL, . 1983; Hatzidinutriouet a!., 1985).Eachzoneto be con-revised and updatedin the framework of the sideredin the comparativeanalysiswascomposedItalian GeodynainicProject,1983).The datarela- of oneor moreof the subregionsshownin Fig. 2..tive to the Balkan region were taken from A quantitative and objective estimateof theGalanopulos(1963), Karnik (1969, 1971), Sheba- level of similarity betweenthe annualenergyre-lin et a!. (1974),ConstantinescuandMarza(1980), leaseseries in each pair of zonesexaminedwasPapa.zachosandComninakis(1982).The most re- obtainedthrough the cross-correlationtechniquecentdatawerealso takenfrom NEIS and CSEM (Dz.iewonskiand Hales, 1972; Clark andThomp-bulletins, son,1979; Bath,1984;Liang-yuandChien-chung,

Welimited our analysisto seismichistory from 1984). Somepreliminary tests have shown that1600 onwards, since very little information is applyingthismethodto the rawdatamay leadtoavailableaboutprevious seismicity. Someincom- misleading results since the correlation coeffi~pletenessalso affects the availablecataloguesfor cients appearto be almostcompletelycontrolled~1600—1800 (Shebalin et al., 1974; Prochazkova, by the highestpeaksin the annualenergyrelease.1980;Mulargia andTinti, 1985; Hatzidimitriou et To mitigate this effect, we attenuatedthe ampli-al., 1985).However,we decidedto takethis infor- tudecontrastswithin the time series.Furthermore,mationinto account,whereavailable,eventhough to increasethe continuity of energyreleasepat-

266

I 1-

.

• ‘ •~ ..• •~. • O • ~ 8c~t~

• o 4~N

•o •• • • % •a•••~•• %oo 0

S - •S.•••. • . _ ~0:~•:j~

IO~E 15 20 25

Fig. 2. Distributionof epicentresandsubdivisionof theinvestigatedarea.

ternsandthe weight of seismiccriseswith respect nificant if greaterthan 2M 1/2 where M is theto isolatedevents,we applieda smoothingproce- numberof d~tain the time series(Liang-yu anddureto the data. Chien-chung,1984). The confidenceintervals of

The relationbetweenthe new time senesF( z) correlatioi~coefficientswere computedafter the

andthe raw onesE(i) is methodof Kendall andStuart(1973).i+(K—1)/2 This investigationenabled us to identify the

F( 1) = K1 ~ E( j pair of zonesshowingthe bestcorrelations.Afterj-.,—(K—1)/2 this, anew seriesof testswascarriedout for each

of thesezonesto try to optimize the correlationThe value of K determinesthe length of the throughananalysisof moredetailedboundanes

movmgwindow usedfor smoothingandthat of p The quantitative seismicity analysis describedcontro!~the amplitudeattenuationThe numerous abovewas accompaniedby a quahtativemspec-tests carried out have shown that a reasonable tion of the timeseriesto gain abetterunderstand-compromisebetweenthe different needs•involved ing of the influence of the single features ofin our problemis achievedwith the valuesK = 5 seismicity patternson the values of correlationand p = 5 (Mucciarelli et al., 1985).The valuesof coefficients.Among thesefeatures,we considered,thecorrelationcoefficientswereconsideredas sig- in particular, the time-spacedistribution of the

267

strongestshocks,the proportionalitybetweenthe especiallyregarding the inland borderswith theamplitudesof well-correlatedevents,the regularity southernApenmnesand Sicily. The extensionofof time delays,the numberof uncorrelatedevents this zoneto include part of the lonian Seawasandtheir distribution in the seismichistory. suggestedby correlation tests, but thereis little

evidenceto supportsuch a choice,sinceinforma-tion aboutseismicityin this areais only relatingto

4. Main findings thelastcentury.(2) The westernboundaryof North Aegeanzone

The most interesting correspondencepointed is fairly well definedby thepresenceof analmostout by thisanalysisis that betweenthe Calabrian aseismicareain the loman Sea.The inclusioninArc andthe northernmostAegeanregion. Figure3 this zoneof the Albanian—Dinancregionsor ofshows some significant correlogramsconcerning thoseregionslying southof the Anatolian Troughthe analysisof this relation.The geometryfinally systematicallylowers the correlationdegreewithadoptedfor the zonesinvolved and the respective the CalabrianArc. The identification of the east-patternsof annualenergyreleasesaregivenin Fig. em borderis, on the otherhand,moreuncertain.4. A list of themost significanteventsis laid out Thecorrelation testsclearly indicatethat~this zonein TableI. must certainly comprise the seismic belt around

The identification of the Calabro—Peloritanre- the lonian islands and the moreinternal seismicgion (i.e., the emergedportion of the Calabnan areaaroundthe Serbo—Macedonianmassif.Arc) as an independentseismotectoniczonehas Concerningthe time delaysbetweenCalabrianalreadybeensuggestedon the basis of geological seismiccrisesand those in the Balkans,we haveandtectonicevidence(Amodio Morelli et al., 1979; obtainedthe highestvaluesof correlationcoeffi-Funiciello et al., 1981;GhisettiandVezzani,1982) cientsfor time lagsrangingbetween0 and3 years.and by seisniicityanalysis(Barbanoet al., 1978; The patternsof annual seismic energy releaseinMantovani and Boschi, 1983; Gasparini et al., the two regions (Fig. 4) show that the seismic1985). behaviourof the two zones is characterisedby

Concerningthe Balkan area, the most signifi- periods of intenseactivity and others of relativecant tectonic featureswe investigatedas possible quiescence,which synchronouslyalternatein theboundariesare: (1) the Anatolian trough (Fig. 1) lower andupperhistograms.The factthat seismicwhichseemsto separatethe northernmostAegean energyreleasein the CalabrianArc systematicallyregion from the southwestwardextrusionof the decreasesduring the period of minimum activity

Aegean block (McKenzie, 1978; Dewey and of the North Aegeanzonecorroboratesthe hypo-Sengor,1979; Le PichonandAngelier, 1979); (2) thetisedrelationandseemsto indicatethe absencethe Albanian zone which marks the transition of other important external influences for thebetweentwo sectorsof the easternAdriatic de- Calabrianseismicity.formed margin charactensedby different de- Some considerationson Table I may give aformationpatternsandseismicbehaviour;(3) the more precise idea about the correspondencebe-transition zone between the Alpine belts (Di- tween significant events in the two zones andnarides and Hellenides) and the southern de- about the practical usefulnessof the observedformedmargin of the Europeancontinent(Balkan relationshipfor earthquakepredictionpurposes.range). In five cases(1783,1829,1867,1904 and1905)

Theindication providedby thenumerouscorre- seismic crises in the North Aegean zone havelation testscarriedout with different shapesand compriseda shockmagnitude � 7.3. Correspond-dimensionsfor the two zonesmay be synthesised ingly, the CalabrianArc has beenaffected fouras follows: times(1783, 1870, 1905 and 1908) by earthquakes(1) The geometry initially assumed for the of M ~ = 6.6 and once (1832) by an event ofCalabrianArc did not needsignificant modifica- M = 5.6 (followed in 1835 and1836 by two othertion after the analysis of seismicity correlation, shocksof M = 5.6 and M = 5.9).

268

0.7

0.6

z

~0. 5 ‘I”-_~”

1)0,4z

0.2 /~0. 3 ,1 ________

-to -s 5 10a) TIME LAG (yeara)

0.~ I

0.6 ,~.

0.5 I~-~ ,----~IL ‘~‘—. ~ ii

~4 ~//~0.3 .,

[2 IS ______________

za~ 0.2‘C

~0.1 I 2 I0

-tO —5 0 5 10b) TIME LAG (ysara)

Fig. 3. Significant examplesof correlogramsobtained from the comparativeanalysisof Calabrian and Balkan seismicity~(a)Investigationof thebestgeometryfor theCalabrianArc seismic zone.(b) Investigationof thebestgeometryof theNorth Aegeanzone. Theannualseismicenergyreleasesfrom 1600 to 1983 were used.If we take into accountonly the data relative to last twocenturies, thecorrelationcoefficientvaluesonly showa slight increase.However,a periodlengthof about400yearspermitsgreatlyincreasedconfidencein theobservedrelation.

In 10 of the 11 casesin which the maximum ‘replied’ with shockmagnitudesgreaterthan 5.6.magnitudesof Balkan earthquakeshave been The only exceptionis the Balkan eventof 1953,greaterthanor equalto 7.0, the CalabrianArc has which was followed in 1954 by the Calabrian

earthquakeof M = 5.3.

269

22 Calabrian Arc

~21‘C

~2O

18 _______~ . i, . dl,, ~, i1i~1iii U hilt ii_i~._i1iliii,,1600 1700 1300 1900

TIME (years)

~ 23 North Aegean Zone

Si0

~ ~i .~ ~ ~ . ~ ~ I ~~. ~ .~~l~i~~1600 1700 1800 1900

TIME (years)

Fig. 4. Finalgeometry assumed for the Calabrian Arc and North Aegean seismic zones (small picture in theupperright-handcorner

of the Figure) and relativepatternsof annual seismic energy releases.The energyvalues have been derivedfrom magnitudesaccordingto Gutenberg(1956).

In 20 of the 27 cases in which the North betweenthe beginning of eachcrisis in the Ba!-Aegeanzonehas beenaffectedby earthquakesof kans and the main shock of the correspondingmagnitudesbetween6.5 and7.0, the seismicity of crisis in the CalabrianArc has always been lessthe CalabrianArc has undergonea significant than 5 years,with a dominantvalueof 2 years.increasecharacterisedby shock magnitudesbe- Thesequalitative observationsand avisual in-tween 5.0 and 6.1. It should be noted that the spectionof seismicitypatternsof Fig. 3 are suffi-sevencasesof ‘no reply’ in the CalabrianArc all cient, in our opinion, to supportthebelief that therelateto before1800. observedrelation betweenCalabrianand Balkan

The CalabrianArc experienced23 earthquakes earthquakesis not merely casual.However, it isof magnitudegreaterthan5.5 and20 of themwere possibleto show that the observedcorrelation ispreceded,within a few years, by Balkan earth- statisticallyrathersignificant. To this purpose,wequakesof magnitudegreaterthan6.5. applied to our case a simple statistical analysis

In mostof the casesgiven in TableI, the main proposedby Shimazaki (1978). For this test, weshockof eachcrisis in the CalabrianArc occurred took into accountonly the periodsof maximumafter the first earthquakein the Balkans.The only activity in the CalabrianArc during the last twosignificant exceptionoccurredin 1791, when the centuries (1783—1787, 1831—1836, 1869—1870,Calabrianearthquaketook place about a month 1905—1909)andthe main crisesin North Aegeanbeforethe correspondingonein the North Aegean zonewith at least one earthquakeof M ~ = 7.3zone. (1783, 1829, 1867, 1904). These events are as-

Finally, it shouldbe noted that the time delay sumed to be mutually independentand to occur

270

TABLE I

List of major earthquakes(M> =6 for theNorth Aegeanzone and M> 5 for theCalabrianArc) which occurredin two areasduringthetimeinterval 1600—1983.Eacheventis identifiedby theyearandmagnitude(in parenthesis).Tofacilitatethediscussioninthetext we havearbitrarilygroupedtheeventsin ‘seismic crises’.

North Aegean Calabrian NorthAegean Calabrianzone Arc zone Arc

1609 (5.1) 1840(6.8) 1842 (5.1)1612 (6.8) 1850 (5.1)1613 (6.8) 1613 (5.6)

1854 (6.4) 1854 (5.6)1622 (6.6) 1858 (5.1)1625 (6.8) 1626 (5.6)

1862(6.7)1630 (6.9) 1864 (6.4, 7.0) 1865 (5.6, 5.1)1633 (6.9) 1634 (5.6)

1866 (7.0)1636 (7.2) 1638 (6.1, 5.6) 1867(6.6,7.3, 6.3)

1640(5.1, 5.1) 1869(6.9) 1869(5.6)1870 (6.6)

1641 (6.9)1879 (5.6)

1649 (5.1)1883 (5.1)

1658 (6.9) 1659 (6.1) 1886 (5.1)1664(6.7) 1669 (5.6,5.1) 1887(6.7) 1887 (5.5)

1889 (5.0)1687(5.1)

1893(6.5,6.7, 6.4, 6.5) 1894(5.7,5.1, 5.3, 5.1)1696 (6.5)1704 (6.8) 1895 (6.6,6.2) 1897 (5.2)1710 (6.5) 1898(6.6) 1898 (5.1)1714 (6.6,6.5) 1717(5.6) 1902(6.5)

1904(7.0,7.8)1720(5.1) 1905(6.4,7.4) 1905 (7.0)

1906(6.0) 1907 (5.9,5.1)1735 (6.1) 1736 (5.1) 1908 (7.1)

1739 (5.6, 5.1, 5.1)

1911 (6.6)1912 (6.4) 1913 (5.6)

1740(6.1) 1914(6.1) 1914 (5.9, 5.1)1742 (6.5) 1915 (6.4, 6.5, 6.4, 6.0) 1917 (5.3)

1743 (6.9) 1743(5.6, 5.1)1744(5.1)

1919 (6.0) 1920 (5.0)1750 (6.7)

1923(6.3) 1925 (5.2)1759 (6.5, 6.5)

1928(6.8,7.0) 1928 (6.0)1765 (6.6) 1929 (5.1)1766 (6.7)1767 (7.2) 1767 (5.6) 1931 (6.0, 6.6)1769 (6.5) 1932 (6.9,6.1) 1932 (5.4)

1933 (6.3)1781 (6.3) 1783 (7.1,6.1, 6.7, 6.7)1783 (7.5) 1784 (5.1) 1941 (6.1) 1941 (5.4)1784(6.6) 1784 (5.1) 1942 (6.0)

271

TABLE I (continued)

North Aegean Calabrian NorthAegean Calabnanzone Arc zone Arc

1785 (6.6) 1786 (5.6,5.1) 1943 (6.0) 1947 (5.6)1787 (6.3) 1778 (5.1,5.1)

1948 (6.5, 6.4) 1950 (5.2)1791 (7.0) 1791 (5.6)

1953 (6.1, 6.5, 7.0, 6.0, 6.0)

1797 (6.6)1804 (6.7) 1805 (5.1) 1954(6.0) 1954(5.3)1813 (6.4) 1958 (6.1)1815 (6.5) 1818 (5.6) 1959 (6.5)

1960 (6.4) 1961 (5.0)1820(6.6, 6.6) 1962 (6.3)1821 (6.5) 1821 (5.1) 1963 (6.0)1823 (6.9) 1823 (5.9, 5.1)

1966 (6.3)1825 (6.8) 1828 (5.1) 1967 (6.1) 1968 (5.5)1829 (6.7, 7.3) 1831 (5.1) 1978 (6.1) 1978 (5.5)

1832 (5.6, 5.1, 5.1)1983 (6.7, 6.2)

1835 (5.6, 5.1)1876 (5.0, 5.6)

randomly in time. The probability that such an prediction is 82% and the probability that aearthquakein the Balkansoccursduring the active Calabnanearthquakefails to havea precursorisperiods of the CalabrianArc or in the 3 years 21%.before, is given by By consideringthe period from only 1800 on-

4!.(8/200)(9/200)(5/200)(8/200)= 0.00004 wardwe haveS=25,FA=0,FP=4.

thus the correlationis statisticallysignificant at a . . .The probability of a successfulprediction isconfidencelevel of 99.996%. 96% andof failed precursorsis 16%.

The fairly good regulantyof the precursorac-tivity in the Balkansmight representauseful toolfor middle-term earthquakeprediction in theCalabnanregion. To make an estimateof the 5. Discussionandconclusionsprobability of successfulprediction we followedthe approachproposedby Rhoadesand Evison Seismicity in the CalabrianArc seemsto bein(1979), which relatesthesequantitiesto the num- some way connectedwith that in North Aegeanber of successfulpredictions(S), of false alarms Zone.This hypothesisis mainly supportedby the(FA), andof failedprecursors(FP). In our case,a fact that all largestearthquakesin the Calabriansuccessfulprediction is assumedto be the occur- Arc (1638, 1659, 1783, 1870, 1905, 1908, 1928)rencein the North Aegeanzoneof oneeventof haveoccurredwithin a few yearsafter the stron-M ~ = 6.0 followed, within 5 years,by an earth- gest shocks or very active periods in the Northquakeof M ~ = 5.0in the CalabnanArc. Taking Aegeanzone(1636, 1658, 1783, 1866—1869,1904,into accountthe period since 1600 (Table I) we 1905,1928), andby the good correlationbetweenhave theseismicitypatternsin the two zonesrelativeto

S = 36 FA = 7 FP= 9 thelast four centuries.The high statisticalsignificanceof the observed

It comesout that the probabilityof asuccessful interrelation suggeststhe existenceof a tectonic

272

connectionbetweenthe two regionsinvolved. It is Paglionico, A., Perrone,V, Piccarretta,G., Russo,M.,

obviously difficult to understand the precise Scandone,P., Zanettin-Lorenzoni, E. and Zuppetta, A.,1979. L’ Arco Calabro-Peloritanonell’ orogene Ap-mechanismsof this connection; one may, how-

peninico—Magrebide.Mem. Soc. Geol. Ital., 17: 1—6.ever, advancesomespeculativehypotheses. Anderson, D.L., 1975. Accelerated plate tectonics. Science,

From the generalpoint of view, the Mediter- 187: 1077—1079.

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regime due to the convergenceof African and Fonte, G., Ghisetti, F., Lanzafame,G., Lombardo, G.,Pantane’,0., Riuscetti,M., Tortorici, L. and Vezzani, L.,Europeancontinentalmargins. This convergence 1978. Elementi per unacartasismotettomcadellaSicilia

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Hellenidesbelts. Within this framework, seismic- Constantinescu,L. and Marza, V.!., 1980. A computer-corn-ity in the Hellenic—Balkanregionmight imply a piled and computer-oriented catalogue of Romania’s

partial attenuationof the obstacleswhich resist earthquakes during a millennium (984—1979).Rev. Roum.Geol. Geophys.Geogr.,24: 193—234.the rotation of the Adriatic microplate.This per- Dewey, J.F. andSengor,A.M.C., 1979. Aegeanandsurround-

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