T h e 8 e i s m o t e c t o n i c r e g i m e in G r e e c e A . G . GALANOPOULOS Ricevuto il 3 Novembre 1966 SUMMARY. — Evidence is presented which favours the theory that the earthquake activity in the area of Greece should bo ascribed to horizontal currents flowing off to the side of the secondary sedimentary arc, from a minor mantle current rising under the primary volcanic arc of the Southea- stern section of the Dinaric Alps. RIASSUNTO. — Vengono portate testimonianze in favore della seguente teoria: l'attività sismica in Grecia si deve attribuire a correnti orizzontali che scorrono ai margini dell'arco sedimentario secondario, provenienti da da una corrente minore del mantello che ha origine al di sotto dell'arco vulcanico primario della sezione sud-orientale delle Alpi Dinariche. A c c o r d i n g to Geologists ( A u b o u i n ( a )), the m o u n t a i n ranges of Greece, v i z . the southeastern section of the D i n a r i c A l p s , k n o w n under the n a m e " H e l l e n i d e s " . , consist of f o u r m a i n units, i.e. of the eastern inner zones (eugeoanticline-eugeosyneline) and the western outer zones (miogeoanticline-miogeosyncline). T h e A l p i n e orogenesis started in the upper Cretaceous proceeded successively f r o m the eastern inner zones to the western outer zones of the A l p i n e geosyneline, being bor- dered on a c o m m o n f o r e l a n d ( A p u ñ a n r i d g e ) w i t h the A p p e n i n e s and a c o m m o n hinterland ( R h o d o p i a n massif) Avith the Balkans. A f t e r the A l p i n e f o l d i n g and overthrusting of the surface layers, v i z . at the end of Miocene started a dislocation of the Eastern M e d i t e r - ranean which created large f a u l t zones running in t w o p r e v a i l i n g direc- tions: NNW-SSE and WSW-ENE. Seisinological and Geological stu- dies r e v e a l the f o l l o w i n g features: on the faults along the fracture zone running in WSW-ENE direction, the m o t i o n is right-lateral and down- t h r o w is on the N o r t h side. On the faults along the other zone, running 110 A . G. G A L A N O P O U L O S in NNW-SSE direction, the motion is left-lateral, and downthrow is on the southwest side ( A k i I1)). I n the case of intermediate shocks, the motion along the same fault zones seems to be opposite (Hodgson and Wickens ( u ) ) . A careful investigation of the earthquake history of Greece over the 250-year interval, 1710-1959, revealed that 642 earthquake f o c i were active in the area limited b y 34° and 42° latitudes and 19° and 29° longitudes during the period considered. Of the 642 active f o c i which released shocks of magnitude 5> 4.7 in the Greek area 389 f o c i were active only once, 203 foci were active 2 to 5 times, 29 foci 6 to 10 times and 21. foci were active more than 10 times (Galanopoulos ( ' ) ) . I n the interval 1960-1963 the number of earthquake foci which releas- ed shocks of magnitude > 4.7 in the area of Greece increased b y 146. Thus the number of earthquake foci which w e r e active in the area of Greece during the period 1710-1963 total 788. T h e vast m a j o r i t y of these foci, i.e. 620 of them, were active in the period 1951 to 1963. This shows that in active regions, such as Greece, even a short interval is good enough to reveal w i t h a high degree of readability the pattern of strain release. In v i e w of this and of the f a c t that the strain accu- mulated in any region might be releaved either suddenly b y almost one m a j o r shock or gradually b y a series of minor shocks, one might get a more realistic idea about the " q u a k e a b i l i t y " of a seismic region f r o m the distribution of the earthquake foci being denoted b y the "cumula- t i v e magnitudes", which correspond to the sum of the strain energy released f r o m each one in a g i v e n period. T h e a d v a n t a g e of this Tech- nique is that it is v e r y simple and devoid of any assumption. A map showing the distribution of the earthquake foci denoted b y the cumulative magnitudes allows to see i m m e d i a t e l y in which regions the time of recurrence of strong shocks is relatively short. Areas with a thin population of earthquake foci of minor importance do not indicate that these are not subject to strong shocks, but merely that the t i m e of recurrence of strong shocks in these regions is probably v e r y long. This holds especially for the borders of the cratons, i.e. of the forelands and hinterlands and of the intermediate crystalline masses. T h e clu- stering and the delineation of the earthquake foci, on the other hand, permit to draw some conclusions about the trend or the continuation of the deep seismotectonic lines which lack of surface expression. T o the best of m y knowledge no other kind of mapping of the earthquake a c t i v i t y allows such a conjecture. T I I E S E I S M O T E C T O N I C l i E G I M E I N G R E E C E 111 The delineation of the earthquake epicentres in the area of Greece indicates that the Northern Anatolian dextral shear fault system con- tinues through the Trikkeri-Canal fault into the Gulf of Patras and the Fig. 1 - Seismotectonic map showing the large conjugate fault-system which presumably is responsible for the major part of the earthquake activity in the area of Greece and the distribution of the earthquake foci during the period 1951-1964. shear fault zone of Cephalonia-Zante (s. P i g . 1). A s known, the Cepha- lonia fault zone entertains one of the t w o permanent centres of higher 112 A . G. G A L A N O P O U L O S earthquake a c t i v i t y being assessed in the area of Greece (Galanopoulos (')). A d d i t i o n a l corroborative evidence f o r the extension of the Northern Anatolian fault system into Greece is the migration of earth- Fig. 2 - Bathymetric chart showing the principal features of the submarine topography of the Eastern Mediterranean, after 0 . B. Michailof (12). quake foci along the big dextral fault zone and the Eastward offset of Cephalonia, Central Greece, Magnesia and Mysia (Northern part of Asia M i n o r ) over a distance of about 20 km in reference to Zante, Pelo- ponnesus, Euboea and Lesbos, respectively. The large shocks of T I I E S E I S M O T E C T O N I C l i E G I M E I N G R E E C E 113 M a r c h 18, 1953, in Asia M i n o r and A u g u s t 9-12, 1953, on Cephalonia, as w e l l as those of Thessaly (1954, A p r i l 30-1955, A p r i l 19 and 21-1957 M a r c h 8) and the recent a c t i v i t y in the N o r t h e r n Sporaden (1965, 45,000- - 44,000 43,000- 0 - J 1,000- Q 2,000- M A G N E T I C GRAVITY w + 1 HO t̂ o a> 1-60 £ - 6 0 5 -MO Î - o 0 S o u t h 100 200 Miles 300 400 North Fig. 3 - A North-South profile of magnetics, gravity, and bathymetry along the meridian 26° East from the African coast through Crete to the central part of the Aegean Province (X. Lat. 39°). After K . 0. Emery et al. (5). M a r c h 4 and 1967, M a r c h 9), f o r m a striking e x a m p l e of migration of v e r y a c t i v e f o c i along the d e x t r a l shear f a u l t zone in question. T h e offset of Cephalonia is p r e t t y well e v i d e n c e d b y the sharp E a s t w a r d b e n d i n g of the I o n i a n f a u l t escarpment between Z a n t e and Cephalonia (s. P i g . 2). This eastward bending is consistent w i t h the p r e v a i l i n g almost horizontal m a x i m u m tension in S 27 ° W direction in the area of Greece (Scheidegger ( " ) ) . T h e a v e r a g e rate of displacement is presumed to be of the order of 1 to 2 m m per year. T h e association of the Ionian centre of higher earthquake a c t i v i t y w i t h the end of the big 114 A . G. G A L A N O P O U L O S shear fault zone and the sharp bending of the Ionian fault escarpment indicates the existing relationship among them. T h e second large fault zone responsible for the earthquake a c t i v i t y observed in Greece is the Cretan furrow. The Eastern wing of the fur- row entertains the second large centre of higher earthquake a c t i v i t y asserted in the area of Greece. The centre is associated with a fairly steep g r a v i t y gradient (s. F i g . 3). In a distance of about 100 km the g r a v i t y falls f r o m + 8 0 mgals to —100 mgals. E a s t w a r d of Maleas Cap the Cretan furrow bends to the NNW direction into the Gulf of Argolis. Morphological evidence and delineation of foci indicates that the fault zone continues through the Trichonis L a k e and the Gulf of A m b r a k i a into the valleys of Drinos-Vojussa R i v e r s and reaches into the coast of the A d r i a t i c Sea. The t w o fracture zones that cross each other at the entrance of the Gulf of Corinth f o r m a large conjugate fault system responsible for the m a j o r part of the earthquake energy released in Greece (Galanopoulos (8)). A similar fault zone parallels the Southern coast of Rhodes, K a r p a - thos and Crete and crosses the Ionian fault zone W e s t w a r d of Crete. The postulation made b y A . Philippson ( w ) that the Ionian fault zone extends to the North-Northwest up to the Western coasts of the Adriatic Sea is invalidated b y the lack of earthquake foci in the struc- tural trend considered N o r t h w a r d of the 38 ° N parallel. T h e relief of the sea-bottom, as revealed b y the isobaths (Michailof (12)), shows a bifurcation of the Ionian fault zone N o r t h w a r d of the parallel of the strait Zante-Cephalonia. T h e Eastern branch trends parallel to and East of the strait of Otranto. T h e Western branch seems to be an extension of the East side of the Gulf through of Taranto. Of the t w o branches merely the Eastern one appears to be active. The measurement of g r a v i t y in the Eastern Mediterranean revealed the existence of a large zone of positive isostatic anomalies ( + 1 0 0 mgals) in the Southern Aegean Sea and a long belt of negative isostatic ano- malies (—140 mgals) bordering the island arc Ivy thera-Crete-Rhodes. According to V . Fleischer (6), the g r a v i t y anomalies in the Eastern Mediterranean indicate that " the ¡»went morphology of thin urea has probably developed from an uplifting of the sea bottom, of the Aegean Sea and a depression outside the island are Khitera-Crete-Rhodes ,,. Taking into account this notion, the overriding of the sediments of the eUgeosyncline furrow of Pindus over the miogeanticline ridge of Gavrovo-Tripolitza and the pattern of strain accumulation, viz. the distribution of the earthquake foci (Delibasis and Galanopoulos (4)), T I I E S E I S M O T E C T O N I C l i E G I M E I N G R E E C E 115 Table I - CATALOGUE OF MAIOR SHOCKS (M > 7) OCCURRED IN THE AREA OF GREECE SINCE 1800. MAGNITUDES PRIOR TO 1900 FROM MAXIMUM INTEN- SITY AND FELT AREA. Date Time Location Depth Magnitude 1805, July 3 36 ° N , 24 °E i 7.6 1810, Febr. 16 — 35V2 ° N , 25 °E i 8.2 1846, March 28 15 36 ° X , 25 °E i 8.1 1846, June 10 15 37 ° N , 22 °E 11 7.2 1851, Oct. 12 — 4 0 7 , ° N , 1 9 7 2 ° E 11 7.1 1S53, A u g . 18 8 V , 38V4 ° N , 231/j °E n 7.2 1856, Oct. 12 3U 3572 ° N , 26 °E i 8.6 1863, A p r . 22 20 V , 3672 ° N , 28 °E i 8.5 1867, Febr. 4 381/4 ° N , 207„ °E i 7.9 1867, Sept. 20 03:15 361/2 ° N , 2274 "E i 7.6 1874, N o v . 16 — 36 ° N , 28 °E n 7.3 1886, A u g . 27 211/. 37 ° N , 2174 °E i 8.4 1887, July 17 774 36 ° N , 26 °E i 7.7 1897, May 28 2274 37V2 ° N , 2072 ° E i 7.6 1903, Aug. H 04:32:54 36.0 oN, 23.0 °E 100 8.3 1905, N o v . 8 22:06:12 40.0 ° N , 24.0 °E » 7.8 1910, Febr. 18 05:09:18 36.0 ° N , 24.5 ° E 150 7.0 1911, Apr. 4 15:43:54 36.5 ° N , 25.5 ° E 140 7.0 1912, Aug. 9 01:29:00 40.5 ° N , 27.0 °E n 7.75 1926, June 26 19:46:34 36.5 ° N , 27.5 °E 100 8.3 1926, A u g . 30 11:38:12 36.8 ° N , 23.3 °E 100 7.0 1947, Oct. 6 19:55:37 37.0 ° N , 22.0 °E n 7.0 1948, Febr. 9 12:58:15 35.5 ° N , 27.0 °E 40 7.1 1953, March 18 19:06:14 40.0 ° N , 27.3 °E n 7.25 1953, A u g . 12 09:23:53 38.3 ° N , 20.8 "E n 7.1 1954, Apr. 30 13:02:37 39.0 ° N , 22.0 «E n 7.0 1956, July 9 03:11:40 37.0 ° N , 26.0 °E n 7.8 1957, A p r . 25 02:25:36 36.5 ° N , 29.0 °E 11 7.1 119 A . G. G A L A N O P O U L O S the writer m a y hazard the theory that the earthquake a c t i v i t y in the area of Greece should be ascribed to horizontal currents flowing off to the side of the secondary sedimentary arc, f r o m a minor mantle current rising under the p r i m a r y volcanic arc of the Southeastern section of 18° 20° 22° 24° 26° 28° 30° Fig. 4 - Pressure direction in the area of Greece, as determined by Livin Constantinescu et al. (3). Numbers are the identification numbers of earthquake mechanism solutions in the Constantinescu's catalogue. The long heavy line represents the general trend of the Alpine folding. the Dinaric Alps. T h e stress pattern at the foci of about t w o scores of Greek earthquakes (s. F i g . 4), as revealed f r o m the fault-plane solutions (Constantinescu et al. (3)), the association of the m a j o r i t y of the foci of m a j o r shocks ( M 7) with the p r i m a r y volcanic arc (s. F i g . 5 and 6), and the fact that the earthquake volume of the m a j o r shocks extends to T I I E S E I S M O T E C T O N I C l i E G I M E I N G R E E C E 117 depths f a r below the Moho-discontinuity (Galanopoulos (9)) g i v e e v i d e n t l y a fair support to the theory. T h e theory is in harmony with the suggestion made b y J. B . Hersey (10) " that vertical movements Fig. 5 - Distribution of the earthquake foci of major shocks ( M 5» 7) oc- curred in the area of Greece since 1800 to 1900. Gravity contours after V . Fleischer («). of the earth's mantle, in addition to horizontal shearing stress, might better characterize Mediterranean tectonics than horizontal shears alone •'. T h e spread of low mantle-velocities and " cobblestone areas " i.e. areas of many small blocks of fractured material, found so far in the Eastern Mediterranean seem to g i v e a f a i r l y strong support to the suggestion that " upwardmoving mantle assimilates some of the crustal 118 A. G. GALANOPOUJjOS rock, and causes other parts to move laterally under gravitational forces ''. Fig. 0 - Distribution of the earthquake foci of major shocks ( M > 7) oc- curred in the area of Greece since 1901 to 1966. Gravity contours after V. Fleischer (6). A C K N O W L E D G M E N T S . T h e research r e p o r t e d in this d o c u m e n t has been sponsored b y the " A i r F o r c e O f f i c e of Scientific Research " under c o n t r a c t A F 61(052)-803 t h r o u g h the " E u r o p e a n Office of A e r o s p a c e Research ( O A R ) " , " U n i t e d States A i r F o r c e " , as part of the A d v a n c e d Research P r o j e c t s A g e n c y ' s P r o j e c t Vela U n i f o r m . T I I E S E I S M O T E C T O N I C l i E G I M E I N G R E E C E 119 R E F E R E N C E S (*) A K I K . , Study of Love and Rayleigh Waves from Earthquakes with Fault Plane Solutions or with known Faulting, Part 2, Application of the Phase Difference Method, " Seism. Soc. A m . " , 54, No. 2, 529-558, (1964). (2) AUBOUIN J., Esquisse paléogéographique et structurale des chaînes alpines de la Méditerranée moyenne, " Geol. Rundschau " , 53, lieft 2, 480-534, (1964). ( 3 ) CONSTANTINESCU L . , RUPRECIITOVA L . si ENESCU D., Mecanismul cutremu- relor mediteraneene-alpine si implicatiile lor seismotectonice, " Studii Cer- cetäri de Geologie, Geofizicä, Geografie " , Ser. Geof., 3, 173-191, (1965). (4) DELIBASIS N . and GALANOPOULOS A., Space and Time Variations of Strain Release in the Area of Greece, " Ann. Geol. Pays Hellen. " , 18. 135-146, (1965). ( 5 ) EMERY K . 0., BRUCE C. HEEZEN and T. D . ALLAN, Bathymetry of the Eas- tern Mediterranean Sea, " Dcep-Sea Research", 13, 173-192, (1966). (°) FLEISCHER U., Schwerestörungen im östlichen Mittelmeer nach Messungen mit einem Aslcania-Seegravimeter, " Deutseh. Hydrogr. Zeit. " , 17. 4, 153-164, (1964). ( ' ) GALANOPOULOS A., On Mapping of Seismic Activity in Greece, " Annali di Geoflsica", 16, 1, 37-100, (1963). ( 8 ) GALANOPOULOS A . , The Large Conjugate Fault System and the Associated Earthquake Activity in Greece, " Ann. Geol. Pays Hellen. " , 18, 119-134, (1965). ( 9 ) GALANOPOULOS A . , Evidence for the Seat of the Strain-producing Forces, " A n n a l i di Geoflsica", 18, 399-409, (1965). (1 0) IIERSEY B. J., Sedimentary Basins of the Mediterranean Sea, " Submarine Geology and Geophysics " , publishers: Butterworth & Co. Ltd., (Lon- don, 1965), 75-89. ( U ) HODGSON J. H . , and A . J . WICKENS, Coniputer-Determined P-Nodal Solu- tions for the Larger Earthquakes of 1959-1962, " Publ. Dom. Obs. " , 31, No. 5, 123-143, (1965). (12) MICHAILOF 0 . B., Relief of the Mediterranean, " Basic Characteristici of the Geological Structure, the Hydrological System and the Biologie of the Mediterranean " , Edition Nauka, 10-19, (Moscow, 1965). (1 3) PHILIPPSON A., La tectonique de VEgeide, " Ann. Geogr. " , 7, No. 32, 112- 141, (Paris, 1898). (14) SCIIEIDEGGER A . , The Tectonic Stress and Tectonic Motion Direction in and Europe Western Asia as calculated from Earthquake Fault Plane Solutions, " Bull. Seism. Soc. Am. " , 54, No. 5, 1519-1528, (1964).