The 1986 Kalamata (South Peloponnesus) Earthquake: Detailed Study of a Normal Fault, Evidences for East-West Extension in the Hellenic Arc H Lyon-Caen, R. Armijo, J Drakopoulos, J Baskoutass, N Delibassis, R. Gaulon, V Kouskouna, J Latoussakis, K. Makropoulos, P Papadimitriou, et al. To cite this version: H Lyon-Caen, R. Armijo, J Drakopoulos, J Baskoutass, N Delibassis, et al.. The 1986 Kalamata (South Peloponnesus) Earthquake: Detailed Study of a Normal Fault, Evidences for East-West Extension in the Hellenic Arc. Journal of Geophysical Research, American Geophysical Union, 1988, 93, pp.967 - 982. hal-01994159 HAL Id: hal-01994159 https://hal.archives-ouvertes.fr/hal-01994159 Submitted on 25 Jan 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. B12, PAGES 14,967-15,000, DECEMBER 10, 1988 The 1986Kalemate (South Peloponnesus) Earthquake' Detailed Study of a Normal Fault, Evidences for East-West Extension in the Hellenic Arc H. LYON-CAEN,1 R. ARMIJO,2 J. DRAKOPOULOS,3'4 J. BASKOUTASS,4 N. DELIBASSIS,3 R. GAULON,1 V. KOUSKOUNA,3 J. LATOUSSAKIS,4 K. MAKROPOULOS,3 P. PAPADIMITRIOU,1 D. PAPANASTASSIOU,4 AND G. PEDOTTI5 Tectonic and seismological data collected in the field following the September 13, 1986, Kalemateearthquake (south Peloponnesus) are presentedand analyzedto discussthe earthquake rupture process and the regional tectonics. The event occurred on the Kalamata normal fault whose trace was mapped with SPOT images and topographic and field observations. This fault is part of an approximately NNW-SSE en •chelon system cutting through the Hellenic nappes. The fault strikingN15øE on the average,with a dip of about 50ø, has a minimumcumulated Quaternary throw of the order of 1 km. The measured coseismic slip is 6-18 cm over a length of 6 km. Themain shock focal mechanism obtained from long-period waveform modeling (strike=201 ø (+ 10o,_ 20 o ), dip=45o q_ 5 o, rake=283 o (+ 10o,_ 25 o ) ) representsalmost pure east- west extension and is in good agreement with tectonic observations. The centroid depth is constrained to 5-t-3 kmand the seismic moment to ?.0q_2.$x1017 N m. Over700 aftershocks, located by a 16-station network installed after the earthquake for a period of 2 weeks, define two clusters separated by a "gap" of aftershock activity, from the surface to a depth of about 10 km. The main cluster, to the south,defines a 45ø westdipping plane which lies on the downwardextension of the fault mapped at the surface. Focal mechanisms of aftershocks on this fault plane are homogeneous and represent E-W extension as the main shock. In contrast, the majority of focal mechanisms in the uppermost part of the footwall show more or less E-W compression, probably corresponding to postseismic stress release. The northern cluster of aftershocks is very dense and located away from the surface rupture, within a relay zone between the Kalemate and the next en •chelon faults to the NW, the Thouria faults. There focal mechanisms represent extension from about Nl15 ø to N?0ø and N20ø, correspondingmostly to fault reactivationin an areawhere nonrigid deformations prevail. The main shock probably initiated in this relay zone 3-4 s before the rupture front reached the main fault plane and released most of the energy there, the rupture presumably propagating southward. The focal mechanism of the Kalemate earthquake and that of the April 27, 1965, earthquake located to the northwest of Crete, as well as the regional active normal fault pattern, imply that E-W extension oblique to the Hellenic arc is presently the dominant tectonic regime. E-W stretching occurs partly on reactivated NW-SE faults parallel to the Hellenic structures but mostly on newly formed N-S normal faults across those structures. The latter faults are responsible for the apparent segmentation of the Hellenic belt from southorn Peloponnesus to Crete. The existence of active E-W extension in this region implies a recent change in the tectonic regime and consequently a change in boundary conditions at the subduction r.one, probably in response to the incoming margin of Africa. INTRODUCTION Because of the low level of seismicity compared with that of areas nearby like the gulf of Corinth or Thessaloniki and TheSeptember 13, 1986, earthquake (ML=5.5 (Athens), the apparently long recurrence time of medium- or large- M8=5.8(National Earthquake Information Service, NEIS)) size earthquakes, the tectonic activity in this area has been that destroyeda largepart of the city of Kalamatain somewhat overlooked up to now. In particular, no focal southwesternPeloponnesus (maximum intensity X) is the mechanisms were available for this region. The detailed first shallow event in the overridingcontinental crust of study of the Kalamata earthquake presentedhere bears both the Aegeanbetween Corinth and Crete to be wellrecorded on the seismotectonics of the Aegean and on earthquake teleseismicallyand to haveproduced clear surface faulting. rupture processes. The Aegean area has long been recognized as a region of 1 Laboratoirede Sismologie,Institut de Physiquedu Globe, intense and widespread continental extension, but the me- Paris, France. chanismsof this extensionare still a matter of debate[e.g., :2Laboratoire de Tectonique, Institut de Physique du Globe, McKenzie, 1978; Le Piehon and Angellet, 1979; Mercier et Paris, France. al., 1979a;Jackson and McKenzie,1983]. The directionof 3 Divisionof Geophysics-Geothermy,University of Athens, active extension is approximately north-south in Thessalo- Greece. 4 SeismologicalInstitut, National Observatory of Athens, niki [Soufierisand Stewart, 1981; Mercier et al., 1979b;Hatz- Greece. reid et al., 1987],in the gulf of Corinth [Jacksonet al., 1982; 5 IRIGM, ObservatoiredeGrenoble, France Sgbrier,1977] and probablyalso in the Cyclades,the nor- thern Aegean,and westernAnatolia lAngeliefet al., .1982]. Copyright 1988 by the American Geophysical Union. Such north-south extension, despite small episodic or local Paper number 88JB03484. fluctuations, appears to be the dominant tectonic regime 0148-022? / 88/ 88JB- 03484 $05.00 since the Pliocene [Mercier et al., 1979a; Angellet et al., 14,967 14,968 LYON- CAEN ET AL.: THE 1986, KALAMATA, EARTHQUAKE 1981.2.25 CORiN1981.2.24 1981.3.4 •o O o A E G E A N lam S E A OF x MESSENIA• LACONIA / ß ww 36" •) ß ß' (9 • [ 1'•., • 1965.4.27 (9'(9 ß ß '(9 ß ON 22" 28" 100kin I 35 I 22 ø 23" 24" LYON- CAEN ET AL.: THE 1986, KALAMATA, EARTHQUAKE 14,969 1982;Lyberis, 1984]. In contrastwith this apparentunifor- late Helleniccompression (late Miocene-Pliocene?) and sub- mity in most of the Aegean and western Anatolia, the active sequentPliocene-Quaternary extension. Compressionmay and Quaternary tectonics just to the north of the Hellenic have initiated the formation of internal relief by large-scale, trench, from southern Peloponnesusto Crete and Rhodes, gentle NW-SE folding of the nappe structure. This led is more complex: both north-south and east-west extension to updoming of ranges at anticlinoria and downwarpingof has beenobserved lAngelief et al., 1982]. However,local basinsat synclinoria[Institute of Geologyand Mineral Ex- microtectonicmeasurements lAngelief, 1979] and Landsat ploration(IGME), 1986]. Extensionapparently amplified imageinterpretation [Armijo et al., 1986](Figure 1)suggest this smooth, "compressional" relief by large, often en •che- that the most recent movements are being accommodated lon normal faults oriented approximately N-S which roughly by north-south normal faulting and east-west extension. broke at the boundaries between ranges and basins and pro- In this paper we present both tectonic and seismologi- duced more localized uplift and downdrop on them. Parti- cal data collected in the field following the September 13, cularly large Quaternary and active normal faults are clearly 1986, earthquakein Kalamata. We mapped the Kalamata visible on satellite images along the two edges of the Tay- fault and other active faults in the area with the aid of air getosrange (Figure 1) JArmOoet al., 1986]but have not photographs,topographic maps, and high-resolutionSPOT been described in detail so far. The September 13, 1986, images. We evaluatedboth the long-term(structural and earthquake broke along one of those normal faults whose morphological)offsets and the 1986 coseismicslip. We trace runs along the western mountain front of the Tayge- constrain the focal mechanism and the centroid depth of tos massif,only about 4 km east of Kalamata (Kalamata the main shock using waveform modeling and present an fault, Plate I and Figures2, 3, and 4) (Plate I is shown analysisof the locationsand individualfocal mechanismsof here in black and white. The color version can be found in aftershocks recorded by a portable network of 16 stations the separatecolor section in this issue). No surfacedisrup- for a period of 10 days. Papazachoset al. [1988]already tion or earthquake had been observed and associated to this described the main shock focal mechanism from first motion Quaternary fault before the 1986 event. readingsand the resultsof a 3-day aftershocksstudy with a network of five portable stations. The quality and large Surface Faulting of the September13, 1986, Earthquake amount of data enable us to constrain precisely the geome- Small but sharp surface faulting occurred along the trace try of the aftershockzone. We then combineseismological of the Kalamata fault during the earthquake. This was and tectonic observations for a discussion of both the rup- immediately noticed by the inhabitants because the main ture process of this earthquake and the regional east-west fault break crossedthe road to Eleochori(site 5 in Figure extensional tectonics.