SEISMIC DEFORMATION IN THE SOUTH WESTERN HELLENIC ARC: PRELIMINARY RESULTS FROM ACTIVE AND PASSIVE SEISMIC OBSERVATIONS

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1 SEISMIC DEFORMATION IN THE SOUTH WESTERN HELLENIC ARC: PRELIMINARY RESULTS FROM ACTIVE AND PASSIVE SEISMIC OBSERVATIONS Papoulia J. 1, Makris J. 2, Tsambas A. 1, Fasoulaka Ch. 1 1 Hellenic Center for Marine Research, Athens, nana@ath.hcmr.gr 2 GeoPro GmbH, Hamburg, Germany, info@geopro.com Abstract In Fall 2006, we deployed a combined on/offshore seismic network consisting of 17 4C ocean bottom seismographs and 15 3C land-stations in the Kyparissiakos gulf and the surrounding area, of western Peleponese and observed the seismic activity for a period of 2 months. We located more than 3,500 earthquakes by using arrivals from a minimum of 6 stations at a time, applying a local velocity model obtained from active seismic observations. For 581 earthquakes we also defined local mechanisms. Seismic activity is closely associated with the active fault zones of this area: The shallow seismicity (0 to 15 km ) is mainly confined at the continent-ocean crustal transition approximately 70 km west of the island of Zakynthos, onshore Zakynthos and the area of Pylos, western part of Messinia (SW-Peleponese). All three zones are deforming by thrusting of the westwards moving Hellenic napes, which form topographic uplifts and are frequently associated with exposed metamorphic limestones. The orientation of this fault system is parallel to the collision front. Seismicity associated with the deeper part of the crust (15 30 Km) coincides with the tectonically uplifted blocks of Messinia and that of the island of Zakynthos. Deeper crustal seismicity follows the subduction of the oceanic, Ionian lithosphere below western Peleponnese with constantly increasing hypocentral depths towards east. We also recorded subcrustal seismicity, extending to 90 km depth at the central part of the Kyparissiakos gulf. This deep seismicity, is confined to a narrow zone of extention less than 8x8 sq km located in a transtentional basin. We believe that this is associated with fracturing of the oceanic lithosphere, in the subduction zone below the western Hellenides, and affects the lithosphere over its complete thickness of

com Abstract In Fall 2006, we deployed a combined on/offshore seismic network consisting of 17 4C ocean bottom seismographs and 15 3C land-stations in the Kyparissiakos gulf and the surrounding area,

2 nearly 50 km. Focal mechanisms show dextral strike slip in the crust and the active faults dominated by this deformation are perpendicular or near perpendicular to the collision front. The active seismic experiments that mapped the geometry of the sediments and crust clearly show that the internal Hellenic zones are thrusted over the external ones and that the uplifted units exposed on the island of Zakynthos and that of Messinia are highly elevated blocks of the thrust belts. Περίληψη Στα πλαίσια του ευρωπαϊκού ερευνητικού προγράµµατος SEAHELLARC του 6ου Kοινοτικού Πλαίσιου Στήριξης έγινε εγκατάσταση 15 ψηφιακών σεισµογράφων ξηράς και πόντιση 17 ψηφιακών σεισµογράφων θαλάσσιου πυθµένα στη ευρύτερη περιοχή του Κυπαρισσιακού κόλπου και δυτικής Πελοπονήσσου για την καταγραφή της µικροσεισµικής δραστηριότητας. Σε χρονικό διάστηµα δύο µηνών (23/09/ /11/2006), το δίκτυο κατέγραψε περισσότερα από 3500 σεισµικά γεγονότα, µε ελάχιστο κριτήριο την καταγραφή τους από 6 σταθµούς. Για τον προσδιορισµό των επικέντρων εφαρµόστηκε ένα τοπικό µοντέλο ταχυτήτων που προέκυψε από ένα ενεργό σεισµικό πείραµα. Καθορίστηκαν συνολικά 581 µηχανισµοί γένεσης σεισµών και πραγµατοποιήθηκε διάκριση των πιο αξιόπιστων λύσεων. Η σεισµική δραστηριότητα µικρού βάθους (0-15 χλµ) υποδεικνύει την πολύπλοκη παραµόρφωση των επιφανειακών ιζηµάτων και τµήµατος του ανώτερου φλοιού. Σε µεγαλύτερα βάθη (16-30 χλµ) η εντονότερη σεισµική δραστηριότητα συµπίπτει µε το χώρο νότια της Ζακύνθου και το τεκτονικά ανυψωµένο τέµαχος της Μεσσηνίας. Οι σεισµοί ενδιαµέσου και µεγάλου βάθους χαρακτηρίζουν τη ζώνη υποβύθισης κάτω από τη δυτική Πελοπόνησσο. Σε µια καθορισµένη ζώνη που συνδέεται µε την διεφελκυστική υποθαλάσσια λεκάνη βορειοανατολικά των Στροφάδων τα βάθη των σεισµών ξεπερνάνε τα 90 χλµ. Αυτή η βαθιά σεισµικότητα σχετίζεται µε θραύση της λιθοσφαιρικής πλάκας και διαχωρισµό της σε δύο επιµέρους τεκτονικές µονάδες, που κινούνται µε διαφορετικές ταχύτητες βύθισης. Οι µηχανισµοί γένεσης των σεισµών βάθους υποδεικνύουν κυρίως δεξιόστροφες κινήσεις σε οριζόντιας µετατόπισης ρήγµατα. Από τη χαρτογράφηση της γεωµετρίας των ιζηµατογενών στρωµάτων καθώς και των βαθύτερων στρωµάτων του φλοιού όπως αυτή προκύπτει από πραγµατοποίηση σεισµικών τοµών στη περιοχή, υποδεικνύεται η επώθηση της προ Απουλίας ζώνης

exposed on the island of Zakynthos and that of Messinia are highly elevated blocks of the thrust belts.

3 πάνω από την Ιόνιο, ενώ τα τεκτονικά τεµάχη της Ζακύνθου και της Μεσσηνίας αποτελούν τα ανώτερα τµήµατα της επωθούµενης ζώνης. 1. Introduction The south western part of the Hellenic arc is one of the most seismically active areas in Greece and the entire Mediterranean region (McKenzie, 1972; Makropoulos, 1978; Makropoulos and Burton, 1981; Jackson and McKenzie, 1988). This area has been repeatedly affected by large magnitude earthquakes that have caused severe destruction and human loss (Papazachos and Papazachou, 1997) (i.e Philiatra M7.3, 1893 Zakynthos-Keri M6.5, 1899 Kyparissia M6.5, 1947 Pylos M7.0, and 1997 Gargaliani M6.6). Some of the largest regional tsunamis in the Mediterranean Sea have also been observed in association with large earthquakes (i.e and 1866), affecting near field as well as remote coastal segments in western Peloponnese, Crete, and as far as Alexandria (Egypt), Adriatic Sea and east Sicily. Despite the significant progress in construction and earthquake engineering standards, the population growth and extensive urbanization have caused the risk from earthquakes to increase significantly in recent years. This situation requires urgent solutions for an effective risk management and mitigation plan. For this, it is essential to study the local seismic activity and define the active fault zones on/ and offshore with high accuracy and resolution required for a reliable seismic and tsunami hazard assessment. In the following paper we will present first results of an active and passive seismic study on/ and offshore western Peleponnese and we will discuss the tectonic implications. 2. Passive and Active Seismic Experiment To accurately locate the seismic activity we deployed a combined on/offshore network consisting of 17 ocean bottom seismographs (OBS) and 15 stand-alone land stations (fig. 1).

Burton, 1981; Jackson and McKenzie, 1988).

4 Figure 1 Location of the on/offshore seismic stations used to record microseismic activity in the south western Hellenic arc. Stations are represented by yellow (land) and green (marine) triangles. The area of SEAHELLARC array is shown in the inserted regional map of the eastern Mediterranean region. Both marine and land stations used were equipped with the SEDIS III seismic recorder of GeoPro, Hamburg (Makris and Μοeller, 1990). Within the two-month period of field observations we recorded more than 3500 microearthquakes above a threshold of magnitude of M L 0.3, observed by a minimum of six stations. Magnitudes were defined by the coda-length of the recorded events, (Crosson, 1972) previously calibrated using earthquakes recorded also by the Seismograph Network of the National Observatory of Athens. To locate hypocenters we used the HYPOINVERSE software (Klein, 2002). HYPOINVERSE allows the application of local velocity models for the hypocenter location. The model used in the present study was adapted to a controlled source seismic experiment (Makris and Papoulia, 2008) and is listed in Table I.

Both marine and land stations used were equipped with the SEDIS III seismic recorder of GeoPro, Hamburg (Makris and Μοeller, 1990).

5 TABLE I Local Velocity Model after Makris and Papoulia (2008) Velocity Vp (Km/sec) Depth (Km) Fig. 2 represents the error distribution of located events within and outside the seismic array. Within the array, time-residuals after location did not exceed an RMS 0.2s. This corresponds to an epicentral accuracy of the order of ±2 Km, whereas the hypocentral locations are affected by an error twice this value. Figure 2 Error distribution of located events within and outside the seismic array The distribution of the located events is shown in Fig. 3. It is obvious that the major part of the seismicity is crustal. Shallow activity (0-15 Km depth) is mainly concentrated around Zakynthos island, western Messinia and the offshore area 70 km west of Zakynthos. Seismicity is associated with intense deformation of the sediments and upper crust in the zones of crustal collision, and coincides with the tectonically uplifted blocks of Messinia, Zakynthos and the one 70 km west of it. Seismicity associated with the deeper part of the crust (15 30 Km) deepens gradually from west to east, from 20 to over 30 km, following the crustal thickening and the trend of the subduction zone.

This corresponds to an epicentral accuracy of the order of ±2 Km, whereas the hypocentral locations are affected by an error twice this value.

6 Subcrustal seismicity follows the subduction below western Peleponnese with gradually increasing hypocentral depths eastwards. Below the Saronikos gulf, east of Peleponnese, hypocentral depths reported by Makris et al. 2002, exceed 100 km. Figure 3 Microseismic activity in the south western Hellenic arc, recorded by the SEAHELLARC array. Period of observation: 23/9 to 26/11/2006. To better demonstrate the focal distribution of the located events and their relation to the crustal and tectonic structures we plotted two cross sections, across and parallel to the main geographic features ( see profiles I and II in fig. 1) All events located 20 Km on either side of the two profiles are projected on the cross sections. Profile I crosses from the deep Ionian sea, over the island of Zakynthos towards Vartholomio, west Peloponnese (see fig. 4). Two major clusters of crustal seismic activity were delineated between Km 28 and 48, and Km 60 and 80, respectively. The western one correlates with the northernmost end of the Mediterranean Ridge, as mapped by active seismics. The eastern cluster coincides with the Zakynthos uplift, built by the lower limestone of the pre-apulia zone, thrusted over the Ionian (see fig. 5, Makris and Papoulia, 2008). The subduction of the Ionian oceanic lithosphere below the continental crust of Peloponnese is reflected by the increasing hypocentral depths of the seismic foci towards the east, from 20 to 40 km depth.

Figure 3 Microseismic activity in the south western Hellenic arc, recorded by the SEAHELLARC array. Period of observation: 23/9 to 26/11/2006.

7 Figure 4 Depth distribution of located seismic foci along Profile I, crossing from deep Ionian sea over Zakynthos towards Peloponnese In figure 5 we have plotted the hypocentres of figure 4 in the crustal cross section, derived by active seismic observations (Makris and Papoulia, 2008) The two main concentrations of seismic activity are coinciding with two deformation belts of thrusting and crustal uplift. The one is at the continent-ocean transition of the oceanic Ionian crust colliding with the continental edge of the western Hellinides (Aegean plate). The second is at the intraplate crustal shortening of the area of Zakynthos, where the pre Apulian zone has been thrusted over the Ionian one. It is now obvious, that tectonic deformation does not affect the upper crust and sediments only. The entire crust is fractured and the stress field of the deformation is very complex. Figure 5 Velocity/Depth Model along Profile I, crossing from deep Ionian sea over Zakynthos island towards Peloponnese, revealed by active seismic observations (Makris and Papoulia, 2008) and distribution of recorded seismic events

crustal uplift. The one is at the continent-ocean transition of the oceanic Ionian crust colliding with the continental edge of the western Hellinides (Aegean plate).

8 Profile II is oriented parallel to the main collision from the gulf of Patras (offshore Kyllini) to offshore Pylos, western Messinia. The seismicity along this profile is presented in figure 6. All events are crustal except for a major zone with deep seismicity that extends to nearly 90 Km depth. This coincides with a transtensional basin between Zakynthos and Pylos, northeast of the island of Strofades. This deep activity is probably associated with a major fracture of the oceanic lithosphere that is displacing vertically two different lithospheric units that are getting subducted below the western Hellenic collision zone. Focal mechanisms associated with this fault zone demonstrate mainly dextral strike slip movements (see fig. 7), indicating that the subducted slab is fractured and adjacent lithospheric elements move at different speeds. The strongly confined space affected by the deep seismicity is probably the fracture surface between two different lithospheric units, dipping at different angles of subduction. Figure 6 Depth distribution of located seismic foci along Profile II, striking NNW-SSE from offshore Kyllini towards Pylos, parallel to the Kyparissiakos basin The active seismic experiment along this profile mapped the geometry of the sediments and crust. It shows that the Kyparissiakos gulf is flanked by two uplifted blocks; that of Zakynthos to the NW and to the one of west Messinia Pylos block to the SE. The in between area is divided in two sub-basins. The northern one coincides

$This deep activity is probably associated with a major fracture of the oceanic lithosphere that is displacing vertically two different lithospheric units that are getting subducted below the western$

9 with the transtensional basin mapped by multibeam data (Wardell, personal communication), having a rhombic shape and a very steep flank to the NW, associated with strike slip and normal faulting (see fig. 7). Figure 7 Focal mechanisms of selected events The thickness of the sediments exceed 7 km. The south eastern basin is flanked by normal faults, nearly symmetrical at both sides. This basin is filled with 4 Kms of sediments. In summary, the whole area extending from Zakynthos to Messinia is extensional, limited by two main fault zones, the one striking NE-SW, offshore Kyllini, dipping to the south east, and the other striking EW, offshore Pylos, dipping to the north (see fig. 8). Both fault zones are active, and are the offshore extensions of EW trending faults of western Peloponnese (see Mariolakos et al., 1985; Fountoulis, 1994; Papanikolaou et al., 2007).

This basin is filled with 4 Kms of sediments.

10 Figure 8 Velocity/Depth Model along Profile II. Main active faults are inidcated 3. Conclusions From active seismic observations performed in the frame of the SEAHELLARC project, and also by past seismic experiments (Makris and Papoulia, 2008), it became evident that the western Hellenides are developing by progressive thrusting to the southwest. Thrust fronts are accommodated laterally by strike slip faults, as revealed by solutions of focal mechanisms of local and regional earthquakes (see also Kiratzi and Louvari, 2003; Papoulia and Makris, 2004). The strike slip deformation favors the development of extensional basins between the thrust fronts. The one mapped in the Kyparissiakos gulf is an example of this process. Shallow seismic activity, showing the deformation of sediments and upper crust, is well correlated with the thrust blocks and uplifted segments at Zakynthos and Messinia. Subcrustal seismicity on the other hand follows the subduction of the Ionian oceanic lithosphere below western Peloponnese with systematically increasing depths towards the east. 4. Acknowledgments This study is a contribution to the EC, Frame Work Program 6, Project SEAHELLARC Field operations were accomplished by the R/V AEGAEO and R/V EXPLORA, and observers of GEOPRO Hamburg, HCMR Athens and OGS Trieste.

Hellenides are developing by progressive thrusting to the southwest.

11 5. References Crosson, R. S Small earthquakes, structure and tectonics of the Puget Sound region. Bull. Soc. Am. 62, Fountoulis, I., Neo tectonic evolution of the Central-Western Peloponnese. Ph.D. Thesis, Faculty of Geology, National and Kapodistrian University of Athens, GAIA 7, 386pp. (in Greek, abridged English version). Jackson, J., and McKenzie, D., Rates of active deformation in Aegean Sea and surrounding regions, Basin Research 1, pp Kiratzi, A. and Louvari, E., Focal mechanisms of shallow earthquakes in the Aegean Sea and the surrounding lands determined by waveform modeling: a new database, Journal of Geodynamics 36, pp Makris, J., and Moeller, An Ocean Bottom seismograph for general use. Technical requirements and applications., In Hoefeld, J., Mitzlaff, A. Polomsky, S. (eds.), Proceedings of Symposium Europe and the Sea, Hamburg. Makris, J., and Papoulia, J., The continent ocean transition offshore Zakynthos island, western Greece, revealed from wide aperture seismic data (in preparation). Makropoulos, C., The statistics of large earthquake magnitude and an evaluation of Greek seismicity. PhD thesis, Edinburg Univ., 193pp. Makropoulos, C. and Burton, P.W., A catalogue of the seismmicity in Greece and adjacent areas, Geophys. J. R. Astr. Soc., 65, Mariolakos, I., Papanikolaou, D., and Lagios, E., A neotectonic geodynamic model of Peloponnesus based on: morphotectonics, repeated gravity measurements and seismicity, Geologisches Jahrbuch Reihe B, Heft 50, pp

Jackson, J., and McKenzie, D., 1988. Rates of active deformation in Aegean Sea and surrounding regions, Basin Research 1, pp. 121 128. Kiratzi, A. and Louvari, E., 2003.

12 McKenzie, D., Active tectonics in the Mediterranean region, Geophysical Journal of the Royal Astronomical Society 30 (1972), pp Papazachos, B., and Papazachou C., Earthquakes in Greece. Editions Ziti, Thessaloniki. Papanikolaou, D., Fountoulis, J., and Metaxas, Ch., Active faults, deformation rates and Quaternary paleogeography at Kyparissiakos Gulf (SW Greece) deduced from onshore and offshore data, Quaternary International, , pp Papoulia, J., and Makris, J., Microseismicity and active deformation of Messinia, SW Greece, Journal of Seismology 8 (2004), pp

Active faults, deformation rates and Quaternary paleogeography at Kyparissiakos Gulf (SW Greece) deduced from onshore and offshore data, Quaternary

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