5th International Symposium on Eastern Mediterranean Geology Thessaloniki, , 14-20 April 2004

The 14 August 2003 (Ionian sea) earthquake

Pavlides S.B.1, Papadopoulos G.A.2, Ganas A.2, Papathanassiou G.1, Karastathis V.2, Keramydas D.1 & Fokaefs A.2 1Department of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece, [email protected] 2Institute of Geodynamics, National Observatory of Athens, 11810 Athens, Greece

Keywords: Lefkada earthquake, macroseismic effects, active tectonics, landslide, soil lique- faction

A strong earthquake (Mw=6.2, Ms=6.4) occurred offshore NW of Lefkada Island (Ionian sea) on August 14, 2003 (05:14:53.9 GMT). The shock was felt over the , west and central Greece and also in the region from Peloponnese to Albania. No important foreshock activity was noted before the 2003 earthquake. On the contrary, intense aftershock activity was recorded by the NOAGI national seismograph system. Up to 22 September 2003 more than 270 aftershocks of ML ≥ 2.9 (or Ms ≥ 3.4) were recorded. The strongest of them (ML = 5.2) occurred on 14.08.03 at 16:18:03.9 GMT a few hours after the mainshock. The parame- ter b=1.10 of the G-R relationship and the aftershock decay exponent, k=1.24 , of the Omori law were found to have typical values for Greek earthquakes. Because of the main shock and its strong aftershocks many houses have been destroyed (about 120 out of 3000), almost the half of the buildings of the island were severely or light damaged. Only one building in the center of the town was collapsed. The maximum intensity has been evaluated as Io=VII+ to VIII in the town of Lefkas. For many other villages on the island the intensities ranged from V to VII. All intensities were assessed in both the 12-point EMS (European Macroseismic Scale) and MM scales. Fortunately, there were no deaths. Ex- tensive ground failures (rock falls, liquefaction, cracks) were observed, especially along the northwest part of the island forming a compound failure zone, reveal intensity peculiarities in association to neotectonics and morphotectonics of the area. The basement of the Lefkada island consists of: 1) carbonate sequence of the Ionian zone, 2) Limestone of Paxos zone restricted in the SW peninsula of the island, 3) few outcrops of Ionian flysch and Miocene marls – sandstones in the northern part of the island. Pleistocene and especially Holocene coastal deposits are extended in the northern edge of Lefkada, where the homonymous capital town is founded, in Vasiliki valley and Ligia-Nydri coast. The inherited tectonic structures are thrusts of the Ionian limestone on the Miocene marls from NE towards SW and a system of Neotectonic faults. Since kinematic indicators contained on the fault surfaces define the kinematic history of faults, quantitative analyses of striation data have shown a stress pattern of strike-slip or transtensional type (fig.1). The N-S and NE-SW trending neotectonic main faults are normal structures with significant right-lateral component (oblique slip), while some minor faults NW-SE trending show left-lateral character. Typical o o o o normal dip-slip faults are oriented ENE-WSW. That is, σ1 = 90 /80 axis, σ2 = 250 /10 and σ3 o o = 341 /3 trending NNW-SSE. T is compatible to σ3 and P to σ2 axes. Fault plane solutions determined teleseismically indicated the next nodal planes: NP1 (strike/dip/slip): 1040 / 820 / 60 (USGS), 170 / 600 / -1750 (Harvard); NP2 (strike/dip/slip): 130 / 840 / 1720 (USGS), 2850 / 860 / -300 (Harvard). The NNE-SSW elongation of the aftershock area implies that the nodal plane 2 of the USGS solution and the nodal plane 1 of the Harvard solution represent the fault plane. Then, it results that the motion in the seismic fault was a typical right lateral strike-slip with a small dip-slip component. For the Lefkada island the focal mechanisms of the 2003 event and the focal mechanism of

Ref: T5-34 5th International Symposium on Eastern Mediterranean Geology Thessaloniki, Greece, 14-20 April 2004 small earthquakes in association with the sea floor topography and the neotectonic pattern on land, support the existence of a right lateral strike-slip fault which strikes NNE-SSW (5-15o). According to seismological data it is a 40km long structure. This fault can be considered as the northern Lefkada fault segment, the northern edge of the Cephalonia transform fault. Tak- ing into account the western Lefkada linear coastal morphology and strike, it can be divided into two shorter segments separated by Sesoula Islet, the southern and northern ones di- rected NNE-SSW and NE-SW, respectively. Taking into account the fault Length – Magnitude empirical relationships the surface fault length capable to produce co-seismic ruptures (topog- raphic expression) corresponding to magnitude M 6.2 to 6.5 earthquakes is of 8 to 18 km. That is about one half of the Lefkada fault zone, which has been activated during the last earthquake, the northern segment. A working hypothesis is that the two Lefkada segments behave independently.

Figure 1. Stereographic projection (equal area, lower hemisphere) of representative striated neotectonic faults of northwestern-central Lefkada (as curves with arrows) and the strain-stress principal axes σ1 σ2 σ3, which are compatible to P, T and N axes aris- ing from the focal mechanism of the main shock. The Ionian islands have suffered from many destructive earthquakes. There is reliable detailed information for 24 events at least, since 1612 for Lefkada. Our evaluation of past earthquakes is based on the original manuscripts or second hand historical writings, (Newspapers “Aeon”, “Filomathon”, “Eho”; Archive of Lefkada for the seimicity of the island, File no 5; Barbiani D.& Barbiani G. 1864; Stamatelos 1870; Eginitis 1915; Maravelakis 1948; Galanopoulos 1955; Rondoyannis P. 1995; Papazachos & Papazachou, 1989/1997). A first conclusion arising from the list of historical events is that earthquakes appear in couples (twin or cluster events) with time period of occurrence ranging between 2 months and 5 years e.g. 1612-1613 (16 months); 1625-1630 (5years); 1722-1723 (10 months); 1767-1769 (2 year); 1783-1783 (2 months, possi- ble aftershock); 1867-1869 (2 years); 1914-1915 (2 months); 1948-1948 (2 months). The island suffered extensive ground failures and macroseismic effects, either to its northern part mainly, like the present event and the 1625,1630,1704, 1914 shocks, or also to its southern. The 2003 earthquake can be considered as the result of the rejuvenation of the northern Lefkada fault segment. The most characteristic co-seismic effects were typical ground failures like rock falls, soil liq- uefaction, ground cracks and particularly landslides. Rock falls and scree sliding were wide- spread on the whole island and especially on the western and central area. The most typical landslides were observed on the road net across the natural or artificial slopes, that is toppling and sliding, cm to 2m in length. Boulders up to 3-4m in diameter were observed along the 6km road of Tsoukalades - Pefkoulia - (epicentral area) accompanied by gravel, small rock and soil slides (row material). The falls follow a 10 to 40m high slope (200-300m morphological slope) in NNE-SSW fault zone, characterized by typical deep-slip normal and oblique faults, which are responsible for the high-angle dipping coast and deep-sea bathym- etry. Similar but less widespread phenomena were appeared in Kalamitsi, Dragano, Komilio,

Ref: T5-34 5th International Symposium on Eastern Mediterranean Geology Thessaloniki, Greece, 14-20 April 2004

Figure 2. Ground failures observed after the 14th of August strong earthquake:,2,3,5 Rock falls in Pefkoulia and 7 in Agios Nikitas. 4, sand crater 1.5m long in Pefkoulia. 1, dock cracks and sand-silt flow in Lefkas town.6, craks -lateral spreading in Lygia. 9. landslide in Agios Petros village. 8,10 liquefaction in Nydri and Vassiliki. Agios Petros, . In most cases rock falls are directly associated to pre-existing tectonic discontinuities and steep slope within the thick or thin bedded limestone. Ground cracks (excessive displacements 10 to 100cm), deformation and damage to quay walls and backfills observed mainly in Lefkas town quay, Lygia, Nidri and Vassiliki (western part of the island mainly), were associated with compaction of the foundation soil, liquefaction and/or settlement. Liquefactions were: a) Lateral spreading with sand and silt flow, while some of them have shown water ejected during the event according to eye-witnesses, and b) "sand volcanoes" (or sand boils), with small craters (diameters of few centimetres) in Vasiliki (southern edge of the Island) or larger (up to 2 meters diameters) as in Peukoulia-Agios Nikitas, Myloi and Kathisma beach (waterfront sites). They were important phenomena for some docks, pave- ments and seawalls damages, especially in the Lefkas town, e.g. 5 to 12cm horizontal shift was observed. Soil amplification effects in Lefkas, Lygia and Nydri coastal zones were re- sponsible for extensive damage, because of the soft lacustrine and recent alluvial soils. The recorded peak ground acceleration was 0.42g (ITSAK). This high acceleration was observed on a site with alluvial soft sediments, alternating clay-silt-sand layers, which overlie relatively estiffr marls. Sandy silts arew of media to high plasticity (CH-CL).Water table is in 1 to 2m depth. From the liquefaction sites were taken representing samples, whose gradation having the possibility of liquefaction. Specifically the percent finer than 0.002 mm is 5.11% in the sample from Lefkas town, 10.40% for the sample from Basiliki and 4.98% from the Lefkas port sample. Also the liquid limit LL of these three samples is 25, 25.2 and 24.5 respectively. All these soils had moisture bigger than their LL. According to the Chinese Criteria these soils are considered to be of high potentially liquefiable type. The ground deformation results of the 2003 earthquakes are remarkably similar with that of same historical shocks e.g. 1625, 1630, 1684, 1704, 1769, 1820, 1825, 1869 and especially 1914(Ms=6.3) and 1948(Ms=6.5) earthquakes, that is tend to confirm the characteristic earth-

Ref: T5-34 5th International Symposium on Eastern Mediterranean Geology Thessaloniki, Greece, 14-20 April 2004 quake model. Id NAME LATDD LONGDD REMARKS 1. NIKIANA 38.7745 20.7229 ROCK FALLS 2. LYGIA 38.7873 20.7210 LIQUEFACTION –settlement 3. AMMOGLOSSA 38.845 20.714 LIQUEFACTION- Settlement, mud ejection 4. BRIDGE 38.839 20.711 GROUND CRACKS 5. PARKING 38.8315 20.7080 LIQUEFACTION – mud ejection 6. PORT 38.8320 20.7086 LIQUEFACTION 7. PORT 38.8327 20.7103 LIQUEFACTION – Settlement 8. PORT 38.8342 20.7119 Settlement – ROAD 9. PORT 38.8350 20.7119 Settlement – PIER 10. PORT 38.8355 20.7096 Settlement 11. TSOUKALADES 38.8179 20.6413 ROCK FALLS 12. KARYA 38.76 20.645 ROCK FALLS 13. KAVALLOS 38.7918 20.6540 ROAD CRACKS 14. ASPROGERAKAS 38.8028 20.6340 SLIDE 15. ASPROGERAKAS 38.7909 20.6505 ROCK FALLS 16. DRIMONAS 38.7877 20.6352 ROCK FALLS 17. KALAMITSI 38.7704 20.6171 SLIDE 18. AGNIKITAS 38.7898 20.6150 ROCK FALLS 19. PEFKOULIA 38.8035 20.6310 LIQUEFACTION – Sand crater 20. NYDRI 38.7115 20.7173 LIQUEFACTION –Sand boils 21. NYDRI 38.7079 20.7122 LIQUEFACTION 22. BASILIKI 38.6272 20.6069 LIQUEFACTION 23. BASILIKI 38.6255 20.6066 LIQUEFACTION -Settlement 24. BASILIKI 38.629 20.60 LIQUEFACTION – Mud Volcanoes 25. AG.PETROS 38.6825 20.5968 SLIDE – ROCK FALL 26. TSELIGAS 38.6285 20.5617 SLIDE OVER EGREMNOI BEACH Table 1. Detailed list of sites and villages with remarkable ground failures (fig. 2)

Reference Karakostas V.G., Papadimitriou E.E. & Papazachos C.B. (2003). Properties of the 2003 Le- fkada, Ionian Islands, Greece, earthquake seismic sequence and seismicity triggering (Submitted). Margaris et al. (2003). The Lefkas (Greece) earthquake of August 14, 2003: preliminary re- ports on strong motion data, geotechical and structural damage, 1st International Work- shop on Earthquake Prediction -Athens, November 6-7, 2003 (Short report 4 p.). Papadopoulos A.G., Karastathis V., Ganas A., Pavlides S., Fokaefs A. & Orfanogiannaki K. (2004). The Lefkada, Ionian Sea (Greece), Shock (Mw 6.2) of 14 August 2003: Evi- dence for the Characteristic Earthquake from Seismicity and Ground Failures, Earth, Planes and Space (in press). Rondoyanni-Tsiambaou Th. (1997). Les seismes et l’environnement géologique de l’île de Lefkade, Grèce: Passe et Futur, In: Marinos P. et al. (Eds.), Engineering Geology and the Environment, Balkema, 1469-1474.

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