Preliminary Observations on the August 14, 2003, Lefkada Island (Western Greece) Earthquake
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EERI Special Earthquake Report — November 2003 Learning from Earthquakes Preliminary Observations on the August 14, 2003, Lefkada Island (Western Greece) Earthquake Following the earthquake, EERI strongly felt in other Ionian Islands heavily affected by the strong members and their colleagues from (Kephalonia, Zakynthos, Ithaki, etc.) ground motion (settlements, hori- three institutions in Greece went into and in a large area on the mainland zontal displacements, lateral the field. Teams from the Institute of of Greece. The shock was also felt spreading, liquefaction, loss of Engineering Seismology and Earth- as far away as Athens (Δ=280 km). shear strength). The area affected quake Engineering (ITSAK), the The epicenter was located in the by the mainshock falls in Zone IV National Technical University of Ionian Sea, about 30 km east-north- of the Greek seismic code (EAK Athens (NTUA), and the University east of the town of Lefkada. Within 2000), with a design acceleration of of Athens (UoA) participated in this the next 12 hours, three major after- ag=0.36g, the highest for Greece. reconnaissance study. shocks were felt. Lefkada is third in size among the A few hours after the earthquake A peak ground acceleration of Ionian Islands at 302.5 km2. The and during the subsequent three ag= 0.42g was recorded at the ac- island is only 50 m from the main- weeks, teams of experts (seismolo- celerograph installed in Lefkada, land of Greece, and is connected gists and civil engineers of the re- close to the epicenter. Despite the by a pontoon bridge. According to search staff of ITSAK) visited the very strong ground motion recorded the 2001 national census, there are meizoseismal area. ITSAK teams in Lefkada, only one reinforced con- 22,506 permanent residents on the consisted of engineering seismolo- crete building collapse was reported. island. The capital is the homony- gists B. Margaris, C. Papaioannou, There were no deaths, and only a mous town of Lefkada. and N. Theodulidis; geophysicist A. small number of injuries. Moderate Savvaidis; geotechnical engineers damage was observed in villages Tourism is highly developed on the A. Anastasiadis, N. Klimis, and K. of the central and western part of island of Lefkada and it swarms with Makra; and structural engineers M. the island as well as in the town of thousands of visitors during the Demosthenous, C. Karakostas, V. Lefkada. summer months. The earthquake Lekidis, T. Makarios, T. Salonikios, happened during the culmination of and S. Sous. The primary and secondary road the tourist season. At the time of the network of the island was damaged main event, Lefkada had an esti- NTUA and UoA formed one team by landslides and rock falls. A num- mated combined population of about consisting of P. Carydis of the Labo- ber of ports were moderately to 80,000 residents and tourists. ratory for Earthquake Engineering of NTUA and E. Lekkas, S. Lozios, E. Skourtsos, and G. Danamos of the Laboratory for Tectonics and Geo- logical Mapping of UoA. The publication of this report is supported by EERI’s Learning from Earthquakes Program under National Science Foundation grant # CMS-0131895. Introduction On August 14, 2003 at 08:15 local time (05:15 GMT) a magnitude 6.4 earthquake struck close to the island of Lefkada in Western Greece (Fig- Figure 1. Location and tectonic setting of the August 14, 2003, Lefkada ures 1 and 2). The earthquake was earthquake. 1 EERI Special Earthquake Report — November 2003 EERI Special Earthquake Report — November 2003 Repeated GPS measurements car- ried out between the northern Ionian Islands and Crete have shown rates of crustal motion of the Aegean reaching 35 mm per year, oriented SW relative to southern Italy (figure 1). Lefkada is 70% mountainous, with the highest peak being Stavrota (1182 m) in the center of the island. The western side of the island is very steep, with rather narrow sandy Figure 2. beaches, and the sea deepens rap- Location map idly. On the eastern side there are and fault plane large flat areas and the sea is shal- solutions of low with many islets. Inland there the main shock are narrow plateaus, fertile valleys, and the largest and deep ravines with opulent veg- aftershocks of etation. the Lefkada earthquake. The main tectonic structure of the island is an emplacement of carbon- ate rocks of the Ionian Unit over of more than 3000m. From marine the Pre-Apulian Unit, along a major The Seismotectonic Setting geophysical data it has been sug- N-S thrust fault (figure 3). Many gested that this trough represents neotectonic active faults, striking The tectonic setting of the wider a transform fault that demarcates to the NNE-SSW or E-W direction, area is determined by the continen- the northwestern end of the Hel- tal collision between northwestern lenic Subduction Zone. This has Greece in the east and the Apoulian been reinforced by earthquake focal platform in the west, as well as by mechanisms, microseismic studies, the subduction of the African plate and geodetic measurements that under the Aegean microplate along have shown that the KFZ is a right- the active Hellenic Arc in the south- lateral transform fault (figure 1). west (figure 1). The Ionian Islands are situated in a transitional zone Microseismic studies and GPS between the northwestern end of measurements (Yannick et al. 1998; this active subsidence and the con- Louvari et al. 1999) have shown that tinental collision in the north. This the horizontal movement continues transitional zone is characterized by in the sea area west of Kephalonia a high crustal deformation as re- and Lefkada (figure 2). This north vealed by the high seismicity of this segment of the KFZ was named the zone, which is the highest in the Lefkada segment and is character- Aegean. ized by dextral strike-slip motion with a thrust component. It strikes The main tectonic structure of this in a north-northeast direction, dips transitional area is the Kephalonia to the east-southeast, and has a Fault Zone (KFZ), which represents length of ~ 40 km. This fault length the active boundary between the corresponds to an earthquake mag- SW-moving Aegean microplate and nitude ~ 6.8, which is equivalent to the Apulian platform (figure 2). It is the magnitude of the largest event an offshore fault system to the west that struck Lefkada during the last of the island of Kephalonia, an area four centuries. The typical focal Figure 3. Alpine and neotectonic with a deep bathymetric trough, mechanism for an earthquake has structure and natural hazard map of striking at N20E, with water depths strike =14º, dip= 65º, rake =167º. Lefkada Island (Lekkas et al. 2001). 2 3 EERI Special Earthquake Report — November 2003 EERI Special Earthquake Report — November 2003 cross-cut the island. They represent mainly normal or strike-slip faults. All the alpine rocks are highly fractured and deformed, especially along the active faults and the major neotec- tonic and alpine structures. The poor quality rocks and the steep slopes result in landslides and rock falls, which cause frequent damage to roads and the respective infrastruc- tural works (Lekkas et al. 2001). Seismological and Strong- Motion Data Seismological Data. The area of planes are NP1: 11/60/165 and damage (Papazachos and Papaza- the three islands (Lefkada, Kepha- NP2: 109/77/31. chou, 1997). lonia, and Zakynthos) from north to south (figure 4) has the highest Table 1 summarizes the results for The aftershock sequence was ac- shallow seismicity in all of Greece the source parameters of the main tive during the first 24 hours, but and in the whole western Eurasia shock determined by various institu- diminished quickly over the follow- area (Papazachos 1990; Papaza- tions. The fault plane solutions of ing days. Within 12 hours of the chos and Papazachos 1997). Papa- these agencies indicate that the re- main shock, there were three major zachos and Papaioannou (2000) cent Lefkada earthquake was gen- aftershocks: the first was Mw = 5.3 separated the seismic activity in the erated by a right-lateral strike slip (ETHZ), the second Mw = 5.4 and islands into three seismogenic fault trending NNE-SSW. This is in the third Mw = 4.7. Surprisingly, the sources, Lefkada’s being character- agreement with the typical mecha- fault plane solutions of these major ized by high seismic activity with nism proposed by Papazachos et aftershocks, as provided by ETHZ, maximum magnitude M=7.1 and al. (1998). Since 1612, 16 strong correspond to thrust faults, which dominated by a strike-slip fault. The (M>6.0) earthquakes have been re- differs from the strike-slip solution of typical fault plane solution for this lated to this fault; the previous two the main shock (figure 2). seismogenic source has been pro- strongest ones during the instru- posed by Papazachos et al. (1998): mental period were in 1914 (M=6.3) Recorded Accelerograms and Re- the parameters of the two nodal and 1948 (M=6.5) and caused great sponse Spectra. Near the capital city, low-resolution digital accelero- graphs (Kinemetrics QDR-11 bits) had been installed by ITSAK, seven Figure 4. Perma- of which recorded the main shock. nent and temporary The triggering threshold of the net- strong motion array work instruments is 2%g, and all of ITSAK deployed in stations in the broader area were the broader epicentral found in operation. The station area (black squares: VAS1 was filled by noise and did permanent network; not record the earthquake. On Au- gray squares: tempo- gust 15, six digital high-resolution rary array deployed accelerographs (Guralp CMG-DM24 after August 15, 2003. with CMG-5 accelerometers) were Lefkada mainshock deployed on the island both to cap- is denoted by star.