A significant earthquake sequence was initiated on 26 January 2014 at the western part of the Kefallinia Island. The study area is located in the Ionian Sea (W. Greece) at the northwestern end of the Hellenic Arc – Trench, in a region dominated by the Kefallinia - Lefkada transform fault, which exhibits dextral strike–slip motion at a rate of 2–3 cm year−1During the historical period, until 1900, 13 earthquakes with magnitude Μ≥6.0, have been reported in the Kefallinia region. The strongest event, of magnitude 7.4 and intensity X at Lixouri, occurred on 4 February 1867 (Papazachos and Papazachou, 2003; Stucchi et al., 2012). Major destructions were reported at the villages of the Paliki peninsula, while in Lixouri only two houses did not collapse. Ground ruptures were observed, as well as a tsunami of small height. Rock falls and liquefactions also occurred. In the Paliki peninsula 2612 houses collapsed, while only 4 in Argostoli, the capital and major town of the Kefallinia Island. Since 1900, 11 earthquakes with magnitude Μs≥6.0 occurred (Makropoulos et al., 2012) in the region. Five of them took place in 1953, four of which during August. The largest event had a magnitude equal to 7.3 and maximum intensity X+ in Argostoli. This earthquake was preceded by two strong events (Ms=6.1 and 6.8). The Ionian islands of Kefallinia, Zakynthos and Ithaca suffered very severe damages. Among the 33300 houses, 27659 collapsed. The highest intensities (IX-X) were observed, among other locations, at Argostoli, Lixouri and Valsamata. Thirty years later, on 17 January 1983, an event with magnitude Ms=7.0 occurred approximately 30 km SW of Lixouri, causing moderate damage. The most recent moderate event (Mw=5.6) occurred on 25 March 2007, 5 km NE of Mirtos Bay. (Kahle et al., 1996; Cocard et al., 1999). This region is situated between the Hellenic subduction zone to the south and the Apulia - Eurasian collision zone to the north. Focal mechanisms reveal right-lateral strike-slip motion (Anderson and Jackson, 1987; Jackson and McKenzie, 1988), coherent with geodetic data, according to which the slip motion has a NNE–SSW direction (Cocard et al., 1999; Jenny et al., 2004). On Sunday 26 January 2014 (13:55 GMT) a strong shallow earthquake of magnitude Mw=6.1 occurred in the study area. The epicenter is located about 2 km NE of Lixouri. It is worth noting that no significant earthquake sequence was recorded at the Paliki Penisula during the last decades. The main shock and the aftershock sequence were recorded by seismographs of the Hellenic Unified Seismological Network (HUSN), which involves the Seismological Laboratories of the Athens and Patras Universities, the Department of Geophysics of the Thessaloniki University and the Geodynamic Institute of the National Observatory of Athens (GI-NOA). Ground motion of the 26-1-2014 mainshock (Mw=6.1) was recorded by three permanent accelerographs located in Argostoli, Lixouri and Vassilikiades (ITSAK-EPPO, GI-NOA) with the response spectra in Lixouri indicating high horizontal acceleration. Moreover, the vertical component exhibited high spectral acceleration at a lower period when compared to the horizontal ones. Similar pattern is observed for the next strong event, which occurred on February 3, 2014 (Mw=5.9). A temporary accelerograph installed complementary to the permanent stations by GI-NOA in Chavriata, south of Lixouri, recorded an effective acceleration of 1g for the latter event, surprisingly exceeding the Greek Seismic Code provisions (0.36 g), being the highest recorded in Greece. The aftershock sequence was intense, while five hours after the mainshock, an aftershock of magnitude Μw=5.2 occurred. This sequence continued for a week with more than 30 events having magnitude greater than 4.0, till the occurrence of a strong earthquake of magnitude Μw=5.9. Its epicenter was located at the northwestern part of the Paliki peninsula. The earthquake sequence consists of more than 2000 events, the focal depths of which range mainly between 4 and 18 km. The aftershocks spatial distribution indicates that the activated seismogenic area is about 30 km length, in a NNE direction, located onshore in Paliki peninsula. At least three clusters can be distinguished. It is bounded to the south by the Vardiani islet and to the north by Myrto’s bay. The source parameters determination of the two strongest events were determined using body-wave modeling and teleseismic recordings. Synthetic waves are calculated by the trial-and-error method to determine the focal mechanism, the focal depth, and the seismic moment for a single trapezoidal source time function (Papadimitriou et al., 2006). Focal mechanisms of the strongest aftershocks have also been determined using the moment tensor inversion method developed by the Seismological Laboratory of the University of Athens (Papadimitriou et al., 2012). The data used are digital waveforms, recorded in regional distances by stations of HUSN. The observed seismograms were band-pass filtered between 0.02 Hz to 0.08 Hz. Following, Green Functions were calculated using the method proposed by Bouchon (1981). The inversion method revealed strike-slip focal mechanisms, in agreement with the fault plane solutions of the two main shocks. The results point out a dextral strike-slip rupture, oriented in a NNE-SSW direction. Taking into account both the aftershock spatial distribution and the focal mechanisms, it is concluded that the activated area does not coincide with the regional catalogue seismicity that is attributed to the Kefallinia - Lefkada transform fault, located offshore. Hence, activation of a sub-parallel major fault can be considered, compatible with the active tectonics of the region, which is crucial for its seismic hazard.
Lefkada Island is situated off the west coast of the Greek mainland and belongs to the complex of the Ionian Islands, one of the most seismically prone areas within the SE Mediterranean region, dominated by the right lateral Cephalonia Transform Fault Zone. As it is reported from historical and instrumental catalogues the study area has suffered numerous devastating local earthquakes with M>6.0. The most recent strong earthquake occurred on 14 August 2003, having M=6.2, at a distanceof only 10 km from the town of Lefkas where a peak ground acceleration of 0.42g was recorded, being one of the largest observed in Greece during the last decades. Although the earthquake producedlimited damage to the building stock, it likely induced damages of geotechnical character, associated with extensive liquefaction mainly in the waterfront of the town. Coseismic failure observationsprovided an expansive database which tempted us and triggered a multitasking project toward loss assessment in the Lefkada Old Town (LOT), comprising traditional construction practices of architectural significance and high seismic behaviour as a rule. The project employs several modules which were performed successively during the last years: (A) Macroscopic analysis of the buildings stock in LOT by conducting a comprehensive in-situ inventory of the buildings; (B) Vulnerability classification of the building stock using EMS98; (C)Ground Motion Parameters (GMP) determination for regional hazard assessment; (D) Site effects resulting from a detailed ambient noise study conducted in LOT; (E) Analysis of available geotechnical information and in-situ measurements of the local soil properties; (F) Employment of damage distributions during the recent 2003 earthquake; (G) Development of an ARC-GIS platform to include and allow the process of the massive data collected. In this paper we present a first attempt to synthesize the above modules in order to investigatesoil-structure interactions in LOT, assuming a linear response for both. By using various seismic sources whose characteristics are inferred by the active tectonics and the probabilistic hazard assessment of the study area, strong ground motion has been simulated both stochastically anddeterministically for hard bedrock conditions. GMPs were constrained beneath the foundations of the buildings by convolving the simulated ground motion with the soil transfer functions deduced frommicrotremors. According to this, for each scenario a damage potential has been assigned at each building. The obtained patterns were proven to be fairly comparable with the damage distribution due to the 2003 event, hence constituting a provocative motivation for further investigation and other applications.
Anelasticity of the Earth crucially affects the propagation of seismic waves especially, in the long period range. However, even though the elastic properties of the Aegean deep lithosphère and upper mantle have been thoroughly investigated, their quantitative anelastic properties that influence the long period wavefield are still largely unknown. This work is towards contributing to the better knowledge of the deep structure of the Aegean by introducing experimental anelastic parameters via the study of long period Rayleigh waves attenuation. For this scope, fundamental mode attenuation coefficients (γ%) have been obtained for different two-station great-circle paths across the Aegean. The data used were provided by a broadband array installed in the area for 6 months in 1997. More than 1100 seismograms were analyzed in the 10-100 s range to obtain 17 sets of path average γR(T) functions. The attenuation coefficients are in the range 2.5*10~3 — 0.15 x 10' km' and correlate sufficiently with both experimental measurements in active tectonic regions elsewhere and synthetics generated with the use of an attenuation reference model inferred from other sources. By applying a stochastic uncoupled causal inversion method an average joint Qß'1 and shear velocity model representative of the under study area was obtained. Furthermore, path average JR(T) functions were combined in a continuous regionalization tomographic scheme to obtain local yR(T) and tomograms were constructed in the range 10-60 s. The most prominent feature in the tomograms is a high attenuation region in the central and north Aegean. This region is located south of the North Anatolian Trough and correlates well with a low shear velocity zone inferred from surface wave phase velocities. Moreover, it is associated with observed intense extensional deformation rates, mantle olivine anisotropy, recent volcanism and high heat flow.