Lefkada Island is situated off the west coast of the Greek mainland and belongs to the complexof 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 andinstrumental catalogues the study area has suffered numerous devastating local earthquakes withM>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, beingone 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, associatedwith extensive liquefaction mainly in the waterfront of the town. Coseismic failure observationsprovided an expansive database which tempted us and triggered a multitasking project toward lossassessment in the Lefkada Old Town (LOT), comprising traditional construction practices ofarchitectural 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-situinventory of the buildings; (B) Vulnerability classification of the building stock using EMS98; (C)Ground Motion Parameters (GMP) determination for regional hazard assessment; (D) Site effectsresulting from a detailed ambient noise study conducted in LOT; (E) Analysis of availablegeotechnical information and in-situ measurements of the local soil properties; (F) Employment ofdamage distributions during the recent 2003 earthquake; (G) Development of an ARC-GIS platform toinclude 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 seismicsources whose characteristics are inferred by the active tectonics and the probabilistic hazardassessment of the study area, strong ground motion has been simulated both stochastically anddeterministically for hard bedrock conditions. GMPs were constrained beneath the foundations of thebuildings 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 eachbuilding. The obtained patterns were proven to be fairly comparable with the damage distribution dueto the 2003 event, hence constituting a provocative motivation for further investigation and otherapplications.

On May 21st , 2013, a large series of small earthquakes initiated a cluster formation, few km to the southeast of the city of Aigion, on the southwestern coast of the Gulf of Corinth in Central Greece. Over the next ten days more than 250 shocks with M<3.4 had been reported and on May 31st a M=3.7 earthquake was strongly felt and was accompanied by more than 100 smaller shocks within a 24 hour period. On July 13th-14th another outburst was reported with the occurrence of four M=3.0-3.5 events, followed by more than 250 earthquakes for the next three days. Thereafter, the activity diminished and terminated in the beginning of August. This unexpected phenomenon alarmed the local citizens because the seismic history of the area involves the occurrence of the great earthquake of 373 B.C, which extinguished the ancient city of Heliki, as described in great detail by Aristotelis in 330 B.C. The recent seismic history of the region has indicated that the Gulf of Corinth produces significantly high strain rates and it is ranked as the 'fastest’ continental rifts in the world and the most seismically active part of the Mediterranean. After the most recent catastrophic earthquake in 1995 with Ms=6.2 to the north of Aigion city, several seismological and geophysical networks have investigated the area and these have provided valuable scientific information concerning the regional seismotectonic regime. Shortly after the initiation of the May 21st, 2013 activity in Aigion, a local network of 10 portable seismographic stations was installed in the area, by the Institute of Geodynamics of the National Observatory of Athens and the Seismological Laboratory of the National Kapodistrian University of Athens. This network has been transmitting real-time data to the Hellenic Unified Seismological Network and recorded about 1000 events significantly improving the detectability of local earthquakes and the associated seismic hazard evaluation. In this study we investigate the dynamics and spatio-temporal characteristics of the sequence.For this purpose we performed relocation of the whole sequence using catalogue and waveform data and an optimized velocity structure, which improved the initial hypocentral solutions by the order of amagnitude. A large number of focal mechanisms was computed using P-wave first motion polarities of the local recordings, implying for shallow E-W normal faulting, compatible with regional tectonics. Furthermore, we employed a scheme involving the temporal frequency-magnitude and stress field distribution aiming to interpret the causative and triggering mechanism of the activity.

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.

This work is part of a multi-parametric research towards representation of damage scenarios in theLefkada old town. The study area lies in the most seismically active zone of the Greek territory. Mostof its buildings were built with local practices and have been designated by the European CouncilCultural Heritage Unit as representative earthquake resistant constructions. The August 14th, 2003(Mw=6.2) local earthquake produced several damage in the old town of Lefkada, with aninhomogeneous spatial distribution. In this paper we investigate the correlation of the observeddamage pattern with soil characteristics using data collected during an ambient noise survey conductedin 2007. Those were analyzed following Nakamura's HVSR methodology and soil response wasapproximated by the dominant frequencies and quasi-amplification factors of the resulted HVSRcurves. The latter were further inverted using a Monte-Carlo approach and best-fitting site specificgeotechnical models were determined. The obtained results are reasonably consistent with boreholetest data and show a remarkably good correlation with the 2003 damage distribution.

In this paper, we present a detailed study of a shallow seismic swarm which took place in the area of Oichalia (SW Peloponnesus), between August and December 2011. The seismic crisis started on 14/8/2011 with an Mw=4.8 earthquake and was followed by more than 1600 events, several of which having magnitude over 4.0. The activity was recorded by local temporary and regional permanent seismic stations. Thousands of records were collected and routinely analyzed. P- and S-wave arrival times were manually picked and incorporated in the HYPOINVERSE algorithm together with a new optimum local velocity model. Hypocentral solutions were improved by applying a double-difference method. Focal mechanisms show that the activated fault zone is dominated by dip-slip normal faulting, trending NNW–SSE, with the average T-axes orientation being N70°E, consistent with regional tectonics. We have investigated towards stress triggering and fluid diffusion, by employing Coulomb stress transfer, spatio-temporal and Frequency–Magnitude Distribution (FMD) analyses. The negligible Coulomb stress transfer and seismicity rate changes that were calculated imply for a stress deficit in the broader study area, hence an external triggering mechanism is required to justify the observed pattern. The b-values increase towards the SSE, compatible with the similarly directed migration of seismicity, showed that the Oichalia swarm could possibly be adapted to an Epidemic Type Aftershock Sequence model (ETAS). Fluid diffusion is reflected in the spatio-temporal hypocenter migration. Clustering analysis, combined with the temporal distribution of b-values, has shown that the swarm evolved in three major phases, the first two being initiated by major events, which were probably triggered externally due to fluid injection that brought the seismogenic volume into a critical state, likely followed by afterslip. The last phase signified a relaxation period, with dispersed seismicity throughout the area and the b-values gently diminishing towards unity.

On 10 April 2007, three moderate earthquakes with Mw=4.9–5.1 occurred in the vicinity of Trichonis Lake (W. Greece). A local network composed of 12 three-component digital seismographs was installed in the epicentral area and recorded more than 1600 events. The double-difference algorithm HYPODD, incorporating both catalog and waveform cross-correlation differential travel-time data, was applied for the successful relocation of 1490 earthquakes. The latter led to the distinction of a main NW-SE trending and NE-dipping zone, as well as of three neighboring faults; a conjugate NW-SE striking and SW-dipping marginal fault mapped along the northeastern flanks of the lake; a E-W trending and south-dipping low-angle normal fault, possibly related to the major Agrinio Fault Zone (AFZ), parallel to the northern bank of the lake; a NE-SW striking and NW-dipping normal fault, likely related to a segment of the active Evinos fault, located south of the lake. Calculation of the Coulomb stress induced by the combination of the 1975 Mw=6.0 event and the three largest events of 10 April 2007 on the inferred structures, reveals that most of the seismicity lies within the “stress-loaded” region, except for the westernmost activity, which probably belongs to the deep part of the AFZ. A total of 178 reliable focal mechanisms were determined by regional and local body-wave modeling (5 largest events) and P-wave first motion polarity data. The types of the obtained focal mechanisms are predominantly normal and strike-slip, however, numerous earthquakes were found to exhibit reverse faulting. Inversion of focal mechanism data showed that the prevailing principal horizontal component σ3 is quite homogeneous throughout the activated area with a roughly NW-SE trend, parallel to the strike of the Hellenides. On the contrary, the compressional field σ1 appears in two patterns: NE-SW trending onshore and NW-SE trending beneath the lake. This apparent rotation of σ1 by 90° reveals a complex system enclosed by the suggested NW-SE trending antithetic faults in depths between 7 and 9km. The calculated stress ratios beneath the lake imply that vertical forces are close to the overburden pressure. The overall inferred stress pattern is rather linked to topographic variations, locally imposing increase or decrease of the vertical forces. The presence of the water in the lake possibly plays an additional important role, penetrating through the bedrock, reducing the friction coefficient, while the pore pressure and, consequently, the effective stress increase. Thus, shearing along mature fractures is enhanced, likely yielding the observed diversity.