Athanassios Ganas

The Lefkada, Ionian Sea (Greece), shock (Mw 6.2) of 14 August 2003: Evidence for the characteristic earthquake from seismicity and ground failures

The earthquake (Mw 6.2, Ms 6.4) of 14 August 2003 which occurred in the Lefkada segment of the Cephalonia Transform Fault, Ionian Sea (Greece), was associated with dextral strike-slip faulting striking NNE-SSW. Reevaluation of instrumental and documentary sources show that the 1914, 1948 and 2003 earthquakes ruptured the same fault segment and that all had similar size, which implies that this segment produces characteristic earthquakes. This is verified by the magnitude-frequency diagram which for the instrumental period of 1911–2003 exhibits a relatively narrow range of magnitudes near the maximum (∼Ms 6.4), deviation from the log linear relationship and a gap in the moderate-magnitude range. Field observations indicate that the 2003 earthquake and past strong shocks caused on Lefkada island impressively similar ground failures at exactly the same sites: extensive landslides and soil liquefaction, which signifies comparable strong motion features as an additional evidence of the characteristic earthquake. However, while the maximum seismic intensity for the 1914 and 1948 strong shocks is estimated as I max= IX t - X (MM scale), the impact of the 2003 shock was less severe (Imax= VIII) possibly due to building strengthening after 1948.

GIS-based statistical analysis of the spatial distribution of earthquake-induced landslides in the island of Lefkada, Ionian Islands, Greece

This is the first landslide inventory map in the island of Lefkada integrating satellite imagery and reports from field surveys. In particular, satellite imagery acquired before and after the 2003 earthquake were collected and interpreted with the results of the field survey that took place 1xa0week after this strong (Mwu2009=u20096.3) event. The developed inventory map indicates that the density of landslides decreases from west to east. Furthermore, the spatial distribution of landslides was statistically analyzed in relation to the geology and topography for investigating their influence to landsliding. This was accomplished by overlaying these causal factors as thematic layers with landslide distribution data. Afterwards, weight values of each factor were calculated using the landslide index method and a landslide susceptibility map was developed. The susceptibility map indicates that the highest susceptibility class accounts for 38xa0% of the total landslide activity, while the three highest classes that cover the 10xa0% of the surface area, accounting for almost the 85xa0% of the active landslides. Our model was validated by applying the approaches of success and prediction rate to the dataset of landslides that was previously divided into two groups based on temporal criteria, estimation and validation group. The outcome of the validation dataset was that the highest susceptibility class concentrates 18xa0% of the total landslide activity. However, taking into account the frequency of landslides within the three highest susceptibility classes, more than 85xa0%, the model is characterized as reliable for a regional assessment of earthquake-induced landslides hazard.

The problem of seismic potential assessment: Case study of the unexpected earthquake of 7 September 1999 in Athens, Greece

Lessons learned form the disastrous earthquake (MW = 5.9) that hit the metropolitan area of Athens, Greece, on 7 September 1999, are examined particularly as for the seismic potential considered before the earthquake occurrence. A general belief was created in the past decades that the seismic potential in Athens was very low. Fault plane solutions of the 1999 shock indicate that it was associated with a normal fault trending WNW-ESE and dipping to SW. Field geological observations conducted after the event in the Fili neotectonic fault, situated at 15–20 km to the north of Athens, imply that it has possibly been the seismogenic structure of the main rupture, and that it reactivated in very recent geological times. Archaeoseismological observations performed in the ancient Fili Fort, revealed repaired structural damage that was very likely caused by an earthquake occurring in palaeochristianic or Byzantine times. From a new catalogue of historical earthquakes it results that the main events of 1705, 1805 and 1889 could be tentatively located within a distance of ∼30 km from Athens although the little macroseismic information available makes their locations quite uncertain. During the instrumental period of observation, only few small shocks were recorded in the Athens region. It is obvious that should a research effort had been undertaken before the 1999 earthquake, certainly it would be concluded that at least one strong earthquake took place in historical times in the broad region affected in 1999, and that the Fili fault is active and is capable to produce strong shocks in the future. However, such a study was never conducted by the scientific community beforehand.

Near-source high-rate GPS, strong motion and InSAR observations to image the 2015 Lefkada (Greece) Earthquake rupture history

The 2015/11/17 Lefkada (Greece) earthquake ruptured a segment of the Cephalonia Transform Fault (CTF) where probably the penultimate major event was in 1948. Using near-source strong motion and high sampling rate GPS data and Sentinel-1A SAR images on two tracks, we performed the inversion for the geometry, slip distribution and rupture history of the causative fault with a three-step self-consistent procedure, in which every step provided input parameters for the next one. Our preferred model results in a ~70° ESE-dipping and ~13° N-striking fault plane, with a strike-slip mechanism (rake ~169°) in agreement with the CTF tectonic regime. This model shows a bilateral propagation spanning ~9u2009s with the activation of three main slip patches, characterized by rise time and peak slip velocity in the ranges 2.5–3.5u2009s and 1.4–2.4u2009m/s, respectively, corresponding to 1.2–1.8u2009m of slip which is mainly concentrated in the shallower (<10u2009km) southern half of the causative fault. The inferred slip distribution and the resulting seismic moment (M0u2009=u20091.05u2009×u20091019 N m) suggest a magnitude of Mw 6.6. Our best solution suggests that the occurrence of large (Mwu2009>u20096) earthquakes to the northern and to the southern boundaries of the 2015 causative fault cannot be excluded.