Papanastassiou, Dimitris

Il contesto geodinamico in cui ricade la Grecia fa di questa la regione sismicamente più attiva ed ha più alto rischio sismico d’Europa (Tsapanos, 2008). I processi tettonici in atto nella subduzione dello slab nel mar Egeo, controllano la distribuzione sia della sismicità sia del vulcanismo in Grecia (Karagianni et al., 2002, 2005). Oltre il 60% delle sismicità in Europa è localizzata in Grecia è forti terremoti hanno colpito più volte tale regione. La sismicità in Grecia è monitorata da quattro centri sismologici: l’Istituto di Geodinamica e i laboratori di sismologia delle università di Atene, Thessaloniki e Patras. Agli inizi del 2005, nell’ambito di un progetto nazionale chiamato “Hellenic Unified Seismological Network-HUSN” finanziato dal Ministero dello Sviluppo, le vari reti sismologiche greche sono state unificate. Tutti i segnali registrati dalle stazioni dei vari parterns, sono oggi raccolti dall’Istituto di Geodinamica e ritrasmessi agli stessi. Ad oggi la HUSN è composta da 88 stazioni digitali e molte altre saranno aggiunte negli anni a seguire. Nel presente lavoro abbiamo applicato il metodo SNES (D’Alessandro et al.) alla HUSN per quantificarne la performance di localizzazione e la magnitudo di completezza Le mappe SNES sono state determinate in funzione della magnitudo (ML 2, 2.5 e 3) fissando la profondità ipocentrale a 10 km e il livello di confidenza al 95%. Per piccole magnitudo (ML 2) solo due piccole aree in Macedonia e tra la Grecia centrale e il Peloponneso risultano coperte. Per magnitudo di poco maggiore (ML 2.5) quasi tutto il territorio Greco appare ben coperto con valori di RES di circa 2.5 km. La mappa della Magnitudo di Completezza mostra che tutto il territorio greco è coperto già per ML>2, con un valore minimo di circa 1.6 tra la Grecia centrale e il Peloponneso.

The three-dimensional attenuation structure beneath the Aegean sea and the surrounding regions was determined by inversion of seismic intensity data. A large number of seismic intensity data have been accumulated in a uniform scale in the Aegean region, where the seismic activity is much higher than that of the other Mediterranean regions. Nearly 11000 seismic intensity data from 47 earthquakes that have occurred in these regions were used to determine the seismic attenuation structure. The resulting structure reveals a remarkable contrast of attenuation. In the top layer (depth 0-20 km), low Q is dominant in the central Aegean sea, while high Q is dominant in the surrounding land areas, except for Southwestern Turkey. The low-Q regions correspond to areas of Neogene-Quaternary grabens where the high seismicity of shallow earthquakes appears. In the lower layer (20-40 km) low-Q areas are located in the southeastern part of the Hellenic arc. Some low-Q spots corresponding to the distribution of volcanoes exist along the volcanic arc. The low-Q spots might correspond to diapirs causing subduction volcanism.

In this paper we analyze the location performance of the Hellenic (Greek) Unified Seismological Network (HUSN) by SNES method (Seismic Network Evaluation through Simulation). This method gives, as a function of magnitude, hypocentral depth and confidence level, the spatial distribution of the: number of active stations in the location procedure and their relative azimuthal gaps and confidence intervals in hypocentral parameters regarding both the geometry of the seismic network and the use of an inadequate velocity model. Greece is located on a tectonically active plate boundary at the convergence of the Eurasian and African lithospheric plates and exhibits a high level of seismicity. The HUSN monitors the seismicity in Greek territory from 2007. At present it is composed by 88 seismic stations appropriately distribute in the area of Greece. The application of the SNES method permitted us to evaluate the background noise levels recorded by the network stations and estimate an empirical law that links the variance of P and S travel time residuals to hypocentral distance. This latter permitted us to assess the appropriateness of the velocity model used by the HUSN in the location routine process. We constructed SNES maps for magnitudes (M_L) of 2, 2.5 and 3, fixing the hypocentral depth to 10 km and the confidence level to 95%. We also investigated, by two different vertical sections, the behavior of the errors in hypocentral parameters estimates as function of depth. Finally we also evaluated, fixing the hypocentral depth to 10 km and the confidence level to 95%, the Magnitude of Completeness. Through the application of the SNES method, it is demonstrated that the HUSN provides the best monitoring coverage in western Greece with errors, that for M_L=2.5, are less than 2 and 5 km for epicenter and hypocentral depth respectively. At magnitude 2.5, this seismic network is capable of constraining earthquake hypocenters to depths of about 160 km and more, and provides a threshold of completeness down to magnitude 2 for most of Greek territory. Some seismogenic areas of southern Greece, that probably are not adequately covered by HUSN were delineated. The upgrading of the network in these areas could be optimized using the SNES technique.

The Atalanti fault bounds to the southwest the Evoikos Gulf, one of the major extensional basins of central Greece. This fault ruptured during the 1894 earthquakes, producing at the surface a complex, ca. 30-km-long rupture. Paleoseismological trenching performed at three sites along this fault provided the first insights on its seismogenic behavior. Unfavorable trench stratigraphy and scarcity of datable material made the identification and characterization of individual paleoearthquakes quite difficult. However, by integrating paleoseismological, geological, historical, and archaeoseismological data, we defined three surface-faulting earthquakes. The most recent event is the 1894 earthquake; the penultimate occurred during the Middle Ages between A.D. 770 and 1160, whereas the third event back occurred in Roman times between 50 B.C. and A.D. 230 and is interpreted to be the Opus earthquake of A.D. 105. These results suggest that 1894-type earthquakes repeat each 660–1120 yr. The average minimum slip per event and vertical slip rates are of the order of 45 cm and 0.4–1.6 mm/yr, respectively. These values are in agreement with other geological estimates and with geodetic measurements. Because of the short time elapsed since the 1894 earthquake, the Atalanti fault does not appear to contain an important seismogenic potential. On the other hand, these results may shed light on the potential of other seismogenic sources threatening the area.

Geomorphic observations focused on landforms of marine and fluvial origin such as notches, beachrocks, stream channel shifts, alluvial terraces and knickpoints, when combined with historical and archaeological information are able to date seismic events that took place in the past in some places of the Peloponnesus. At thc Eastern Gulf of Corinth, a seismically active area, all the geomorphic observations fit quite well with the deformation field induced by the action of an offshore fault. At Mycenae, a seismically inactive area with no historical evidence of earthquakes, the archaeological information is the only evidence for past earthquakes while geomorphic data indicate the most probable activated fault. At Sparta, an area of low seismicity but with historical evidence of destructive earthquakes, the geomorphic evidence helps to identify the most likely ruptured fault. At Eliki, a seismically active area with well documented historical activity, the geomorphic data serve to define the causative fault.This paper shows that although historical and archaeological data provide evidence far the occurrence of past earthquakes and often their date, geomorphic observations must be used to identify the causative fault.

This article presents the results of new field and aerial photo surveys of the Atalanti fault and of the mesoseismal area of the 20 and 27 April 1894 earthquakes. Coupled with a reanalysis of contemporary reports and previous investigations, these are used to gain a better understanding of the faults responsible for these events and their seismic behavior. The first shock was smaller and probably located inshore or offshore the Malessina peninsula. No resolving field evidence has been found to locate the seismogenic structure responsible for this shock. On the basis of the limited information available, we suggest the Malessina escarpment, a 12-km-long, ENE-trending, NW-dipping fault as a possible structure responsible for this event. On the other hand, the second and largest shock is definitely related to the Atalanti fault sensu stricto, a main WNW-trending, N-dipping active fault extending between the Platirema valley (a few km NW of the town of Atalanti) and Larymna. The total length of the rupture recognized in the field is about 32 km, but it can be extended further SE up to 40 km. No evidence for a longer rupture extending some other 20 km to the NW, between the Karagiozis river and Ag. Kostantinos, is found. The complex geometry of the fault with bends and step overs appears to be controlled by preexisting transverse structures. Minimum coseismic vertical throws, measured in the field after more than a century elapsed from the earthquake, are 30–80 cm, thus consistent with contemporary reports indicating 1-m average. Slip rates are not well constrained. The available estimates fall in the range 0.1–0.5 mm/yr confirming the smaller amount of crustal extension taking place in this area with respect to other nearby regions such as the Corinth gulf. No new data are available to define the average recurrence interval typical of the Atalanti fault. However, a reconsideration of the existing information induced us to rule out the possibility that the famous 426 B.C. earthquake occurred on the Atalanti fault. On the basis of the extent and size of the rupture recognized in the field, a M 6.8 is estimated for the second and largest shock.