S. Pavlides

Extensional tectonics of northwestern Macedonia, Greece, since the late Miocene

Tectonic field studies in the Florina-Ptolemais basin (northwestern Greece) were carried out in an attempt to define the regional stress tensors in selected areas affected by Neogene and Quaternary faulting, using recently proposed quantitative methods. The analysis allows us to distinguish two extensional phases in the area: a late Miocene-Pliocene one with a NESW average direction of extension: and a Pleistocene-Recent one with a NWSE direction of extension. We conclude that the principal stress axes α2 and α3 were interchanged in passing from one phase to the other. These results are reasonably consistent with results of studies carried out in other parts of the Aegean area, especially the south Aegean back-arc domain.

Late Cainozoic geodynamic evolution of Thessaly and surroundings (central-northern Greece)

Abstract In the framework of the late Alpide deformation of Greece and of the recent and active extensional tectonism of the Aegean region, the geotectonic evolution of the Thessaly region (central-northern Greece) has been examined, using a quantitative and qualitative structural analysis; stratigraphie, sedimentological, morphotectonic and seismological data. The geometry of the faults, their architecture and the knowledge of the stress pattern are used to explain some aspects of the tectonics and crustal dynamics of Thessaly and the surrounding area. The oldest compressional phases taken into account show a mean ENE-WSW trending direction of shortening and have been defined as late Alpide (early Aquitanian and Langhian). A later (Late Miocene-Pliocene) NE-SW oriented extensional phase has been related to the Hellenic post-orogenic collapse which developed behind the collisional front between the Aegean (Eurasia) and African plates. This phenomenon diachronically migrated from the east (Central Macedonia, Thermaikos Basin) towards the west (Epirus, Albania) where it is still active. As a consequence of this second phase, the area forms a basin and range like structure. The third, and last, phase (Middle Pleistocene-present) is characterized by a N–S direction of extension and affects the entire Aegean region. It generated new E–W trending basins, superimposed on the inherited ones. This gave as a final result, the complex block pattern we can see today. The recent and active right-lateral strike-slip movements along the North Aegean Trough seem to stop in the Sporades Basin and do not affect the uppermost crust of mainland Greece. A further WNW-ESE directed extension observed occasionally in central and northern Greece could be explained by local events or as block-related deformation.

Seismotectonics of the Aegean region

Abstract Fault plane solutions, neotectonic field observations, and in-situ stress measurements have been used to determine the stress field associated with the active deformation of the lithosphere in the Aegean and surrounding regions. A stress gradient and a zonal pattern of different tectonism away from the trench axis has been postulated. Pure thrust faulting occurs in the fore-arc side while the back-arc side is mainly dominated by almost N-S extensional tectonics. A narrow belt of strike-slip faults with predominantly thrust component separates these tectonic zones. This belt seems to represent a transition from the thrust-type faults to normal faults. Fault plane solutions of the whole region in question indicate that both the large and small magnitude shocks are caused by the same regional stress-field. A body of 33 focal mechanisms indicates that the intermediate depth shocks along the sinking Mediterranean slab are associated with thrust-type fracturing. Orientation of kinematic axes favours the suggestion that shear zones within the descending slab cause these shocks. The active crustal shortening along the Hellenic consuming boundary and the sinking of the Mediterranean lithosphere beneath the South Aegean area explain satisfactorily the seismotectonic features of the fore-arc and the southern back-arc sides. On the contrary, geophysical evidence concerning the Cenozoic geodynamic evolution of the Aegean is needed to interpret features of the north back-arc area.

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.

The large 1956 earthquake in the South Aegean: Macroseismic field configuration, faulting, and neotectonics of Amorgos Island

Abstract New field observations of the seismic intensity distribution of the large (M s = 7.4) South Aegean (Amorgos) earthquake of 9 July 1956 are presented. Interpretations based on local ground conditions, structural properties of buildings and peculiarities of the rupture process lead to a re-evaluation of the macroseismic field configuration. This, together with the aftershock epicentral distribution, quite well defines the earthquake rupture zone, which trends NE-SW and coincides with the Amorgos Astypalea trough. The lateral extent of the rupture zone, however, is about 40% smaller than that predicted for Aegean earthquakes of M s = 7.4. This discrepancy could be attributed to sea-bottom topography changes, which seem to control the rupture terminations, and to relatively high stressdrop with respect to other Aegean earthquakes. Fault plane solutions obtained by several authors indicate either mainly normal faulting with a significant right-lateral strike-slip component or predominantly strike-slip motion. The neotectonism of Amorgos Island, based on new field observations, aerial photograph analysis and fault mechanisms, is consistent with the dip-slip interpretation. The neotectonic master fault of Amorgos and the 1956 seismic faulting appear to belong to the same tectonic phase (NE-SW strike and a southeasterly dip). However, the significant right-lateral strike-slip component supports the idea that the Amorgos region deviates from the simple description for pure extension in back-arc conditions.

Structural characteristics of two strong earthquakes in the North Aegean: ierissos (1932) and Agios Efstratios (1968)

Abstract Structural analysis has been carried out on the volcanics of the island of Agios Efstratios and along the highly faulted zone of SE Chalkidiki, south and north of the North Aegean Trough, respectively (northern Greece). The areas have been affected by strong earthquakes and a comparison is made between the available structural and seismological data, together with new seismotectonic information. At Agios Efstratios, the earthquake of 1968 (M7.1) occurred on a principal displacement zone with dextral strike-slip movement, while the fault geometry of the whole area supports the view of transtensional deformation. The strong (M7.0) earthquake of 1932 at Ierissos was directly connected with an E—W-trending normal fault. Striations on the seismic fault surfaces and the corresponding fault mechanism indicate a N-S direction of active extension. The hangingwall is also affected by smaller antithetic E-W- and NW-SE-trending structures.

The Fault that Caused the Athens September 1999 Ms = 5.9 Earthquake: Field Observations

On 7 September 1999 the Athens Metropolitan area (Greece) was hit by a moderate size (Ms = 5.9) earthquake. The severely damaged area is localized in the northwestern suburbs of the city, at the foothills of Mt. Parnitha (38.1°N, 23.6°E), about 18 km from the historic centre of Athens. In this paper, we present our results on the surface expression of the seismogenic structure. Methods applied were: field observations, geological mapping, fault geometry and kinematics, evaluation of macroseismic data, interpretation of LANDSAT images, construction of a DEM and application of shading techniques. Aftershock distribution and fault plane solutions were also considered. Our results suggest that the earthquake source is located within the NW-SE trending valley bearing a few outcrops of Neogene-Quaternary sediments across the south foothills of Mt. Parnitha, never known in the past to have been activated by such strong earthquakes. The earthquake occurred along a 10 km long normal fault, striking N110°–133° and dipping 64°–85°SW, extending from the Fili Fort (4th century BC) in the NNW to the Fili town and then to Ano Liossia, to the SSE. Tensional stress field with σ3 axis almost horizontal striking NNE-NE prevails in the area. The fault strike and the extensional direction (σ3) are compatible with the focal mechanism of the main shock.

The large tsunami of 26 December 2004 : Field observations and eyewitnesses accounts from Sri Lanka, Maldives Is. and Thailand

Post-event field surveys were conducted and measurements were taken in Sri Lanka and Maldives about two weeks after the catastrophic Indian Ocean tsunami of 26 December 2004. The measurements taken were cross-checked after interviewing with local people. In the southwest, south and east coastal zones of Sri Lanka maximum water levels ranging from h = 3 m to h = 11 m a.m.s.l. were estimated. The highest values observed were in the south of the island: Galle h ∼ 10 m, Hambantota h ∼ 11m. Maximum inundation of d ∼ 2 km was observed in Hambantota. The heavy destruction and thousands of victims caused in coastal communities, buildings and infrastructure, like railways and bridges, is attributed not only to physical parameters, like the strength of the tsunami hydrodynamic flow, coastal geomorphology and the wave erosional action in soil, but also to anthropogenic factors including the increased vulnerability of the non-RC buildings and the high population density. Local people usually described the tsunami as a series of three main waves. The leading wave phase was only a silent sea level rise of h ≤ 1.5 m and d ≤ 150 m, while the second wave was the strongest one. The first two waves occurred between 09:00 and 09:30 local time, depending on the locality. It is well documented that near Galle, southern part, the strong wave arrived at 09:25:30. In the west coast the third wave was a late arrival which possibly represents reflection phases. In Maldives, three waves were also reported to arrive between 09:00 and 09:30 local time. Maximum water level was only h ∼ 3 m in Laamu Atoll, which is interpreted by the wave amplitude damping by the coral reef to the east of the island complex as well as to that the tsunami did not arrived at high tide time. Damage was observed in several islands of Maldives but this was minimal as compared to the heavy destruction observed in Sri Lanka. About 25 Greek eyewitnesses, who happened to experience the tsunami attack in Padong and Blue Lagoon Port of Phuket island as well as in Maya Bay, Phi-Phi islands, Thailand, were interviewed on the basis of a standard questionnaire. The first sea motion was a retreat of at least 100 m. Then, two main waves arrived, the first being the strong one occurring at about 09:55–10:05 local time, with h ∼ 6m in Padong causing significant destruction and human victims. The collected information clearly indicates that the tsunami propagated as the leading crest wave to the west side, e.g. in Sri Lanka and Maldives, and as the leading trough wave to the east, e.g. in Thailand.

Seismic fault geometry and kinematics of the 13 May 1995 Western Macedonia (Greece) earthquake

Abstract During the devastating earthquake of 13 May 1995, in the Kozani-Grevena area (Western Macedonia, Greece), many surface ruptures formed in the epicentral area. Most of these fractures were due to faulting, but some were secondary ground ruptures and landslides. Geological field work in the area has shown that the Aliakmon river neotectonic fault consists of several (three or more) fault strands: the Servia, the Rymnio and the Paleochori-Sarakina strands. Using geological criteria, all of these fault strands were judged to be active faults affecting recent (Holocene) deposits and scree. The main new surface fractures caused by the earthquake, and particularly those clearly of tectonic origin, follow systematically the traces of the last two neotectonic fault strands, forming a new fracture line. This tectonic line, trending ENE-WSW (N 70 °), coincides with the focal mechanism solution and the satelite image major lineament. Both the geological and instrumental seismological data suggest that the seismogenic fault is a segment of the Aliakmon river neotectonic fault zone situated among the villages of Rymnio, Paleochori, Sarakina, Kentro and Nisi. The total length of the reactivated fault segment is about 30km long overall and is separated from the non-activated Servia fault segment by a geometrical seismic segment barrier near the village of Goules. The seismic fault is a normal fault trending ENE-WSW and dipping to NNW, with high angle at the surface and low angle at depth. The majority of the epicentres of the seismic sequence were distributed on the hangingwall of this reactivated fault segment. Additionaly a series of subparallel antithetic surface fractures, mainly striking E-W or ENE-WSW and dipping to the South, following previous neotectonic strike-slip faults, were reactivated during the earthquake with the geometry of normal faults antithetic to the main seismic fault. The most important of these are the Chromio-Varis-Myrsina fracture line (length 15km), along the Vourinos corridor dextral strike-slip structure and the Felli fracture line (length 6 km) along the Felli sinistral strike-slip fault. An interpretation of the geometry and kinematics of the reactivated faults is shown in the proposed geological model with simplified cross sections.

Radon Activity Levels And Effective Doses In The Perama Cave, Greece

Abstract— An investigation of atmospheric radon levels in the Perama Cave, North-western Greece, has been carried out using CR-39 detectors. The detectors were placed at various locations along the guided cave pathway and exposed during different sampling periods. Mean concentrations amounting to 925 ± 418 and 1,311 ± 352 Bq m−3 were recorded in the summer and winter months, respectively. As the Perama Cave is one of the most popular in Greece, attracting more than 85,000 tourists per year, the quantification of effective doses to staff and visitors was an issue of importance. Doses less than 5.1 μSv per visit were calculated for tourists and around 1.8 mSv y−1 for seasonal guides, employed for periods of high visiting frequency. The annual exposure of permanent guides was estimated to fall between 3 and 10 mSv, which is the range of action levels recommended by the ICRP.