Variscan basic dykes in the Pelagonian (Northern Greece and south FYROM): Geodynamic significance based on petrological, geochemical and geochronological studies

Dolerite dykes intruding Variscan plutonites were studied in terms of mineralogy, petrology, geochemistry and geochronology. The main mineral constituents were studied and the sequence of crystallization has been derived. The geochemical characteristic indicate mantle origin of the dolerites and magma sources different from the hosting granitoids. From SHRIMP analyses of five spots on four different zircon crystals, resulted a 292.0±4.1 Ma age that is interpreted as the time of crystallization of the dolerite. The hosting granitoids are probably the result of mixing between two possible end-members: enriched mantle and acid metaigneous or lower crustal metasediments.

The Variscan age of the dolerites, in combination with the geochemical characteristics, indicated that the enriched mantle basaltic material should be the source of the dolerite veins. These mantle-derived basaltic melts may represent the underplated material, which probably provided the necessary thermal input to the dehydration melting in the lower crust. The dolerites should have intruded the newly formed batholiths before or at the first stages of their uplift, recording the last events of the Variscan subduction.     

DEM-based morphometry of range-front escarpments in Attica, central Greece, and its relation to fault slip rates

In this paper, we apply current geological knowledge on faulting processes to digital processing of Digital Elevation Models (DEM) in order to pinpoint locations of active faults. The analysis is based on semiautomatic interpretation of 20- and 60-m DEM and their products (slope, shaded relief). In Northern–Eastern Attica, five normal fault segments were recognized on the 20-m DEM. All faults strike WNW–ESE. The faults are from west to east: Thriassion (THFS), Fili (FIFS), Afidnai (AFFS), Avlon (AVFS), and Pendeli (PEFS) and range in length from 10 to 20 km. All of them show geomorphic evidence for recent activity such as prominent range-front escarpments, V-shaped valleys, triangular facets, and tilted footwall areas. However, escarpment morphometry and footwall geometry reveal systematic differences between the “external” segments (PEFS, THFS, and AVFS) and the “internal” segments (AFFS and FIFS), which may be due to mechanical interaction among segments and/or preexisting topography. In addition, transects across all five escarpments show mean scarp slope angles of 22.1°±0.7° for both carbonate and metamorphic bedrock. The slope angle equation for the external segments shows asymptotic behaviour with increasing height. We make an empirical suggestion that slope angle is a function of the long-term fault slip rate which ranges between 0.13 and 0.3 mm/yr. The identified faults may rupture up to magnitude 6.4–6.6 earthquakes. The analysis of the 60-m DEM shows a difference in fault patterns between Western and Northern Attica, which is related to crustal rheology variations.     

Empirical seismic vulnerability,deterministic risk and monetary loss assessment in Fira (Santorini,Greece)

A deterministic seismic risk and monetary loss model is presented for the capital of Santorini volcanic Island, the town of Fira, on a building block scale. A local seismic source of M5.6 inferred from a recent volcano unrest in 2011–2012, detailed seismic vulnerability of 435 buildings and site conditions deduced from free-field ambient noise measurements were combined toward assessing the EMS-98 damage grade and its probability to occur. The seismic scenario yielded no damage or slight damage for 84% of the buildings, 16% of the constructions are expected to present moderate-to-heavy damage, while the economic loss amounts to 4 million euros. Although the model predicts low damage and direct economic loss, interaction with the touristic business activities might produce cascade side effects for the economy of the island and consequently Greece’s GDP, an important part of which emanates from Santorini.     

Testing satellite and ground thermal imaging of low-temperature fumarolic fields: The dormant Nisyros Volcano (Greece)

The Nisyros Volcano (Greece) was monitored by satellite and ground thermal imaging during the period 2000–2002. Three night-scheduled Landsat-7 ETM⁺ thermal (band 6) images of Nisyros Island were processed to obtain land surface temperature. Ground temperature data were also collected during one of the satellite overpasses. Processed results involving orthorectification and 3-D atmospheric correction clearly show the existence of a thermal anomaly inside the Nisyros Caldera. This anomaly is associated mainly with the largest hydrothermal craters and has land surface temperatures 5–10 °C warmer than its surroundings. The ground temperature generally increased by about 4 °C inside the main crater over the period 2000–2002. Ground thermal images of the hydrothermal Stephanos Crater were also collected in 2002 using a portable thermal infrared camera. These images were calibrated to ground temperature data and orthorectified. A difference of about 0–2 °C was observed between the ground thermal images and the ground temperature data. The overall study demonstrates that satellite remote sensing of low-temperature fumarolic fields within calderas can provide a reliable long-term monitoring tool of dormant volcanoes that have the potential to reactivate. Similarly, a portable thermo-imager can easily be deployed for real-time monitoring using telemetric data transfer. The operational costs for both systems are relatively low for an early warning system.     

The Lemnos 8 January 2013 (M w = 5.7) earthquake: fault slip,aftershock properties and static stress transfer modeling in the north Aegean Sea

We investigate mainshock slip distribution and aftershock activity of the 8 January 2013 M _w?=?5.7 Lemnos earthquake, north Aegean Sea. We analyse the seismic waveforms to better understand the spatio-temporal characteristics of earthquake rupture within the seismogenic layer of the crust. Peak slip values range from 50 to 64 cm and mean slip values range from 10 to 12 cm. The slip patches of the event extend over an area of dimensions 16?×?16 km². We also relocate aftershock catalog locations to image seismic fault dimensions and test earthquake transfer models. The relocated events allowed us to identify the active faults in this area of the north Aegean Sea by locating two, NE–SW linear patterns of aftershocks. The aftershock distribution of the mainshock event clearly reveals a NE–SW striking fault about 40 km offshore Lemnos Island that extends from 2 km up to a depth of 14 km. After the mainshock most of the seismic activity migrated to the east and to the north of the hypocenter due to (a) rupture directivity towards the NE and (b) Coulomb stress transfer. A stress inversion analysis based on 14 focal mechanisms of aftershocks showed that the maximum horizontal stress is compressional at N84°E. The static stress transfer analysis for all post-1943 major events in the North Aegean shows no evidence for triggering of the 2013 event. We suggest that the 2013 event occurred due to tectonic loading of the North Aegean crust.     

The April 2007 earthquake swarm near Lake Trichonis and implications for active tectonics in western Greece

We investigate the properties of the April 2007 earthquake swarm (Mw 5.2) which occurred at the vicinity of Lake Trichonis (western Greece). First we relocated the earthquakes, using P- and S-wave arrivals to the stations of the Hellenic Unified Seismic Network (HUSN), and then we applied moment tensor inversion to regional broad-band waveforms to obtain the focal mechanisms of the strongest events of the 2007 swarm. The relocated epicentres, cluster along the eastern banks of the lake, and follow a distinct NNW–ESE trend. The previous strong sequence close to Lake Trichonis occurred in June–December 1975. We applied teleseismic body waveform inversion, to obtain the focal mechanism solution of the strongest earthquake of this sequence, i.e. the 31 December 1975 (Mw 6.0) event. Our results indicate that: a) the 31 December 1975 Mw 6.0 event was produced by a NW–SE normal fault, dipping to the NE, with considerable sinistral strike-slip component; we relocated its epicentre: i) using phase data reported to ISC and its coordinates are 38.486°N, 21.661°E; ii) using the available macroseismic data, and the coordinates of the macroseismic epicentre are 38.49°N, 21.63°E, close to the strongly affected village of Kato Makrinou; b) the earthquakes of the 2007 swarm indicate a NNW–SSE strike for the activated main structure, parallel to the eastern banks of Lake Trichonis, dipping to the NE and characterized by mainly normal faulting, occasionally combined with sinistral strike-slip component. The 2007 earthquake swarm did not rupture the well documented E–W striking Trichonis normal fault that bounds the southern flank of the lake, but on the contrary it is due to rupture of a NW–SE normal fault that strikes at a  45° angle to the Trichonis fault. The left-lateral component of faulting is mapped for the first time to the north of the Gulf of Patras which was previously regarded as the boundary for strike-slip motions in western Greece. This result signifies the importance of further investigations to unravel in detail the tectonics of this region.     

Coulomb stress triggering of earthquakes along the Atalanti Fault, central Greece: Two April 1894 M6+ events and stress change patterns

Two M6+ events occurred 15–20 km apart in central Greece on April 20 and April 27, 1894. We identify the April 27, 1894 rupture (2nd in the sequence) with the Atalanti segment of the Atalanti Fault Zone because of unequivocal surface rupturing evidence reported by Skouphos [Skouphos, T., 1894. Die swei grossen Erdbeben in Lokris am 8/20 und 15/27 April 1894. Zeitschrift Ges. Erdkunde zu Berlin, vol. 24, pp. 409–474]. Coulomb stress transfer analysis and macroseismic evidence suggest that the April 20, 1894 event (1st in the sequence) may be associated with the Martinon segment of the same fault zone. Our stress modelling suggests that this segment may have ruptured in an M = 6.4 event producing a 15-km long rupture which transferred 1.14 bar in the epicentral area of the April 27th, 1894 event, thus triggering the second M = 6.6 earthquake along the Atalanti segment and producing a 19-km long rupture. We also examined three alternative fault sources for the first event; however, all these produce smaller stress stresses for triggering the second event. The proposed slip model for the second earthquake is capable of producing coastal subsidence of the order of centimetres to decimetres, which fits the geological data. The 1894 earthquake sequence was followed by a difference in the timing of subsequent M > 5 events in each of the “relaxed” areas (stress shadows; a negative change in Coulomb failure stress > − 0.6 bar), which terminated between 22–37 years (north) and 80 years (south).     

High-resolution, Digital Mapping of the Seismic Hazard within the Gulf of Evia Rift, Central Greece Using Normal Fault Segments as Line Sources

Seismic hazard within juvenile rift systems ismodelled by a semi-statistical procedure based onfault segmentation theory. Fault segmentationprovides the physical basis for strong earthquakerecurrence in regions undergoing extension, andtherefore, it may control the position and size of rupture areas for future events. The computercode FRISK (McGuire, 1978) is applied to calculateprobabilistic estimates of ground motion parameters. Six normal fault segments, on average 25 km long, aremodelled as the earthquake sources within the Gulf ofEvia Rift, Central Greece. The position and length ofsegments have been determined from image processing ofremotely-sensed data and field work. FRISK output isdirected to a Geographical Information System (GIS)and a series of high-resolution (0.1 by 0.1 of adegree) digital maps are produced. The results for100 year acceleration (63% not to be exceeded) showa 50% increase in the seismic hazard from previouslyestimated magnitudes. Areas of maximum hazard (over0.4 g) are located on the south coast of the Gulf ofEvia between the towns of Kammena Vourla and Arkitsa. The obtained shape of Peak Ground Acceleration (PGA)contours is slightly elliptical with the long axis ofthe ellipse trending E-W, and symmetric, with respectto the rift axis.     

Catchment-wide estimate of single storm interrill soil erosion using an aggregate instability index: a model based on geographic information systems

The main objective of this paper is to estimate interrill erosion after rainfall in the basin of Mourganis river (442?km²; Kalabaka province, Trikala prefecture, Thessaly, Greece). For the estimation of the interrill erosion, the method of Valmis et al. (1988) was used, in combination with Nearing et al. (1989). Input data of the algorithm include the slope angle of the ground surface, the rainfall, the ground cover type, the height of canopy, and the instability of ground of the study area. The spatial data were processed by standard GIS software. Soil samples were collected in the field to calibrate the model. The results comprise soil erosion maps for two specific rainfall scenarios. The first rainfall scenario refers to the most extreme rainfall in this catchment that happened on the 7/21/1959 with 48?mm/h. The second scenario is closer to average as the intensity rainfall is 3.54?mm/h. The total mass of eroded material ranges from 0.048?t/ha (assuming mean rainfall intensity) up to 3.5?t/ha (for the extreme scenario). We note that the western part of the Mourgani basin exhibits higher erosion than the eastern part.