Villy Kontogianni

Coastal uplift, earthquakes and active faulting of Rhodes Island (Aegean Arc): modeling based on geodetic inversion

The geodetic inversion technique was used to model the fault pattern responsible for the uplifted Holocene notches along the NE Rhodes Island coast (SE edge of the Hellenic (Aegean) Arc). Input in the modeling was the elevation of uplifted notches, up to 6000 yr old, which are interpreted to reflect remains of fossil shorelines, to testify to a series of earthquakes producing both uplift and subsidence (i.e. to elastic dislocations) and to correspond to geodetic data senso lato. These shorelines, up to 3.8 m high, were probably originally continuous along the whole of the 75-km-long SE Rhodes coast, possibly locally disturbed by minor normal faults, and reflect the last phases of the uplift and tilting of the island as a rather rigid block since Late Pliocene. Their radiocarbon dating was further refined from archeological data testifying to an about 1-m subsidence associated with the 227 BC earthquake, followed by >3-m seismic uplift. Inversion, constrained by the hypothesis of a compressional fault offshore Rhodes, was based on standard, uniform-slip, elastic dislocation analysis; it proved to depend only weakly even on extreme glacio-isostatic corrections and indicated that coastal uplift is dominated by a main compressional fault zone offshore, sub-parallel to the island coast, with normal faults inland reflecting thin-skin tectonics. This fault pattern, similar to that of western Crete, is confirmed by seismic reflection profiles indicating a major reverse close to the modeled fault and can explain (1) the occurrence of strong (M>7.5) historical earthquakes producing tsunamis and destruction on an East Mediterranean scale, (2) the alternation of uplift and subsidence as a result of fault shift inside the same fault zone, and (3) the long-term uplift of the island as a rather conjugate effect of the formation of an about 4.5-km-deep, 5-Myr-old marine basin SE of Rhodes, bounded to the NW by the modeled fault zone. The seismic risk of Rhodes, higher in its NE part of the island and than what was previously believed, is associated with strong (M>7.5) earthquakes producing widespread destruction, tsunamis, coastal uplift and coastal subsidence.

Predictions and observations of convergence in shallow tunnels: case histories in Greece

Abstract Convergence of shallow tunnels (30–120 m overburden thickness) constructed in Greece in different types of rock masses has been assessed as a function of the Geological Strength Index (GSI classification). Predictions of maximum vertical and horizontal convergence, during or shortly after tunnel excavation, were made using Finite Element Modeling (FEM) and the ‘characteristic line’ theory, and were found to be in good agreement with geodetic observations of convergence collected during a period of approximately 2 months after the section excavation. The results from FEM were found to adequately and reliably predict the expected deformation during tunnel excavation. The theory of the ‘characteristic line’, on the other hand, seems to offer a realistic and reliable upper-bound estimate of the convergence.

Geodetic evidence for slow inflation of the Santorini caldera

Abstract Santorini (Thera) is a volcanic island complex dominated by a partly submerged caldera defined by the islands of Thera and Therasia and the islet of Aspronisi. Santorini is the most important and most active volcano in the Aegean, famous for the Minoan (3600 years old) eruption which buried the ancient but very “modern” town of Akrotiri. Volcanic activity in Santorini was in many cases associated with ground deformations, subsidence and uplift, caused by magma flow at relatively shallow depths. Ground deformations therefore can be regarded as precursors of a future volcanic paroxysm. A geodetic monitoring system aiming at an early identification of a future dilation of the caldera as a result of magma inflation was established in 1994, in the framework of an interdisciplinary project for the surveillance of this volcano. The geodetic monitoring system consisted of a radial EDM network with a central point at the Nea Kameni islet and 10 stations in the Thera and Therasia islands. Between June 1994 and May 2003 eight epochs of baseline measurements at a centimeter-level accuracy were made. A small-scale (up to 10 cm), gradual inflation of the northern part of the caldera (between Nea Kameni and Therasia), possibly associated with magma ascent along a dike has been inferred from these epochs of measurements.

A review of surveying methods for sports and leisure

Abstract Applications of modern geodetic instruments in sports and leisure-related activities are reviewed in this article. Total stations, satellite GPS receivers and laser leveling systems are used to standardize dimensions of sporting grounds, to accurately and digitally record performances in jumps and throws during games, to boost performances of athletes in rowing by recording the details of the vessel kinematics during training, to navigate yachts, boats and mountaineers, and to define heights of high-flying kites.

TOLERANCE OF A LASER REFLECTORLESS EDM INSTRUMENT

Abstract Results of experiments with a laser reflectorless EDM instrument to test its tolerance are reported. Field distance measurements were simulated with measurements to >50 targets, identified with various surfaces, mostly not planar or normal to the sighting axis of instrument. Measured distances, covering the full nominal range of the instrument used, are characterized by systematic errors in the form of fluctuations with a maximum value up to 140mm at the distance of 20m, attenuating at longer distances. It was also found that the colour of target surfaces controls the maximum range of measurements, and some surface such as common mirror lead to erroneous results. Conclusions of our study are broadly consistent with conclusions of experiments with laser scanners.

Rock mass quality assessment based on deformation: experience from recent tunnels

The excavation of a tunnel re-arranges the stress field around the void, which tends to close due to the weight of the overlying rock mass. Tunnel closure is proportional to the void dimensions. For large tunnels (with a radius larger than 10 m and a length longer than 250 m), this takes the form of either a reduction of the tunnel section or a partial or total collapse (Figure 23.1), and thus represents a threat for excavation.