Tectonic structure of Central-Western Attica (Greece) based on geophysical in vestigations-Preliminary results

In an effort to investigate the deep geological structure in the broader area of central-western Attica, that suffered severe damage during the destructive Athens earthquake of September 7th, 1999, the Department of Geophysics-Geothermics of the Faculty of Geology and Geoenvironment of Athens University, in collaboration with the Geodynamic Institute of National Observatory of Athens and the Department of Geological Sciences of Durham University, carried out a combined geophysical survey. For the first time in Attica, seismic and gravity geophysical methods were applied along profiles, in such an extensive scale. Within the framework of this investigation the following tasL· were accomplished: a) Three (3) seismic lines of about 30 kilometres of total length, two (2) in the area of Thriassion plain and one (1) along the Parnitha-Krioneri-Drosia-Ekali-Dionysos axis (Attica plain) and b) 338 gravity measurements distributed along eight (8) gravity profiles, four (4) of which in Thriassion plain, three (3) in Petroupoli-Aharnes-Thrakomakedones region (Attica plain) and one (1) along Parnitha-Krioneri-Drosia-Ekali-Dionysos axis (Attica plain). Preliminary results of the geophysical investigation combined with existing geological and tectonic data are presented in this paper. Significant variation in the elevation of the alpine basement has been detected, expressed by manifestation of low and high subsurface areas which are well correlated with existing fault zones. In the area of Thriassion plain the thickness of post-alpine sediments is estimated of a few hundred meters (<500 meters) and huge thickness of several hundred meters (-800 meters) of post-alpine sediments were detected in Thrakemakedones and Krioneri areas. The relief of alpine basement is more intense in fault zones areas, such as Thriassion plain (WNW-ESE), Pathitha and Kifissos (NE-SW) as well as along the fault zone outline the Fili and Aharnes graben and is covered by postalpine formations

Relation between seismicity and pre-earthquake electromagnetic emissions in terms of energy, information and entropy content

In this paper we show, in terms of Fisher information and approximate entropy, that the two strong impulsive kHz electromagnetic (EM) bursts recorded prior to the Athens earthquake (EQ) (7 September 1999, magnitude 5.9) present compatibility with the radar interferometry data and the seismic data analysis, which indicates that two fault segments were activated during Athens EQ. The calculated Fisher information and approximate entropy content ratios closely follow the radar interferometry result that the main fault segment was responsible for 80 % of the total energy released, while the secondary fault segment for the remaining 20 %. This experimental finding, which appears for the first time in the literature, further enhances the hypothesis for the seismogenic origin of the analyzed kHz EM bursts.

Analytical Estimation of Economic Loss for Buildings in the Area Struck by the 1999 Athens Earthquake and Comparison with Statistical Repair Costs

Reliable loss assessment (in monetary terms) for buildings struck by an earthquake is an essential factor in the development of seismic risk scenarios for a given urban area. The evaluation of loss due to building damage in a certain region depends both on seismic hazard and the vulnerability of the building stock in the area. The study presented herein consists of predicting the loss to selected groups of buildings struck by the 1999 Athens earthquake using an analytical methodology and comparison with statistical repair costs collected after the earthquake. Since no near-field strong ground motion recordings from the main shock were available, a pilot methodology was used for its analytical evaluation for different soil conditions. Different suites of motions were derived, based on various theoretical and semi-empirical approaches, and were then used in analytical investigations of the seismic behavior of the buildings in the examined area, aiming at the prediction of economic losses. An in-situ survey of about 10% of the total building stock was performed, and data regarding the structural type, actual earthquake damage, and corresponding repair costs were collected. The statistically derived repair cost for the area was compared with the economic loss estimation obtained using the analytical procedure and various estimates of the seismic action in the area considered, and was found to agree with it reasonably for some of the seismic hazard scenarios.