Ανάλυση μετρήσεων μικροθορύβου και συσχέτιση με μακροσεισμικά αποτελέσματα της περιοχής Θεσσαλονίκης

Abstract

In the last several decades destructive earthquakes (e.g. Northridge-USA 1994, Kobe-Japan 1995, Athens-Greece 1999) occurred close to populated areas showed that site effects can play an important role to the resulting damage. The evaluation of site effects is then one of the key parameters in microzonation studies. Local site effects may be evaluated by empirical (based on recordings of earthquakes or explosions) or theoretical methods (numerical simulation of the site response based on detailed geotechnical and geophysical parameters). The main disadvantage of these methods is the high cost and the time consumed for obtaining the earthquake recordings or geophysical and geotechnical properties. The horizontal-to-vertical spectral ratio technique applied on ambient noise measurements (Nogoshi & Igarashi, 1971; Nakamura, 1989, 2000) (which hereafter is referred to as HVSR technique) has been used as an alternative method to characterize site response in urban environment. Although the seismological community generally agrees that the HVSR technique gives reliable results in terms of the fundamental frequency of a site, the reliability and applicability of the HVSR results are still in debate. For that reason, the present study attempts to understand better the use of the HVSR technique in microzonation studies. Ambient noise measurements were conducted in the city of Thessaloniki (Northern Greece) for examining the limitations of the HVSR technique for cities of similar location and geological setting. In the first chapter a short description of the main historical periods in the use of ambient noise is attempted. Then, the ambient noise studies that were carried out in Greece and especially in the city of Thessaloniki are mentioned. Lastly, the geological and seismological settings of the broader Thessaloniki area are described. The second chapter deals with the analysis of the Horizontal-to-Vertical technique as is proposed by Nogoshi – Igarashi (1971) and Nakamura (1989, 1996, 2000). The method of acquiring and analysing experimental data is described (SESAME project: Deliverable D23.12, 2004). Finally, an analysis of the methods that are used in the theoretical simulations in the present study is made. In the third chapter the data that were used in the present study are presented. The fourth chapter includes the comparisons and the results found in the present study. In the fifth and last chapter, the main conclusions of the thesis are highlighted. Two appendixes are included at the end of the thesis. The first one contains the table with the information of each measurement point and in the second one the Horizontal-to- Vertical spectral ratios (H/V, NS/V, EW/V) of each measurement point are presented.