Abstract
In this dissertation the results of a study of the Rayleigh wave dispersion for the period range of 5 to 30sec across the Aegean area are presented. The broadband seismometers, with high-quality digital seismic stations from three temporary networks installed in the broader Aegean area, have given the opportunity to present high-resolution maps of lateral group velocity variations of the area, with the use of the records of Rayleigh waves. From these maps a local dispersion curve was extracted for discrete points of the area under study and finally a non-linear program implied to these local dispersion curves in order to determine a shear velocity structure for each discrete examined point. Using the one-dimensional models, a map of Moho depth discontinuity for the study area was constructed and the lateral variations of S wave velocities for the Aegean area for different depths are shown in a set of maps. This dissertation is divided into six chapters. In the first chapter, the basic ...
In this dissertation the results of a study of the Rayleigh wave dispersion for the period range of 5 to 30sec across the Aegean area are presented. The broadband seismometers, with high-quality digital seismic stations from three temporary networks installed in the broader Aegean area, have given the opportunity to present high-resolution maps of lateral group velocity variations of the area, with the use of the records of Rayleigh waves. From these maps a local dispersion curve was extracted for discrete points of the area under study and finally a non-linear program implied to these local dispersion curves in order to determine a shear velocity structure for each discrete examined point. Using the one-dimensional models, a map of Moho depth discontinuity for the study area was constructed and the lateral variations of S wave velocities for the Aegean area for different depths are shown in a set of maps. This dissertation is divided into six chapters. In the first chapter, the basic types of seismic waves are described and more information is given about surface waves that consist the object of the present study. General information about the structure of the Earth’s crust and upper mantle as well as about the discontinuities that have been detected in the internal Earth are discussed. In the last section of this chapter the basic theory of the inversion and the non-linear problem in Seismology is developed. In the second chapter, the main seismotectonic characteristics of the area under study and a review on the research work for the structure of the crust and upper mantle in the Aegean area are developed. In the third chapter, a detailed description of the data that used in the present study is given. Mostly of the data used in this work coming from a large-scale experiment, which took place in Greece during the period January-July 1997. During this project 30 digital threecomponent recorders (mainly Titan and Reftek) (fig. 3.4) have been installed all over the Greek area for a period of 6 months in order to record teleseismic and regional earthquakes. The equipment consisted of Lennartz LE5S (High-Pass 5sec), Guralp CMG-40 (High Pass 20 or 60 sec), and Guralp CMG-3 (High Pass 60 or 100 sec) seismometers, and Reftek 72A06 and Agecodagis TitanDat data loggers, which recorded continuously at a sample frequency of 62.5 or 50 sps. The time was calibrated by GPS receivers in all stations and the instruments have been installed in permanent stations of the Seismological Network of Thessaloniki, of the National Observatory of Athens, and in temporally shelters, where the seismometers were protected from temperature variations. All the stations were visited every month to collect and check the data. Additional data during the period 1996-2000 from other temporary networks have been included in the dataset. Finally 185 seismic events (fig. 3.3) mainly with magnitudes 4.0 M 5.5 w ? ? have been collected and processed. In the same chapter the main principles of the method FTAN that used for the calculation of the observed group velocity curves of Rayleigh waves are presented. For each station-earthquake pair an observed group velocity of Rayleigh waves has been estimated applying the Frequency Time Analysis method (FTAN) to the vertical component of motion (Levshin et al., 1972, 1989, 1992). This method is based on a frequency-time representation of a seismic signal, obtained by passing an input seismic record through a system of narrow frequency band Gaussian filters and representing the amplitudes of the envelopes and instant phases of filter outputs as a 2D complex function of time and period. As the mean averaging path length is in the order of 400km, the Rayleigh waves have been well recorded in the period range from 5s to 30s. Around 700 observed Rayleigh-wave group velocities have been determined along different ray paths covering the region under study. The coverage of the study area for six values of period is shown in Fig. 3.9. As can be observed the azimuthal distribution of the paths is quite uniform and the coverage is satisfactory, especially in the central Aegean area.
show more
