Συμβολή στην ανάπτυξη και εφαρμογή αλγόριθμων στον υπολογισμό σεισμικών επικέντρων και τη σεισμική τομογραφία

Abstract

The purpose of this dissertation is the development and application of algorithms for determining earthquake hypocenters and seismic wave velocity structure, using seismic wave arrival times, by means of inverse theory. Primarily, a modification of the double-difference algorithm is implemented and applied to synthetic, as well as real arrival time data. The double-difference algorithm is an earthquake location algorithm which uses differential arrival times instead of absolute arrival times, maximizing thus the quantity of data used and minimizing the effect of unmodeled velocity structure. Secondarily, the modified algorithm is incorporated in the seismic tomography procedure. The first chapter starts with a short discussion on seismic waves, the largescale structure of the Earth’s interior and the seismic wave velocity structure and active tectonics of Greece and surrounding areas, which is the area where the data originated from and where the newly developed methods were applied. The principles of Inversion Theory are presented in some length, and its application for earthquake location and seismic tomography is described. The second chapter is a qualitative and quantitative description of the seismic wave arrival time data used in the dissertation, including information on the earthquakes and recording stations. The data used consists of arrival times of 140000 earthquakes that occurred in Greece and surrounding areas during the period 1981- 2005. The sources of the data are the bulletin of the Geophysical Laboratory of the Aristotle University of Thessaloniki, the bulletin of the International Seismological Center and several temporary local networks. A procedure for unifying the data in a single set was designed, and implemented as a series of auxiliary programs. The third chapter consists of a description of the earthquake location process, including details on data preprocessing, weighting and inversion specifics. Modifications were made to the original double-difference algorithm, mostly in an attempt to combine accuracy in both relative and absolute locations. These include the use of a 3-D velocity model - which, in turn, lead to a number of technical adaptations of the original code - the use of absolute arrival times in addition to differential arrival times, the use of a variable damping factor and modifications specific to the inversion method. The modified algorithm is applied to synthetic arrival time data and the results are compared to existing earthquake location algorithms including the original double-difference algorithm. Based on the results of the synthetic tests, the optimal parameters for the application in real data are estimated. The fourth chapter is a presentation of the results of the application of the earthquake location method in real earthquake data from Greece and surrounding areas. The results are evaluated and compared to the earthquake locations in the preexisting catalog. The distribution of the relocated earthquake locations displays sharper features compared to the original catalog, within the limitations of the data and method used. This leads to conclusions about the spatiotemporal distribution of the hypocenters and, indirectly, about active tectonics. As the inversion method does not allow for a direct estimation of the errors of the location, a procedure is designed which estimates indirectly the errors of the relative epicenter locations and which, in principle, is independent of the location algorithm. The fifth chapter describes the procedure of seismic tomography and presents the results of the application of the method in the area of the Mygdonia basin (N. Greece), using both synthetic and real arrival times. An attempt is made to exploit the earthquake location algorithm for seismic tomography by relocating earthquakes after each iteration of the tomography algorithm. The results are compatible with the established knowledge of the velocity structure of the area. Finally, the sixth chapter summarizes the results, discusses their interpretation and presents the conclusions reached by the dissertation.