Surface wave tomography of Turkey and surroundings

Abstract

Seismic wave velocities can be obtained by using active or passive sources with appropriate arrays. Seismic reflection and refraction surveys using active sources are the most traditional ones. However the cost as well as inapplicability in urban areas, limits active source reflection and refraction methods in the crustal investigations. Seismic body waves and surface waves emitted from earthquakes are also widely used in seismology in order to constitute the images of the subsurface. However the insufficient path coverage between sources and stations may be the limiting factor. This amounts to the obstruction of obtaining high resolution images in crustal studies with earthquake data. In order to overcome the shortcoming of these techniques, a relatively new concept of “Passive Imaging Technique” is proposed to obtain the surface wave velocity structure of the Earth. Generally, not only in seismology but also in other disciplines which deal with signals, accept noise as an undesired component of the signal. It is commonly believed that noise obscures data and does not contain useful information. However recent developments changed this judgment by indicating that long term correlations of ‘ambient noise’ can also be used as seismic source. This method promises significant improvements in the resolution and accuracy of crustal and upper mantle images. Green’s functions between station pairs can be extracted from long term correlations of seismic recordings. Shear wave velocity distribution can then be obtained from the Green’s functions using the conventional imaging methods. In the frame of this thesis, for a better understanding of the character of the seismic noise, a comprehensive noise analysis has been performed for permanent and temporary broadband stations operating in Turkey and surrounding areas. Power spectral densities (PSD) were computed in the frequency range of 100 sec to 10 Hz. Probability Density Functions (PDF) as a function of noise power, have been analyzed for the stations with available data. Noise maps have been constructed from the power spectral density estimates of selected stations in the region in order to characterize the temporal and geographical variations. Diversities in noise spectra due to different sensors, installation properties and geographical variations are discussed. Ambient seismic noise records are used to determine the group velocity variations in Turkey and surrounding regions. A database for noise correlations was constructed from the continuous recordings of 156 permanent and temporary broadband stations during 2006-2009. The cross correlations of the ambient seismic noise are calculated to determine surface wave Green’s function for station pairs in the region. In order to obtain the group velocity maps from earthquakes a waveform database was formed from 285 earthquakes with magnitudes Mw>4.5 recorded by more than 270 broadband stations. Love and Rayleigh wave group velocity dispersion curves are computed and group velocity maps of Turkey and the surrounding regions have been obtained from local and regional earthquakes. Results from ambient noise were compared with the group velocity maps obtained from earthquakes. The group velocity maps were interpreted in relation to the known geological and tectonic structures in the region. The study shows the existence of significantly different crustal types in the area. Low group velocities at shorter periods (10-20 sec) are observed in local sedimentary basins, the Eastern Mediterranean and the Black Sea. The Eastern Anatolia region is also characterized by low group velocities while Pontides and Bitlis-Pötürge massif display higher group velocities. The Central Anatolia exhibits uniform velocity distribution indicating more homogenous crust. The Isparta Angle is marked by a wedge shaped-low group velocity anomaly. High velocities observed on the maps are associated with metamorphic, magmatic arcs along the orogenic belts of Pontides, Pötürge massif and crustal thinning in the Aegean region. At larger periods (40-50 sec) the Anatolian Block shows low and uniform group velocity distribution while its surroundings display higher group velocities with the exception of the eastern Mediterranean Region.