| dc.description.abstract | The Aegean forms the continental lithosphere thrusting towards the subducting Nubian lithosphere in the eastern Mediterranean region. The extension induced due to the Nubian slab rollback has deformed the Aegean lithosphere and created several rift/fault zones, complex geological structures as well as pathways for fluid migration, which act as heterogeneous media for the propagating seismic waves. A quantitative estimate of how these features across the Aegean impact seismic wave propagation is not yet available, apart from some localized studies. In my thesis, I aim to identify these heterogeneous features and quantify their properties across the Aegean to understand better the underlying processes that generated them.
I use the seismograms and the seismic phase travel-time data from major seismic networks that operated in the Aegean between autumn 2002 to spring 2020 for a three-stage modeling/imaging of the inhomogeneous structure. First, I model the spectral characteristics of the crustal and upper mantle small-scale velocity perturbations using delay times for peak energy arrival in seismograms. Then, I estimate and map the intrinsic and scattering attenuation produced by the crustal structure in different frequency bands using seismogram envelopes. Finally, I perform the travel-time tomography for the Aegean crust for the 3D velocity structure.
Using the above analyses, I find that the Aegean crust-mantle boundary exists as a transition zone and the crust is composed of three main lithological units: extremely low velocity sediments, low velocity silicic units, and high velocity metamorphic units. The crustal inhomogeneities, which generate seismic envelope broadening as well as scattering and intrinsic attenuation, are restricted to four sub-regions: the Corinth rift, the Cyclades, Crete and the Gulf of Gökova. Normal faulting across the Aegean and thrust faulting in the fore-arc likely generates significant seismic envelope broadening and scattering attenuation in the crust. On the other hand, fluid migration from below the volcanic centers as well as within metamorphic core complexes possibly cause high intrinsic attenuation and high Vp/Vs in the crust. Upper mantle inhomogeneities are mainly prominent in the back-arc, where mantle-wedge melting is expected, with small inhomogeneities in the southern fore-arc linked with slab material underplating the crust. The estimates of porosity using the tomography model suggests 4-9 % melt-fraction beneath the volcanic centers with Santorini exhibiting the highest degree of melting. Large earthquakes across the Aegean occur in low Vp and high Vp/Vs zones, indicating fluid activity as their triggering mechanism. | en_US |