Passive seismic interferometry & shear wave splitting in the investigation of the Earth's crust

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Γιαννόπουλος, Δημήτριος

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The Corinth Rift is an active intra-continental structure on the western edge of the Aegean Arc, Greece. It is a region which has been in the limelight of the geo-scientific interest for more than three decades, as it is one of the most seismically active rifts in the Euro-Mediterranean region, presenting one of the highest geodetically measured rates of extension. Despite the large amount of multidisciplinary observations and the proposed models, there is still a considerable discussion concerning the relationship between seismic activity, fault mechanics and the rifting process. In the framework of the on-going research aimed to understand the tectonic evolution of the area, this thesis is trying to investigate the velocity structure and the physical characteristics of the upper crust beneath the western Corinth Rift derived by two different techniques: Passive Seismic Interferometry using ambient seismic noise and Shear-Wave Splitting using locally recorded shear-waves. We use passive seismic interferometric techniques to constrain the velocity structure in the upper crust of the western Corinth Rift. The method of Seismic Interferometry aims to retrieve the Green's function of the medium between two sensors by just utilizing ambient seismic noise recordings at the Earth's surface. Seismic Interferometry is considered as a revolutionary method characterized by a rapid development during the last decade. Seismic Interferometry was initially based on the fact that "by cross-correlating random wave fields of ambient seismic noise recorded on two locations on the Earth's surface, we can retrieve the wave field that would be recorded at one of the locations if there was a source at the other". After the mathematical representation of the previous conjecture, a plethora of studies have been performed using ambient noise Seismic Interferometry showing how useful the Green's function extraction can be in practical applications, either by monitoring temporal changes in material properties associated with volcanic processes and fault zones or by imaging the subsurface of the Earth at different scales (from reservoir to continental scale). Following this rule, we applied Passive Seismic Interferometry technique on long time-series of ambient seismic noise recorded at all available seismic stations which are deployed in the western Corinth Rift. We cross-correlated all vertical-vertical component time-series from all stations, turning each station into a virtual source emitting Rayleigh-waves (Green's functions). Then Rayleigh-wave group velocity dispersion curves were measured for each station-pair by applying frequency-time analysis and finally we performed straight-ray surface-wave tomographic inversions to produce an ambient noise tomography of the western Corinth Rift at different periods between 1 and 6 s. Since no other ambient noise tomography has been performed until today in the area, the first part of this thesis is considered as the first attempt to study the crustal velocity structure of the Corinth Rift by using ambient seismic noise recordings. In general, our results highlight the complexity of the geological and tectonic regime of the western Corinth Rift. Tomographic images at periods up to ~3 s (up to ~2.5 km depth) revealed that the overall distribution of the Rayleigh-wave velocity is mostly coincident with the geology context of the area. Low velocity zones are mostly observed where Plio-Quaternary syn-rift sediments are present, while higher velocities are due to the pre-rift basement structures. At periods above ~3 s (> 2.5 km depth), where Rayleigh-wave velocity measurements are sensitive to deeper structures, the results highlight a low velocity zone located in the southern part of the Corinth Rift with a preferential elongation in the WNW-SSE direction, sub-parallel to the strike of the rift. The low velocity anomaly is the most interesting and profound feature in the velocity maps between 4 and 6 s. Interestingly, this zone is still highlighted at depths even greater than ~5 km (especially around Aigion area), namely much deeper than the estimated sedimentary layer in the area. We interpret the presence of the low velocity anomaly below the southern part of the rift in relation with the present-day active tectonic regime giving also special attention on possible involvement of fluid circulation processes at depth within a highly fractured crust. Prompted by the observations derived from our ambient noise tomography study and motivated by previous studies in the literature noting that possible fluid circulation processes at depth play a key role in the overall evolution of the western Corinth Rift (crustal stress, faulting, seismicity etc.), in the second part of this thesis, we concentrated on locally recorded shear-waves to investigate possible changes in the seismic waves propagation properties of the crust (S-wave anisotropy), related to the occurrence of seismic events in the area. Shear-wave splitting (anisotropy) is a phenomenon in which shear-waves are separated into two components with almost vertical polarizations and different propagation velocities. This can occur during shear-wave propagation through an anisotropic medium. The two splitting parameters that can be measured through a shear-wave splitting analysis are the polarization direction of the fast component of the splitted shear-waves, and the time-delay between the two components. Significant variations of the crustal anisotropy parameters have been observed in relation to earthquakes worldwide, reflecting spatial and/or temporal changes in the characteristics of the medium and the stress field. Towards this purpose, we performed a local shear-wave splitting analysis in a time-period that includes the occurrence of two moderate earthquakes. On January 2010 two earthquakes of MW ~5.2 occurred near the village of Efpalio located at the northern coast of the western Corinth Rift. For the needs of our study, we used a 2-year long dataset, covering one year before and one year after the January 2010 Efpalio earthquakes, to study the temporal variability of the splitting parameters related to the earthquakes occurrence. In general, our analysis revealed the presence of an anisotropic upper crust in the western Corinth Rift, as well as a significant temporal variation of the splitting parameters in relation with the earthquakes. In order to have additional information about the average properties of the medium along the studied ray- paths, we accompanied each shear-wave splitting measurement with apparent Vp, Vs and average Vp/Vs ratio calculations. A distinct increase in time-delay values and Vp/Vs ratios was observed soon after the Efpalio earthquakes, followed by a decrease after the end of the aftershock sequence. The measurements of the apparent Vp and Vs showed that the observed changes in Vp/Vs ratios after the Efpalio earthquakes were due to an increase in Vp and a decrease in Vs. Considering the above, we attempted to interpret the causative factors of the observed temporal variations associated with the Efpalio earthquakes, in terms of the regional stress field and fluids involvement. We suggest that a migration of over-pressured fluids through the earthquake produced fractured damage zone is most probably the main cause of the observed increase in time delays and Vp/Vs ratios. The observed variations in the time-delays and Vp/Vs ratios after the Efpalio earthquakes seemed to be slightly stronger close to the hypocentral areas, which are possibly reflecting minor post-earthquake related changes in the properties of the crust. Fast shear-wave polarizations present a general E-W orientation, which is in agreement with the regional stress field. On the contrary, the Efpalio earthquakes seemed to have little or no influence on this parameter since fast polarization directions did not present any significant change after the Efpalio earthquakes. A notable conclusion drawn from both the shear-wave anisotropy analysis and the noise-based tomography includes evidence of fluids interactions not only within the more seismically active southern margins of the Corinth Rift, but also along the less active northern region. Furthermore, fluids interactions appear to be more intense and permanent in time in the southern part of the rift than the detected ones along the northern part which they mostly triggered by the Efpalio earthquakes occurrence. The observed variations in time delays and Vp/Vs ratio after the earthquakes were slightly stronger close to the rupture areas than outside of them and they appeared to have a decreasing trend, moving to background values a few months later.



Seismic interferometry, Seismic tomography, Seismic anisotropy, Corinth rift