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Investigating seismic anisotropy in Ireland from shear wave splitting and surface wave tomography

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  1. Tez No: 401558
  2. Yazar: GÜLTEN POLAT
  3. Danışmanlar: PROF. CHRIS BEAN
  4. Tez Türü: Doktora
  5. Konular: Deprem Mühendisliği, Earthquake Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2014
  8. Dil: İngilizce
  9. Üniversite: University College Dublin
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 331

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Özet (Çeviri)

The main goals of my Ph.D. thesis are to investigate crustal and mantle anisotropy beneath Ireland by using surface wave tomography and shear wave splitting methods and to increase the understanding of anisotropy within the crust and upper mantle beneath Ireland and the D“ layer by generation and analysis of the shear wave velocity profile and modelling. Previous studies were primarily carried out in southern Ireland. However, for the first time, these methods have been applied to the whole of Ireland in this project. Shear-wave splitting analysis has been carried out on teleseismic data acquired by 40 temporary and 2 permanent broadband stations mainly from 2006 to 2011 in Ireland. It is found that there were two major event clusters: one from East Asia and the other from South America. A significant change in fast polarisation direction was observed between the results from these two event clusters which have nearly opposite back azimuths. This clearly suggests that there is a clear variation in fast polarization directions with back-azimuth. The results show that there are fundamentally different origins for the observed anisotropy beneath Ireland. In contrast, no significant difference in delay time is found for the two event clusters, and for both clusters the average delay time is greater than 1 s. The averaged fast polarization directions from the East Asian events are approximately aligned with Caledonian/Variscan trends but with some significant scatter, particularly in central Ireland. However, the averaged fast polarization directions from the South American events are fairly uniform and show N-S orientations. For this cluster there is not strong alignment of the fast polarization direction with either Caledonian and Variscan structural trends. Rapid spatial variations in splitting parameters from the East Asian events are observed at the stations located in central Ireland. This suggests that the contribution of crustal anisotropy to teleseismic shear wave splitting is quite high and the observed anisotropy beneath central Ireland is probably related to fabrics within deep crustal rocks that preserve a record of deformation during and after the continental collisions. Some events in the East Asian cluster were found to give discrepant SKS-SKKS splitting results. These may be related to anisotropy in the D”layer or other lower mantle effects including core-mantle boundary topography. It was not possible to draw definite conclusions in this regard from the South American events as there were too few SKS-SKKS pairs in the data so far collected. In addition, to determine the distribution of anisotropy with depth, phase velocities of seismic surface waves were measured. An inter-station technique was used to measure the dispersion curves of fundamental mode Rayleigh waves and those of Love waves along 43 and 21 interstation paths, respectively. The observed isotropic phase-velocity heterogeneity reflects moderate crustal thickness and seismic velocity variations across Ireland. Anisotropy of Rayleigh waves at 10-20 s periods shows a NE-SW fast-propagation direction and is the largest (up to 2%) at a 15 s period, at which Rayleigh waves sample primarily the middle and lower crust. The NE-SW trend of the deep-crustal anisotropic fabric is parallel to tectonic trends, in particular the Iapetus Suture Zone, which indicates that suture-parallel flow in the middle and lower crust accommodated the continental collision. The average shear wave-crustal velocity model obtained from combining Rayleigh phase velocity measurements with those of Love wave for Ireland is broadly consistent with the velocity models from controlled source experiement wide-angle results, showing evidence for crustal layers with slightly varying Moho depths. The results suggest that the VARNET profiles are representative of the whole Irish crust. Rapid changes in vertical and lateral shear-speed structure in the whole Irish crust are not observed. The slight variations in shear velocity reflect moderate variations in crustal thickness and radial anisotropy. The absence of rapid changes of anisotropy with depth indicates that there is no change in anisotropic symmetry orientation. The averaged radial anisotropy of Ireland in the crust and uppermost mantle is slightly higher than the global average; it is slightly greater than 2%. Variations in the amount of the observed radial anisotropy within the upper crust to lower crust are small and very smooth. Radial anisotropy indicative of horizontal fabric in the crust and uppermost mantle is preferred by the data collected in this study. This is a robust observation and consistently indicates horizontally oriented fabric due to the alignment of intrinsically anisotropic minerals in the crust and uppermost mantle beneath Ireland. Food Development Authority for their help in finding sites for our stations and the landowners who generously provided them. I would like to thank my best friend, Seda Yelkenci and Hilary O'Donnell for their immense support and help during my stay in Dublin. Last but not least, I would like to thank my family, my mom Seringul, my brother Orhan and my sisters Bahar and Akgul Polat for supporting me with full faith during my life time.

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