ambient noise; crustal structure; coseismic velocity change; 2011 Van earthquake; Sea of Marmara
Exploring the interior of the Earth, particularly at seismogenic depths is a long sought goal for seismologists in order to better characterize (e.g. nucleation, occurence) devastating earthquakes, perform monitoring and better estimate future seismic risks. The success of traditional imaging methods is highly related to how densely the target area is sampled with ray paths between the source and receivers. In case of low seismicity and scarce earthquake-receiver distribution, employing seismic ambient noise, a ubiquitous source, is proved to be an alternative approach. The cross-correlations of seismic ambient noise can be used to estimate the Green’ s function of the medium and thus to determine the Earth’ s structure.
The main goal of this study is to provide a comprehension on spatial and temporal variation of seismic velocity field at different phases of the seismic cycle. To achieve this, a structural investigation was performed applying seismic ambient noise analysis in the eastern Sea of Marmara region. Another application is in order to monitor potential velocity changes associated with a major earthquake occurred in Van region, eastern Turkey.
In the first part of this thesis, once the necessary pre-processing steps applied to the raw data, ambient noise cross correlation technique is used in order to investigate the crustal structure surrounding the Çınarcık Basin (offshore Istanbul), hosting Princes’ Islands segment of the North Anatolian Fault Zone which is in the final phase of the seismic cycle. Low velocity zones of Çınarcık and Thrace Basins in the study area are successfully observed in group velocity dispersion curves, proving ambient noise cross correlation approach to be a feasible and powerful tool for imaging. As a latter step, a 1D S-wave velocity structure is obtained from an average dispersion curve and employed in the inversion process to present the improvement in hypocenter determination.
In the second part, potential variations in the seismic velocity associated with the 2011 M7.1 Van earthquake is investigated by exploting the repeatibility of seismic ambient noise. Six-month data framing the mainshock is investigated. A coseismic velocity decrease directly related to the Van mainshock is observed in the near vicinity of the earthquake hypocenter. This change in velocity is explained by a coseismic stress change at depth taking into account the frequency band of interest. Moreover, a correlation between the coseismic velocity decrease with minimum distance of the respective ray path to the mainshock hypocenter and the amount of coseismic slip is presented.
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