Evaluation of the site effects of the Ankara basin, Turkey

Abstract Recent major earthquakes have explicitly demonstrated that near-surface local site conditions that can generate significant amplifications and spatial variations of earthquake ground motion play a major role in the level of ground shaking and in gathering information on soft soil response. It is therefore highly desirable to develop methods to identify and characterize regions that are prone to this type of site amplification. To determine the subsurface sediment characteristics over a wide area, measurement and analyses of microtremor have been widely employed. Considered to be a relatively easy and economically attractive method for collecting relevant information especially in urbanized areas, microtremor involves utilization of ambient seismic noise to evaluate the local site effects reliably which is one of the vital aspects of seismic hazard assessment. This paper aims to investigate the site response of the sediment characteristics in Ankara, the capital of Turkey through conducting short-period noise recordings of microtremor measurements. A total of 352 microtremor measurements have been performed in the project site within the Plio-Pleistocene fluvial and Quaternary alluvial and terrace sediments in the western part of the Ankara basin. The spectral ratio between the horizontal and vertical components (H/V) of the microtremor measurements at the ground surface has been used to estimate the fundamental periods and amplification factors of the site. The microtremor study was also correlated and complemented by in-situ seismic measurements of dynamic properties, geologic information, and some geotechnical boring information in the project area for evaluating site conditions in an account to estimate site effects. The results of this study identified three main factors that influence site response, namely, the age of the local geological formation, the depth of the soil thickness and soil characteristics in the younger sediments, and non-uniform subsurface configurations. In particular, the H/V results showed that the variation of the fundamental period map agreed well with the maximum value of the amplification as well as with the local site conditions of seismic sediment characteristics that provided reliable estimates of site response of soft soil deposits. Finally, these results were used in zonation studies for reliably determining the local site characteristics.

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