Evaluation of site amplification and site period using different methods for an earthquake-prone settlement in Western Turkey

Seismic micro hazard zonation for urban areas is the first step towards a seismic risk analysis and mitigation strategy. Essential here is to obtain a proper understanding of the local subsurface conditions and to evaluate ground shaking effects. In this study, present and future settlement areas of Yenisehir, which is located in the earthquake-prone Marmara Region of Turkey, were evaluated with respect to site amplification and site period. Borings in conjunction with in-situ penetration tests, seismic velocity measurements, resistivity surveys and microtremor studies were performed, and available data from previous investigations were complied to determine the variation of the soil profile as well as the characteristics of the soil layers within the study site. In addition, new empirical correlations between shear wave velocity (Vs) and number of blows from standard penetration test (SPT-N) were also developed to be used for the estimation of amplification factors. Site amplification was assessed using empirical methods based on estimated values of Vs, 1-D site response numerical modeling program and microtremor measurements. Among the three methods employed, the numerical technique and microtremor method yielded considerably higher amplification factors when compared to those obtained from the empirical method. This situation is considered as a limitation of the empirical methods. The survey of site response suggests ground amplification. The microzonation map based on soil site amplification suggests amplification factors between 1.6 and 5 in the present settlement, while the areas at the north and south of the settlement generally amplify the motion 5 to 9 times. The site periods obtained from microtremor studies vary from 0.51 to 0.8 s throughout the settlement. In addition, the comparison between fundamental site periods and fundamental building periods, which were measured in a few buildings and estimated from an empirical expression, indicate that prime attention should be paid to resonance phenomena, particularly for the northern part of the settlement where high-rise buildings are still in construction.