Estimation of soil dynamic parameters by using ambient noise and scenario earthquake in Aliağa/İzmir (Western Anatolia)

Abstract

As is well known, dynamic parameters are necessary to study the effects of the earthquake because the earthquake is a dynamic phenomenon. The study area is one of the tectonically active regions at Western Anatolia. Microtremor method, which is easy to apply and cheap, was used to reach the dynamic parameters of the study area. The dynamic parameters that were achieved from the microtremor field study applied at 125 points are the predominant frequency of the soil and the dynamic amplification factor. In this study, dynamic amplification factor values were computed and were mapped by joint evaluation of ambient noise and a scenario earthquake. Ambient noise was collected at Aliaga district in Izmir city, and the scenario earthquake was chosen with a distance of 55 km to the study area according to the earthquake distribution. It was observed that the distribution of the soil predominant frequency and the dynamic amplification factor values obtained in the study area were mostly compatible with geological units. These two parameters are expected to change inversely with each other. In some places, they were seen as incompatible. The most important reason for incompatible results was the use of the entire horizontal-to-vertical ratio frequency spectrum during the calculation of the dynamic amplification factor. In contrast, the predominant frequency represents only the frequency at maximum amplitude in the horizontal-to-vertical ratio spectrum not the entire spectrum. Considering the complexity of the geological units and the changes of dynamic amplification factor (DAF) values in the lateral direction, the study area offers heterogeneous structural features. This shows us that the peak ground acceleration (PGA) value calculated by a single earthquake record would be insufficient to represent the entire field of study.