Local site effects evaluation for Aliağa/İzmir using HVSR (Nakamura technique) and MASW methods

Abstract

The site effect plays very important role for planning design of structures and estimating seismic damage of existing structures, especially in seismicity active regions. Aliaga, located in the northern region of the Izmir City, is high seismically active region in western Turkey. We identified the dynamic properties of shallow soil for Aliaga district (western Turkey) in terms of the average shear-wave velocity for the top 30 m of soil (Vs30), predominant period (T 0) and vulnerability index (Kg). We created the microzonation map that classes the shallow soil according to National Earthquake Hazards Reduction Program (NEHRP). Shear-wave velocity (Vs) was evaluated using the 1-D multichannel analysis of surface waves at 34 sites. The microtremor measurements were taken at 58 stations with a grid spacing of 1000 m in Aliaga district. The acquired HVSR results show that the T 0 values change from 0.1 to 1.9 s, while the amplification factor (A 0) values change between 1.5 and 12. Soil classification results illustrate that northern part of the study area, located in alluvial unit, has lower Vs30 values. These values are observed within the range of 100–300 m/s. This part was classified as the D and E types according to NEHRP. In addition, the weak zones of the study area due to the local site effects were indicated by the Kg values which vary from 5 to 55. The northern part of the study area is high vulnerability index zone (Kg > 20). Last of all, we can mention that the northern part of our study area is the weakest zone according to the results of geophysical and geological data.