Dynamic amplification factor concept of soil layers: a case study in İzmir (Western Anatolia)

Abstract

Dynamic soil amplification factor (DAF) defining which ratio earthquake acceleration will reach the soil surface by changing is one of the most important factors in seismic risk studies. When computing the value of DAF at a point without a strong motion station, peak horizontal acceleration values (PGA) at the bedrock and soil transfer function are needed. PGA value at the bedrock can be obtained by using either real seismic records or the earthquake scenario. However, the soil transfer function (soil transfer function) can be computed observationally and theoretically. Observational soil transfer function is defined by microtremor horizontal/vertical spectral ratio. In case of theoretical computation, the density belonging to the soil layers between the bedrock and the soil surface is used together with the change of P-S wave rates with the depth and the damping factor. In this study, the dynamic amplification factor (DAF) has been computed and mapped for 57 points by using observational soil transfer functions obtained by microtremor horizontal/vertical spectral ratio as well as the earthquake scenario at a new city center located within the metropolitan area of Izmir. Also, theoretical soil transfer function at 1 point was obtained through spatial autocorrelation method (SPAC) study and determined to be compatible with observational result. It was observed that both peak period values are higher than 1 s and DAF values are higher than 2 throughout the area. Also, according to the soil profile obtained from the SPAC study, S wave rate changes up to a depth of 1300 m showing that acoustic impedance differences may occur in the frequencies and the magnitudes of the earthquake waves. The fact that peak period values obtained from microtremor studies are higher than 2 s supports this result. This means that acoustic impedance differences likely to occur in these depths should also be taken into consideration while computing the dynamic amplification factor.