Spectral Decay Parameter (k) of Western Iran Using Accelerograms Recorded up to 2018

Abstract

An important parameter that controls strong ground motion at high frequencies (higher than 1 Hz) is the spectral decay or kappa (k) parameter. This parameter is a linear decay factor for Fourier spectrum of acceleration on a linear-logarithmic scale within a certain frequency range, which indicates the range of attenuation of high frequency energy. This parameter is used as a key input for the simulation of strong ground motion through a random method. This study aims to estimate the kappa parameter for the west of Iran that has a high seismic activity. Hence 10 reference earthquakes as well as 76 accelerograms recorded from 1973 until 2018 are used, including the 7.3-magnitude Ezgeleh, Kermanshah earthquake on November 12, 2017. The k is derived for the latest seismotectonic zoning map of Iran to enhance the accuracy of calculations and only the records in which the reference earthquake and accelerograms occurred in the same zone are considered in the k equation. The k parameter is calculated separately for each horizontal component and for the mean. Then, the linear relationships between the kappa and distance are separately calculated and analyzed for each seismic zone. An accumulative equation is also proposed for all zones. The presented equations indicate that the near surface attenuation parameter k0 for the west seismic zones of Iran varies from 0.0331 to 0.0398. This parameter can be used effectively to calculate the strong ground motion in the zones using synthetic methods.