NEAR-DISTANCE SPECTRAL PARAMETER KAPPA CALCULATIONS FOR STATIONS INSTALLED IN N. MACEDONIA

The objective of this study is to estimate the high-frequency decay parameter – kappa (κ) for stations installed on rock (according to EC8) for North Macedonia region. Kappa characterizes the attenuation of ground motion at high frequencies, more precisely defines the rate of decrease of the acceleration amplitude spectrum. At higher frequencies ground motions on rock may be larger than those on soil due to lesser damping from impedance contrasts in the near-surface. Thus, site effects on rock are an important topic for structures that are sensitive to high frequencies. Comparison of kappa estimated values for rock stations is made using strong motion data obtained from different locations. Local site condition parameters, specifically VS,30, are determined for each station based on conducted geophysical investigations and/or microtremor measurements. The accelerometric data used for analysis are obtained from local earthquake events within a magnitude range (Mw) of 3.0 to 5.5, epicenter distances (Repi) less than 50 km, and focal depths (h) less than 40 km. The data were visually inspected, baseline corrected, and a bandpass filter was applied. Earthquake records with SNR<3 were excluded from the analysis. The value of kappa is estimated from the acceleration amplitude spectrum of shear waves from the slope of the high-frequency part where the spectrum starts to decay rapidly to a noise level. Based on the results obtained, conclusions are drawn regarding the impact of this parameter on the attenuation of near-surface crust rock layers, as well as regional and local effects in near-distances. Keywords: kappa, FAS, rock, near-surface attenuation, strong motion data