A STUDY ON HIGH FREQUENCY DECAY PARAMETER (KAPPA) FROM VARZAGHAN-AHAR DOUBLE EARTHQUAKES, NORTHWESTERN IRAN (MW 6.5 & 6.3)

The high frequency decay parameter, kappa and its variations in distance is evaluated using 114 three-component strong motion records from two moderate events in NW Iran. We show that in classical method of estimating kappa, the results are very sensitive to the choices of fE and fX and automated procedures for estimating kappa are likely to lead to a biased estimation. For the present database, we found an obvious concavity in dependency of kappa with distance. The kappa values in distance were regressed to a piecewise bilinear shape. Based on this piecewise bilinear shape the zero distance kappa are 0.044 and 0.023 for horizontal and vertical components respectively