Automatic estimation of earthquake high-frequency strong-motion spectral decay in south Iceland

We present an adaptive automated algorithm for estimating the spectral decay of high frequency waves from recorded acceleration time histories of strong ground motion, along with a simple P- and S-phase picking algorithm to achieve complete automation. Our approach negotiates site resonance peaks and spectral corners through a spectral linearity criterion that on average provides indistinguishable results compared to manual estimates. The overall spectral decay, represented by the so-called “kappa” (κ) parameter, is demonstrated on a dataset of accelerograms from earthquakes of magnitudes 3.7-6.5 at distances of 1–76 km recorded on site conditions classified as rock and stiff soil, respectively, in South Iceland. The automatic procedure gives an average estimate of κ¯=37.2±13.6 ms on rock. The data did not allow the robust determination of linear distance dependence, nor a distinction of κ between the two site classes due to the generally large scatter of κ values. Using a subset of stations for the analysis however, a slight distance-dependence could be observed but is likely due to the influence of the quality factor being proportional to frequency. The results indicate that source and site effects drive κ values in South Iceland, and that a formal inclusion of source contributions to the parametrization and analysis of the spectral decay is needed.