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Vertical Ground Motion in Southern Spain

Instituto Andaluz de Geofísica, Universidad de Granada, Campus de Cartuja s/n, 18071 Granada, Spain flor{at}iag.ugr.es

Robert B. Herrmann

Department of Earth and Atmospheric Sciences, Saint Louis University, 203 0’Neil Hall, 3642 Lindell Blvd., St. Louis, Missouri 63108

José Morales and Daniel Stich

Instituto Andaluz de Geofísica, Universidad de Granada, Campus de Cartuja s/n, 18071 Granada, Spain

We quantify the attenuation of ground motion with distance in southern Spain using 6052 velocity recordings from 1107 local and regional earthquakes registered by the Andalusian vertical-component short-period network. Epicentral distances and magnitudes range from 3 to 500 km and from 1.0 to 5.1, respectively. The observed peak velocity and the root mean square (rms). Fourier velocity amplitudes with distance are parameterized by excitation, distance, and site terms for a set of frequencies ranging from 1 to 12 Hz. We obtain these terms by performing an iterative-damped least-squares regression. From the excitation term, we extract its scaling law with magnitude by using Brune’s source model. The distance terms are fitted by analytic functions that depend on geometrical spreading coefficients and a frequency-dependent attenuation parameter. Apart from the analysis of the entire data set, we perform two separate analyses of two subdata sets: offshore earthquakes (Alboran data set) and inland earthquakes (Betic data set). These data sets have differences in their probed earth structures, showing distance terms clearly distinct, with higher attenuation in the Alboran area (Q₀100) compared to the Betic area (Q₀200). Random vibration theory proved to be a suitable tool to simulate observed peak amplitudes from the obtained spectral scaling laws and signal durations.