Seismic attenuation in Iberia using the coda‐Q method

Abstract

Coda wave analysis is used to obtain frequency‐dependent coda‐Q values for different seismic zones of the Iberian area. Seventeen source regions around the Geophysical Observatory of Toledo and some four seismic events per region have been considered in this study. We have used an iterative Fourier analysis technique to see the variation of the frequency along the coda, also taking into account the instrument response. We have applied a suitable criterion to select the predominant frequency every 5 s along the coda. The variation of the frequency with time for each region is averaged with a second‐degree polynomial, which is compared to master curves obtained directly from the response of the seismograph system, in order to determine the elastic quality factor Q. It has been observed that the frequency‐time curves thus obtained are better explained if Q is considered as an exponential function of the peak frequency. The main result of this work is a set of 1 Hz Q values with a clear indication that frequency dependence of Q exists, although the bandwidth from which our conclusions are reached is only 0.5–1 Hz. The coda‐Q values obtained for the tectonically most stable areas (north Spain) appear somewhat higher, Q > 300, than those corresponding to the seismic active zones (south Spain), Q < 250. Thus, a clear relationship is established between Q values and the two major tectonic provinces in Iberia. These results may be helpful for seismic risk and earthquake engineering purposes. Copyright © 1990, Wiley Blackwell. All rights reserved