Spatial variation of coda wave attenuation in northwestern Colombia

Abstract

SUMMARY One thousand seven hundred and eighty-six vertical-component, short-period observations of microearthquake codas from regional earthquakes recorded by 17 stations belonging to the National Seismological Network of Colombia were used to estimate seismic wave attenuation in Colombia. Local magnitudes range from 2.9 to 6.0 and only events occurring at hypocentral distances up to 255 km were considered for the analysis. The frequencies of interest lay between 1 and 19 Hz and the analysis was performed for each seismic station separately. Coda-wave attenuation (Q −1 ) was estimated by means of a single-scattering method whereas the separation of intrinsic absorption (Q −1 ) and scattering attenuation (Q −1 ) from total attenuation (Q −1 )w as performed using a multiple lapse time-window analysis based on the hypothesis of multiple isotropic scattering and uniform distribution of scatterers. A regionalization of the estimated Q0 (Qc at 1 Hz) values was performed and a contour map of seismic coda attenuation in Colombia is presented, where four zones with significant variations of attenuation related to different geological and tectonic characteristics can be observed. The highest attenuation is linked to the central and western regions (Q0 around 50 and 56) whereas a lower attenuation (Q0 around 69 and 67) is assigned to the northern and eastern regions. Results show that the Q −1 values are frequency dependent in the considered frequency range, and are approximated by a least-square fit to the power law Q −1 ( f ) = Q −1 ( f / f 0) −η . The exponents of the frequency dependence law ranged from η = 0.65 to 1.01 for Q −1 , η = 0.62 to 1.78 for Q −1 , η = 0.28 to 1.49 for Q −1 , and η = 0.53 to 1.67 for Q −1 . On the other hand, intrinsic absorption is found to dominate over scattering in the attenuation process for most of the stations and frequency bands analysed. Some discrepancies have been observed between the theoretical model and the observations for some frequency bands which indicate that it would be necessary to consider models for depth-dependent velocity structure and/or non-isotropic scattering patterns.