Seismic wave attenuation at Valle Medio del Magdalena, Colombia

Abstract

A study of seismic body wave attenuation was carried out for the Valle Medio del Magdalena (VMM) basin, Colombia. A total of 326 high-quality local earthquakes were used to calculate coda Q (Qc) values, and 1463 high-quality local earthquakes were used to calculate Q scattering (Qs), Q intrinsic (Qi), Q total (QT), and other attenuation parameters. The average Qc ranged from 47 for 2.0 Hz to 904 for 12 Hz. The average Q-frequency relation (Qc -f) for the VMM basin was Qc = 50 ± 6.4 f1.1±0.08. Qi, Qs, and QT ranged from 4, 6, and 2.4 for 2.0 Hz to 420,483, and 224 for 12 Hz, respectively, depending on the region, epicentral distance and depth. Intrinsic absorption was predominant over scattering in the VMM basin at shallow crustal depths, although scattering was also remarkable. Qc value is similar to Qs for low frequencies ( ≤ 2 Hz). For high frequencies ( ≥ 3 Hz), Qc is much higher than Qi, Qs, and QT, indicating that at low frequencies Qc is reflecting the scattering process occurring at VMM. In addition, it means that single-scattering approach is valid to measure attenuation at VMM for low frequencies but not for high frequencies, where the attenuation is underestimated by using Qc. The spatial distribution of the average Qi, Qs, and QT shows that, in general, the VMM region has high attenuation at shallow crustal depths, with higher attenuation to the south and north. This high attenuation is possibly due to the presence of oil and water reservoirs as well as the presence of thick sedimentary deposits compared with the surrounding plutonic rocks. The results of this study will be useful for further seismic source, strong motion and seismic hazard studies in the VMM region.