Coda wave attenuation in the Parecis Basin, Amazon Craton, Brazil: sensitivity to basement depth

摘要

Small local earthquakes from two aftershock sequences in Porto dos Gaúchos, Amazon craton—Brazil, were used to estimate the coda wave attenuation in the frequency band of 1 to 24 Hz. The time-domain coda-decay method of a single backscattering model is employed to estimate frequency dependence of the quality factor (Q _c) of coda waves modeled using Q_c = Q₀ fhQ_{rm c} =Q_{rm 0} f^eta , where Q ₀ is the coda quality factor at frequency of 1 Hz and η is the frequency parameter. We also used the independent frequency model approach (Morozov, Geophys J Int, 175:239–252, 2008), based in the temporal attenuation coefficient, χ(f) instead of Q(f), given by the equation c(f)=g+fracpfQ_e chi (f)!=!gamma !+!frac{pi f}{Q_{rm e} }, for the calculation of the geometrical attenuation (γ) and effective attenuation (Q_e-1 )(Q_{rm e}^{-1} ). Q _c values have been computed at central frequencies (and band) of 1.5 (1–2), 3.0 (2–4), 6.0 (4–8), 9.0 (6–12), 12 (8–16), and 18 (12–24) Hz for five different datasets selected according to the geotectonic environment as well as the ability to sample shallow or deeper structures, particularly the sediments of the Parecis basin and the crystalline basement of the Amazon craton. For the Parecis basin Q_c = (98±12)f(1.14±0.08)Q_{rm c} =(98pm 12)f^{(1.14pm 0.08)}, for the surrounding shield Q_c = (167±46)f(1.03±0.04)Q_{rm c} =(167pm 46)f^{(1.03pm 0.04)}, and for the whole region of Porto dos Gaúchos Q_c = (99±19)f(1.17±0.02)Q_{rm c} =(99pm 19)f^{(1.17pm 0.02)}. Using the independent frequency model, we found: for the cratonic zone, γ = 0.014 s − 1, Q_e-1 = 0.0001Q_{rm e}^{-1} =0.0001, ν ≈ 1.12; for the basin zone with sediments of ~500 m, γ = 0.031 s − 1, Q_e-1 = 0.0003Q_{rm e}^{-1} =0.0003, ν ≈ 1.27; and for the Parecis basin with sediments of ~1,000 m, γ = 0.047 s − 1, Q_e-1 = 0.0005Q_{rm e}^{-1} =0.0005, ν ≈ 1.42. Analysis of the attenuation factor (Q _c) for different values of the geometrical spreading parameter (ν) indicated that an increase of ν generally causes an increase in Q _c, both in the basin as well as in the craton. But the differences in the attenuation between different geological environments are maintained for different models of geometrical spreading. It was shown that the energy of coda waves is attenuated more strongly in the sediments, Q_c = (78±23)f(1.17±0.14)Q_{rm c} =(78pm 23)f^{(1.17pm 0.14)} (in the deepest part of the basin), than in the basement, Q_c = (167±46)f(1.03±0.04)Q_{rm c} =(167pm 46)f^{(1.03pm 0.04)} (in the craton). Thus, the coda wave analysis can contribute to studies of geological structures in the upper crust, as the average coda quality factor is dependent on the thickness of sedimentary layer.