INTRINSIC AND SCATTERING ATTENUATION FROM OBSERVED SEISMIC CODAS IN THE ALMERIA BASIN (SOUTHEASTERN IBERIAN PENINSULA)

The anelastic attenuation in the Almeria Basin (southeastern Iberian P eninsula) is investigated by using seismic data collected during the s ummer of 1991. A multiple-lapse time-window analysis is applied to hig h-frequency seismograms corresponding to 20 shallow seismic events wit h low magnitudes (m less than or equal to 2.5) and distances less than 71 km, recorded at six short-period seismographic stations. We have c onstructed corrected geometrical spreading and normalized energy-dista nce curves for the region over the frequency bands 1-2, 2-4, 4-8, 8-14 and 14-20 Hz. A theoretical model for body-wave energy propagation in a randomly heterogeneous medium has been employed to interpret the ob servations. Two parameters describe the medium in this model: the scat tering attenuation coefficient eta(S)=kQ(S)(-1) and the intrinsic atte nuation coefficient eta I=kQ(I)(-1), where k is the wavenumber and Q(I )(-1) and Q(S)(-1) are the intrinsic and scattering attenuation respec tively. This model assumes that scattering is isotropic, including all orders of multiple scattering, and predicting the spatial and tempora l energy distribution of seismic energy. A least-squares fitting proce dure has been used to find the best estimates of the model parameters. The analysis of the spectral amplitude decay of coda waves has provid ed coda Q(C)(-1) values at the same frequency bands. The results obtai ned Q(I)(-1) and Q(S)(-1) and Q(C)(-1) decrease with increasing freque ncy; for frequencies lower than than 3 Hz scattering attenuation is st ronger than intrinsic absorption and coda Q(C)(-1) takes values betwee n intrinsic and total attenuation, being very close to Q(T)(-1). Q(S)( -1) more frequency-dependent than Q(I)(-1); for frequencies greater th an 3 Hz intrinsic absorption is the dominant attenuation effect and Q( I)(-1) and Q(S)(-1) have significant frequency. dependence. In order t o correlate the results obtained with the major geological and tectoni c features of the region, a geotectonic framework for the area is prov ided and the predominant frequency decay in coda waves is analysed in order to obtain the coda Q frequency dependence following a power law Q(C)=Q(0)(f/fo)(v), where f(0) is a reference frequency. In this way w e have obtained regionalized values of coda Q at 1 Hz (Q(0)). Finally, a first-order approach has allowed us to obtain intrinsic and scatter ing quality factors from the obtained eo and v values, leading us to o btain tentative distributions of el, es and eo at 1 Hz for the area. T he derived intrinsic and scattering quality-factor distributions are i n good agreement with the tectonic history and the main geological fea tures of the region. Large scattering and intrinsic attenuation (Q(S) similar to 80, Q(I) similar to 100) are found in the sedimentary Neoge ne and Quaternary basin, while scattering is the dominant effect in th e old Palaeozoic rocks of the mountains (Q(S) similar to 200, Q(I) sim ilar to 1000). Intrinsic Q shows a higher sensitivity to the geologica l characteristics than scattering e.