Intrinsic and scattering attenuation from observed seismic codas in the Almeria Basin (southeastern Iberian Peninsula)

Geophys. J. Int. (1997) 129, 281-291 Intrinsic and scattering attenuation from observed seismic codas in the Almeria Basin (southeastern Iberian Peninsula) L. G. Pujades,' A. Ugalde,' J. A. Canas,' M. Navarro,' F. J. Bada13 and V. Corchete2 Universidad PolitPcnica de Catalufia, Barcelona, Spain. E-mail: pujades@etseccpb.upc.es Vniversidad de Almeria. Almeria. Spain ' Vniversidad de Zaragoza, Zaragoza, Spain Accepted 1996 December 10. Received 1996 December 6; in original form 1996 April 3 SUMMARY The anelastic attenuation in the Almeria Basin (southeastern Iberian Peninsula) is investigated by using seismic data collected during the summer of 1991. A multiple-lapse time-window analysis is applied to high-frequency seismograms corresponding to 20 shallow seismic events with low magnitudes (m 12.5) and distances less than 71 km, recorded at six short-period seismographic stations. We have constructed corrected geometrical spreading and normalized energy-distance 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 observations. Two parameters describe the medium in this model: the scattering attenuation coefficient qs = kq3 ' and the intrinsic attenuation coefficient yi = kq, ', where k is the wavenumber and Qr1 and Q, are the intrinsic and scattering attenuation respectively. This model assumes that scattering is isotropic, including all orders of multiple scattering, and predicting the spatial and temporal energy distribution of seismic energy. A least-squares fitting procedure has been used to find the best estimates of the model parameters. The analysis of the spectral amplitude decay of coda waves has provided coda Q,' values at the same frequency bands. The results obtained show that Q, ', Q2 ' and Q, ' decrease with increasing frequency; for frequencies lower than 3 Hz scattering attenuation is stronger than intrinsic absorption and coda Q,' takes values between intrinsic and total attenuation, being very close to Qy'. Q;' is more frequency-dependent than Q, ; for frequencies greater than 3 Hz intrinsic absorption is the dominant attenuation effect and Q, and Q3 have significant fre- quency dependence. In order to correlate the results obtained with the major geological and tectonic features of the region, a geotectonic framework for the area is provided and the predominant frequency decay in coda waves is analysed in order to obtain the coda Q frequency dependence following a power law Qc = Qo( f/fo)", where fo is a reference frequency. In this way we have obtained regionalized values of coda Q at 1 Hz (Qo). Finally, a first-order approach has allowed us to obtain intrinsic and scattering quality factors from the obtained QO and v values, leading us to obtain tentative distributions of Q,, Qs and QO at 1 Hz for the area. The derived intrinsic and scattering quality-factor distributions are in good agreement with the tectonic history and the main geological features of the region. Large scattering and intrinsic attenuation (Qs N 80, QI - 100) are found in the sedimentary Neogene and Quaternary basin, while scattering is the dominant effect in the old Palaeozoic rocks of the mountains (Qs - 200, QI - 1000). Intrinsic Q shows a higher sensitivity to the geological characteristics than scattering Q. Key words: absorption, attenuation, Q, scattering, seismic coda. INTRODUCTION Seismic waves in the Earth attenuate with travel distance at rates greater than those predicted by geometrical spreading of the wave fronts. This excess attenuation is caused either by intrinsic anelasticity of the rock through which the waves travel, or by inhomogeneities that refract, reflect or diffract seismic energy. Measurements of Q from seismic waves are important as indirect indicators of macroscale Earth heterogeneity, 0 1991 RAS 281

~ 282 L. G. Pujades et al. microscale physical rock processes and the presence of fluids. It is well known that attenuation of seismic energy correlates with tectonic and near-surface soil and sediment conditions (Aki 1969; Aki & Chouet 1975; Sat0 1977; Herrmann 1980; Fehler et al. 1992; Mitchell 1995). The knowledge of the relative contributions of scattering and intrinsic effects to an observed apparent Q is important for correct geological and tectonic interpretations; therefore, the task of Q separation has been a fopic of recent research (Wu 1985; Frankel & Wennerberg 1987; Wu & Aki 1988; Hoshiba, Sato & Felher 1991; Hoshiba 1993; Kang & McMechan 1994). Because of its good theoretical basis (see Hoshiba et al. 1991, p. 67) the multiple-lapse time-window analysis method has become a useful tool to analyse the separation of intrinsic and scattering attenuation from seismic data. In this paper we analyse the seismic attenuation in the Almeria Basin (southeastern Iberian Peninsula) by using seismic data collected in this region during the summer of 1991. The application of the multiple-lapse time-window analysis method (Hoshiba et al. 1991; Fehler et al. 1992) and the spectral amplitude decay analysis method (Aki & Chouet 1975) to bandpass-filtered records provides average values of intrinsic absorption, scattering attenuation and coda attenuation in the area. The change of predominant frequency with time in the coda (Herrmann 1980; Pujades et al. 1990) is then used to provide regionalized coda Q estimates near 1 Hz. Finally, Dainty s (1981) formula, which separates the intrinsic and scattering contributions to total attenuation, is used to estimate tentative QI and Qs distributions in the area. We also analyse the correlation between the attenuation results obtained and the main geological and tectonic features of the area, and with the attenuation estimates reported in earlier work in or near this region. DATA ANALYSIS AND RESULTS From June to September 1991, fieldwork was carried out in the Almeria region. Seven seismic recorders designed by the Instituto Andaluz de Geofisica and Mark Product model L4C geophones were installed in the region. The geophones have a velocity response that is flat above their 1 Hz corner frequency. The vertical components of ground motion were recorded with 12 bits of resolution at a digitation rate of 100 samples per second. The whole velocity response was flat in the 1-20 Hz frequency range. Saturated signals and seismograms with clipped events or poor signal-to-noise ratio were rejected. 61 high-quality seismograms corresponding to 20 shallow seismic events with low magnitudes (~12.5) were selected for this study. Hypocentral distances in all cases are less than 71 km. Fig. 1 shows the epicentre locations (solid circles) together with the seismographic stations (solid squares) used in this study. Intrinsic absorption and scattering attenuation Intrinsic absorption and scattering attenuation are usually quantified by adimensional quantities QI and Qs, called quality factors, or by means of scattering and intrinsic attenuation - 37.5 I - 37 ABRIDE Figure 1. Epicentre locations (solid circles) and seismographic stations (solid squares) used in this study. The main geological features of the region are also shown (see explanation in the text).

coefficients qs and qr, which are related to intrinsic and scattering quality factors by Q; = k- qr and Q, = k- qs, where k is the wavenumber. The seismic albedo BO and total attenuation L;' are alternative attenuation parameters defiled by L,-'=qsSqr. (Wu, 1985). The seismic albedo BO represents the fraction of energy lost by scattering processes, and L, is also called the extinction length. Hoshiba et al. (1991) used an impulsive seismic energy source and the medium model of Wu (1985) and Wu & Aki (1988) to obtain unique estimates of the scattering and intrinsic attenuation coefficients qs and qr. This method exploits the temporal distribution of seismic energy in addition to its spatial distribution and was called the multiple-lapse timewindow analysis by Hoshiba et al. (1991). The basis of the method is to divide a local seismogram into three parts from the S-wave onset: early, middle and late coda. The seismic energy, for each hypocentral distance, is then integrated in the time domain over the three separate time windows. A comparison between synthetic curves corresponding to the timeintegrated theoretical energy as a function of the hypocentral distance and the observed ones leads to a least-squares estimate of BO and L; l. A detailed discussion of the method is given in Fehler et al. (1992), who estimated Q,' and Q;' for the Kanto-Tokai region in Japan. Hoshiba et al. (1991) gave the numerical basis of the method by computing theoretical curves from a Monte Carlo simulation of multiple isotropic scattering. Zeng, Su & Aki (1991) shoaed that the energy density E(r,t) per unit volume, at a point r and time t, for a point source at t = 0 located at ro, obeys the following integral equation: where g = qs is the scattering coefficient, L; = qs + qi and /3 is the wave velocity. The first term on the right side of eq. (2) is the incident seismic-wave energy of the direct wave travelling from ro to the receiver located at r. The second term is the sum of scattered energies for all scatters at rl and all scattering orders. As noted by Jin, Mayeda & Aki (1994), all previously published equations on coda energy are particular approximations to the general integral equation (2). Eq. (2) can be solved by using a hybrid method that combines analytical solutions for the single-scattering case and numerical solutions based on 2-D Fourier transforms for the multiple-scattering case (Sato 1994a). Let Asquared-obs(r, t) be the mean squared amplitudes of bandpass-filtered seismograms measured as a function of lapse time t after the origin time and hypocentral distance r. An example of the processing applied to a seismogram recorded at station ECH is shown in Fig. 2. To obtain the three time integrals, represented as e, (n = 1, 2 and 3), Asquard-obs(r, t) is integrated for the time windows 0-15, 15-30 and 30-45 s, measured from the S-wave onset. 0 1997 RAS, GJZ 129,281-291 Attenuation from seismic codas in Iberia 203 For weak scattering media, the coda-wave energy distribution at a fixed reference lapse time t,,f is nearly uniform (Hoshiba et al. 1991), so the observed amplitudes at a fixed reference lapse time are used to correct for both the local site amplification and the source power (Aki 1980); the site- and source-corrected time integrals &,obs are obtained from the uncorrected ones by means of the expression en(r) En,obs(r) = Asquared-obdrr tref), n=1,2,and3 (3) tref must be chosen in such a way that tref 22rlp for all r, where p is the 5'-wave velocity. A correction for non-uniform energy distribution is made by multiplying eq. (3) by the ratio of the energy at the observation distance to that at the hypocentre, that is Esynth(r, tref)l Esynth(0, tref). Esynth(r, t) is computed from eq. (2). Eq. (3) multiplied by this ratio makes the energy integrals relative to the energy density at the hypocentre for t=tref. By these corrections it is possible to estimate the relative source power and site amplification (Hoshiba 1993). Finally, let En,syn(r) (n=l, 2 and 3) be the synthetic integrated energy for the three time windows. A least-squares fit applied to the theoretical and observed energy integrals leads to estimates of L;' and Bo. The residuals between the predicted and the observed energies are given by the expression where M is the total number of seismograms and En,obs(rm) is the corrected integrated energy for the rn seismogram at a distance rm. En,syn(r) is computed for various combinations of L;l and BO values. The term 4nr2 corrects for geometrical spreading. Bandpass-filtered signals were obtained by applying Butterworth filters in the following frequency bands: 1-2,24, 48,8-14 and 1420 Hz. In order to compute Asquared-obs(rm, t), the mean squared amplitudes were obtained averaging the observed squared amplitudes in a tk2 s window for the frequency band centred at 1.5 Hz, and a t f 1 s window for centre frequencies of 3, 6, 11 and 17 Hz. Only amplitudes with a signal-to-noise ratio > 2 were considered. Fig. 2(b) shows an example of Asquared-obs(rm, t), obtained for a seismogram recorded at station ECH. en(rn,) (n=l, 2 and 3) is then computed by integrating Asquared+,s(rm, t) over three time windows: 0-15, 15-30 and 30-45 s from the S-wave onset. Asquared-obs(rm, tref) were obtained by averaging squared amplitudes in a 5 s length time window centred at 42.5 s lapse time (see Fig. 2b). This particular lapse time is preferred because it satisfies the condition tref > 2rm/v for all r,, allowing us to avoid the contamination of direct S-wave arrivals at all hypocentral distances. Finally, the seismic signals inside this window were clear for all the selected data. Fig. 3 shows the observed energy points, together with the best-fitting theoretical energy curves. A large scatter in the data is observed in the first time window, while the scatter in the two later windows is significantly lower. This fact was also reported by Jin et al. (1994) and was assumed to be due to the simplicity of the model underlying the method. There is probably a need to correct data for the source radiation pattern of direct S waves and for local structure under the seismic

284 L. G. Pujades et al. (4 ts Origin time 1991 /O6/09 12.34:27.1 I Station ECH I Band-pass filtered 2-4 Hz, I, I,,!,, / I I,,, I,,, I,,,,,, I I!,, I,,, / I,, / I,,,,, /,,,, 110 16 210 215 310 i5 410 45 510 55 610 t (s) 10-1- (, I,,,,I /,,,,,,,,,,,,,,,,, /,,,,,,,,, lb 15 210 25 310 35 do 45 510 45 610 I t (s) Figure 2. (a) Example of a selected seismogram recorded at station ECH. The three time windows used to apply the multiple-lapse time-window analysis method are also shown. (b) Asquared-abs(~m, t) obtained for the seismic record presented in Fig. 2(a). station. The source radiation pattern for the direct S waves may not be equal to the effective radiation pattern of coda waves at short distances, and local structure under the station may cause the inadequate energy in the first time window. These two effects would affect the first time window more strongly than the later ones because later coda waves sample greater volumes and average over a greater number of scatterers with a higher degree of azimuthal distribution around the source and the receiver. We may also expect that the scatter in the data of the three time windows is due to the lateral heterogeneities in the area. Fig. 3 presents the results for the 1-2, 2-4 and 4-8 Hz frequency bands. Triangles, crosses and circles represent the observed energy for the 0-15, 15-30 and 30-45 s time windows respectively. Solid lines are the bestfitting theoretical curves. For all the analysed frequency bands, the R ratio (R = residual/minimum residual) represents a unique minimum. The errors in Bo and L;' were obtained from an F-distribution test at a 90 per cent confidence level. 0 1997 RAS, GJI 129, 281-291

Attenuation from seismic codas in Iberia 285 B, L,-'=0.044 km-' B0=0.48 and the inferred Qc', Q,' and Q;' values, together with their 90 per cent confidence intervals. Coda Q <Qc> 8 1 * 2{ 1-4-8 HZ L.-'=0.042 krn-' B,=0.34 Theory... 0-15 s + * * + * 15-30 s 30-45 s ~ 0 ~ 0 ~ - Theory...I 0-15 s + + * * * 15-30 s 0 ~ 0 0 30-45 0 s Coda attenuation is not the real attenuation, but empirically characterizes the coda amplitude decay with lapse time in addition to the geometrical spreading factor. The coda quality factor Qc was first assumed to represent the total S-wave attenuation QT (Aki & Chouet 1975), but later work showed that the quality factor obtained from coda waves may represent the intrinsic quality factor (e.g. Frankel & Wennerberg 1987); application of the multiple-lapse time-window analysis method permits the separation of the scattering and intrinsic attenuation effects, and coda Q may represent total Q, or intrinsic Q, depending on the investigated frequency and region (e.g. Jin et al. 1994). We will see that for the Almeria Basin, coda Q (Qc) is very close to total Q (QT) for low frequencies in the range 1-3 Hz. Since the pioneering works on coda waves of Aki (1969) and Aki & Chouet (1975), the following equation has been widely used to estimate Qel from coda waves: In { t2aobs(i, t)} = C- Q, ' wt, (5) where &bs(t, f) are mean-squared observed amplitudes, r are hypocentral distances, w is the angular frequency, t is the lapse time from the event origin time and C is a constant. Eq. (5) holds for f22ts, ts being the S-wave traveltime (Rautian & Khalturin 1978). A least-squares fit of eq. (5) leads to the determination of average Q,'. For each seismic record, a 20 s time window (from 2ts to 2ts f20 s) was taken to perform the analysis. The average Qe' values are also listed in Table 1. All values and averages are presented in Fig. 4. Fig. 5 shows plots of QE', Q;', Q,' and QF' for the centre frequencies 1.5, 3, 6 and 11 Hz. From Table 1 we choose Qc=89 and Q~=71 at 1.5 Hz (see also Fig. 5). If we take into account the 90 per cent confidence intervals of QT', it is clear that no significant differences between Q,' and Q,' can be established at 1.5 Hz frequency. Table 1. L;I and & values obtained from the application of the multiple-lapse time-window analysis method at different frequency bands, and inferred Qrl, Q;' and QT~ values together with their 90 per cent confidence intervals. fare centre frequencies of the analysed frequency bands and QE' are coda Q-' obtained from the application of the spectral amplitude decay method (Aki & Chouet 1975). O~,n,l,,,q,,,I,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, I,,, 1'0 2b 3b 5b 60 r (kmy Figure 3. Observed energy points together with the theoretical bestfit curves for 1-2, 24 and 4-8 Hz frequency bands obtained from the application of the multiple-lapse time-window analysis (Hoshiba 1993). Triangles, crosses and circles represent the observed integrated energy for 0-15, 15-30 and 30-45 s time windows, respectively. -6.73 The S-wave velocity was assumed to be 3 km SS' and eq. (1) was used to infer Q; ', Qg ' and Q, I. The uncertainties in BO and L,- ' were propagated to obtain the corresponding errors in Q; I, Q, I, and QT I. Table 1 summarizes the BO and L; values 17.0 0.005+0.017 0.98+0 02 0 003+0.003 0.14+0.48-0 002-0.54-0.080-0.09-0.30

286 L. G. Pujades et al.! b I Qo distribution Many authors have found that the net attenuation in a region can be represented in terms of a quality factor Q, which obeys a simple power law Q = eo (i) ", 10 -' 1 i Figure 4. Coda Q-' as a function of frequency for the 1.5,3,6,11 and 17 Hz centre frequencies. Dots represent Qc' values obtained from the analysis of a single seismogram. Squares represent average values for the studied frequencies. By inspection of Fig. 5, we can also see that Q;' varies smoothly between 1.5 and 3.0 Hz, whileits variation is stronger between 3 and 11 Hz. Q,' varies with frequency much more than Q; '. In addition, the 90 per cent confidence interval for Q;' at 1.5 Hz is 4.08~10-~ < Q,' < 11.03~10-~ and at 3 Hz is 2.6~10~~ < Q;' < 6.51~10-~ (see Table 1). Therefore, we have an overlapping interval for Q;' between 4.08~10-~ and 6.5x10d3, indicating that from the data of this study it is not possible to infer an important frequency dependence of QI near 1.5 Hz -, lc -)i 0 I,,'6 11 Hz, 0 A 3 nz, /A 1.5 Hz,' 0,,9' 0 /' 0 HzA 0 0 I I, I I,,, III, --, lo I' -4 10 -' 10-3 10 -I 1 Qc-' Figure 5. QI, QS and QT versus Qc for 1.5, 3, 6, and 11 Hz centre frequencies. Qc is close to QT for low frequencies. where Qo and v are assumed to be constants, and f is frequency normalized by a reference frequency fo. Herrmann (1980) proposed a method to estimate the coda quality factor Qc and its frequency dependence from observations of predominant frequency as a function of lapse time. This method has shown a great sensitivity to lateral heterogeneities and has been used to map coda Q in different regions of the world (e.g. Singh & Herrmann 1983; Jin & Aki 1988). This method was slightly modified by Pujades (1987) to avoid the trade-off between Qo and v. The method, as described in Pujades et al. (19901, will be applied to our data set. The working equation is linearizing, this becomes where Qo is the coda quality factor at a reference frequencyfo, and ( fp, t) are data pairs obtained from the seismograms. t is the lapse time from the origin time and fp is the predominant frequency. fp may be obtained both in the temporal domain from simple readings in the seismic records or in the frequency domain by taking the Fourier transform of the signal contained in a small time window centred at t (Payo et al. 1990); in this case fp is the frequency having maximum energy in the analysed window. I is the instrument magnification curve and I' is its first derivative with respect to the frequency. A leastsquares fit of eq. (8) yields both Qo and v for each available seismic record. This method has been applied to all the available seismograms and, in order to have the best Qo and v determinations, only least-squares fits with correlation coefficients greater than 0.75 are used. In this way, we have been able to obtain accurate values of Qo and v for the region. The Qo values obtained range from 57 to 204, the v values from 0.59 to 0.84, and the averages of the frequencies used from 1.11 to 1.25 Hz. Typical standard errors are in the range 5-10 per cent, and the maximum standard error is about 20 per cent. Several Qo and v values for different epicentre-station paths, contained in homogeneous structures, have been averaged and are presented in Fig. 6 together with their standard errors. To understand better the relation between the geology of the area and the attenuation of the coda waves, an iso-qo line map has been created. A computer software system for the creation of contour maps (Sampson 1988) was used. The inputs of this program are (lon, lat, Qo) points, where lon and lat are the geographical coordinates to which the values of the parameter to be contoured are assigned. In this way, Qo values found for each epicentre-station pair were assigned to the middle point between epicentre and station. A similar procedure was used by Jin & Aki (1988) to map coda Qo values (7)

Attenuation from seismic codas in Iberia 287 Figure 6. Average values with their standard errors for epicentre-station paths contained in homogeneous geological structures. (Nis the number of paths used in the average.) in China and by Canas et al. (1995) to map the coda Qo distribution in the Canary Islands. Fig. 7(a) presents the QO distribution obtained. Areas with a lack of attenuation data were blanked out to avoid meaningless extrapolated QO values. As we will show below, there is an excellent correlation between attenuation and geology, with the Neogene and Quaternary having the lowest Qo. Tentative QI and Qs distributions The net attenuation is the sum of the intrinsic and scattering attenuation. If we knew a priori formulas for the intrinsic and scattering attenuation, QI(~) and Qs( f) respectively, then we could use our knowledge of the net attenuation (i.e. Qo and v) to estimate any unknown parameters in those formulas. However, it all relies on having the correct a priori formulas describing the frequency dependence of intrinsic and apparent attenuation in the actual Earth. The additive relation (Dainty 1981) frequency-independence assumptions; the assumption of constant intrinsic Q contradicts the results of laboratory experiments (Johnston 1981), and the assumption of Q;'( f)- llf is only likely in the high-frequency limit (i.e. the optical limit of the wavelength is much smaller than the scale of the heterogeneities), but actual measurements of heterogeneities in the Earth show it to have significant heterogeneities at all scales. Nevertheless, the application of eq. (9) leads to a first approximation and provides some insight into the contribution of scattering and intrinsic effects to total attenuation. So, by arranging and expanding eqs (6) and (9) in a Taylor series near a reference frequencyfi, assuming that QI and g have a low frequency dependence compared with the f -I frequency dependence of Q;', as found in many regions in the world, and by equating the first-order terms, we obtain the following equations relating Qo, v, QI and Qs at the fr reference frequency: (9) where Q;' is the intrinsic attenuation and g=qs is the scattering coefficient, has been widely used for the separation of scattering and intrinsic contributions (Rovelli 1982; Richards & Menke 1983; Gagnepain Beyneix 1987; Hoshiba et al. 1991; Mayeda et al. 1992; Kang & McMechan 1994). The second term on the right side of eq. (9), Q;', represents the energy loss due to scattering processes in the lithosphere. Some authors assume that QI or g or both are frequencyindependent parameters (e.g. Gagnepain Beyneix 1987; Kang & McMechan 1994). In fact, there is no physical basis for these and inversely: The multiple-lapse time-window analysis does not assume any frequency dependence for total, intrinsic and scattering Q, permitting the investigation of these quality factors at different frequency bands and the empirical inference of their frequency

288 L. G. Pujades et al. 37.,... and scattering attenuation in Southern California. Therefore, eqs (10) and (1 1) can be applied in the studied area at reference frequencies between 1 and 1.5 Hz. By applying eq. (10) to the obtained Qo and v values, and takingfr as the avei-age of the frequencies used in each Qo and v estimation, we have obtained Ql and Qs values for each epicentre-station path. In this way we have mapped intrinsic and scattering distributions in the area. The inferred intrinsic quality factors QI range from 157 to 1135, and the scattering quality factors Qs from 85 to 292; the reference frequencies.fr range from 1.11 to 1.25 Hz. Contour iso-qs and iso-qi lines are shown in Figs 7(b) and (c) respectively. 37 37 Figure 7. Q,, Qs and Qo distributions for the region. (a) Coda Q distribution for the region, at 1 Hz reference frequency Qo. (b) Scattering quality factor Qs distribution in the region inferred from the coda-q frequency dependence near 1 Hz frequency. (c) Intrinsic quality factor Ql distribution in the region inferred from the coda-q frequency dependence near 1 Hz frequency. dependence, and therefore the testing of the applicability of eqs (10) and (11) in the investigated area. As mentioned above, we can see in Table 1 and Fig. 5 that, near 1.5 Hz, coda-q (Qc) is very close to total Q (QT) and that QI has a weak frequency dependence. Similar results were reported by Jin et al. (1994), who estimated intrinsic GEOLOGICAL SETTING The Almeria Basin is located in the southeastern Iberian Peninsula, which is the emergent part of the Iberian Plate. The Iberian Peninsula has been the scene of a variety of tectonic processes because of its unique position relative to the Atlantic and the Mediterranean on its western and eastern sides and between two megaplates on the northern and southern sides. Iberian geology has been dominated by continental rupture from the break-up of Pangaea until the Cretaceous, and by convergent plate-margin phenomena to the present time. Vegas & Banda (1982) proposed a tectonic framework providing a reasonable explanation for the geodynamic evolution of Iberia, for the distribution and geological history of mountain ranges, and for the formation of sedimentary basins and their crustal evolution. This geotectonic framework is followed in this work in order to outline the main geological and tectonic features of the studied region. Despite its moderate size, the Iberian Peninsula shows a diverse assortment of geotectonic units: it is composed of a Hercynian nucleus, the Iberian or Hesperic Massif, to which two Alpine chains have been added, the Betics and the Pyrenees. The Iberian Massif, which is the largest outcrop of the dismembered Hercynian belt of western Europe, is the Iberian part that has remained uncovered by Mesozoic seas. The northern and southern borders of this continental fragment were strongly deformed during the Alpine orogeny. The Betic and Pyrenean chains were built up by compressional and transcurrent movements that took place in the former (extensional and divergent) transform margins of the Iberian Massif. These geosynclinal orogenic areas have traditionally been divided into internal and external zones. We may consider three main tectonic stages in the construction of the actual geological features of Iberia: the first comprises the time-span between the final arrangement of the Hercynian Belt in western Europe and the beginning of the generalized Mesozoic event. These late Hercynian events were forerunners of the Alpine evolution and produced differences in the nature and composition of the Hercynian crust; an intense fracturing took place in Iberia just before the initiation of the Alpine cycle. A second stage may be described as an extensional episode affecting Iberia and southern Europe from the late Permian (230 Ma) to the mid-upper Cretaceous (80 Ma) (this episode corresponds to a sedimentary phase, which initiated the geosynclinal Alpine cycle). The Alpine orogeny is the third and last stage responsible for the main present-day geological features of Iberia.

These different realms, as parts of rift zones related to the opening of the Atlantic, the westward propagation of the Tethys and the mutual interaction of the Tethys and the Atlantic, were created at the borders of the Iberian plate: the Porxguese Basin to the west, the Cantabro-Pyrenean Basin to theaorth and the Betic Basin to the south and southeast. These three rift systems isolated the Iberian Massif and later became the present-day margins of Iberia. The sediments filling these rifts correspond to the Germanic Triassic sequence, which began with fluviatile and lacustrine red beds, evolved to shallow marine water conditions and ended with evaporitic sediments. Volcanic activity is associated with this rifting phase. During the early Jurassic (190 Ma), the rifts subsided below sea level, giving rise to a generalized marine invasion that resulted in the formation of an extended carbonate platform. In some parts of the marine basins the carbonate was rifted, creating oceanic crust. This rifting led to the formation of a continental margin in southern Iberia, the Betic palaeomargin. A change in the spreading direction in the central Atlantic at around 80 Ma marked the beginning of the compressional or Alpine deformational stage. The westward movement of Africa with respect to Europe defines a first period of the compressional stage, which is called the pre-collisional stage. The relative movement of Africa and Europe changed again at about 50 Ma, taking principally a N-S direction from then until 15 Ma. This period corresponds to the collisional phase, during which extra-iberian units impacted the southern margin. This interpretation implies the consumption of oceanic or intermediate lithosphere lying between the Betic units and the southern margin of Iberia. Therefore, the Betic realm constitutes the southern deformed border of the Iberian plate and, like many other chains, has been described in terms of external and internal zones, taking into account the intensity of deformation and metamorphism. The external zone is subdivided into pre-betic and sub-betic zones. The pre-collisional stage is well recorded in these zones. The Betic zone (internal zone) is made up mainly of pre- Mesozoic and Triassic terranes showing a complex tectonic arrangement. In this zone three main units have been distinguished on the basis of the nature of the pre-alpine basement and the Mesozoic cover as well as the grade of metamorphism (Egeler & Simon 1969). These are the Nevado-Filabride, Alpujarride and Malaguide complexes. The Nevado-Filabride complex shows high-grade metamorphism, while the Alpujarride complex is affected by lowgrade metamorphism; the Malaguide complex remained unmetamorphosed. Finally, the period extending from the early Miocene (- 20 Ma) to the present corresponds to the continued convergence of Africa and Europe. This period is also characterized by tensional events in some parts of the Alpine- Mediterranean area. Vegas & Banda (1982) showed how an integration of geophysical and geological data, related to extensional tectonics, points to a rifting episode along the eastern coast of Spain. The post-alpine (mainly Neogene) tensional features cross-cut the Alpine and older structures. The Neogene extensional event caused the dismantling of massive relief zones and resulted in the chaotic geomorphic pattern of the eastern Betic chain. The formation of Neogene basins began in the Tortonian (10 Ma) as characteristic horstand-graben tectonics. Faults in this region can be grouped into Attenuation flom seismic codas in Iberia 289 a conjugate NE-SW and NW-SE fault system, and E-W trending faults. The latter seem to be predominant in the eastern part, delineating the borders of some basins. All these reactivated faults have played an important role as sedimentary controls for the Miocene deposits, which in some places are very thick because they were formed in zones of high relief. Marine deposits were progressively limited to the present near-shore basins during the Pliocene (7 Ma). Volcanic activity, dated at 15-8 Ma, has also been reported in the area (Bellon & Letouzey 1977). In the Almeria basin, calcalkaline volcanic rocks crop out in the Gata Cape, at the Western limit of the Almeria gulf. For the purpose of this work, the different lithologies of the area covered by our data set may be summarized by just three main geological structures. The Filabres range, which is part of the Nevado-Filabride complex, is composed of Palaeozoic materials that have undergone high-grade metamorphism; the age of these paleozoic outcrops is within the 350-300 Ma range. The Alhamilla range is included in the Alpujhrride complex which represents a moderate metamorphism grade, and is mainly composed of Mesozoic-Triassic rocks. The age of the materials corresponding to these geological stages is in the 225-190 Ma range. Finally, the basin was filled with young Neogene and Quaternary sediments, less than 20 Myr in age (for geological time estimation, see Bilal & Van Eysinga 1987). Fig. 1 summarizes the geographical distribution of the three main geological units. SUMMARY AND DISCUSSION We have selected high-quality seismic data collected in the Almeria Basin to analyse the seismic attenuation in the area. First, the multiple-lapse time-window analysis method (Hoshiba et al. 1991) was applied to obtain intrinsic, scattering and total attenuation at several frequency bands. For low frequencies (near 1 Hz), QI depends slightly on frequency. At the 1.5 Hz centre frequency we obtained L;'=0.044 km-' and BO =0.48, which, for an S-wave velocity of b= 3.0 km s-', correspond to Qs = 148, Ql = 137 and QT =71. Second, we applied the spectral-amplitude decay analysis of bandpassfiltered records method (Aki & Chouet 1975) to the same data set and found that near 1 Hz frequency, coda Q is close to total Q. A least-squares fit of the coda Q values in Table 1 and Fig. 4 yields the following expression: QE' = (0.016 ~0.001)f-0~gs*0~03 which is equivalent to Qc = 63f0.8x. Finally, we applied the predominant-frequency decay analysis method (Herrmann 1980; Pujades et al. 1990) to obtain Qo and v for each epicentre-station path, yielding similar QO and v values. The three methods provide similar attenuation estimates but the last method has a greater sensitivity to the lateral heterogeneities of the area. The third method yields QO values in the range 57-204, and v values in the range 0.59-0.84 for reference frequenciesfr between 1.1 1 and 1.25 Hz. The inferred intrinsic and scattering quality factors Ql and Qs range from 157 to 1135, and from 85 to 292, respectively. In order to understand the physical meaning of the values obtained better, we constructed iso-q lines for (20, Q1 and Qs by using standard 2-D contouring software. The maps show a correlation with

290 L. G. Pujades et al. the main geological and tectonic features of the area (see Fig. 7). High intrinsic quality factors are found in the old rocks of the Nevado-Filabride complex, while low QI values are found in the Neogene basin, which is covered by young sediments; intermediate values have been found in areas with intermediate age. Comparing the coda Q and v values obtained here with those reported in earlier work in or near the studied area, it is possible to state the following: Pujades et al. (1990) and Canas et al. (1992) reported low QO and high v values for the southeastern Iberian Peninsula (Qo N 10CL200, veo.8) at a reference frequency of 1 Hz; lower Qo values in Fig. 7 are reasonable because of differences in the range of distances. Akinci, Del Pezzo & Ibaiiez (1995) analysed high-frequency coda waves recorded in a region located about 150 km west of the Almeria Basin, at the western limit of the Betic domain. They analysed the separation of intrinsic and scattering attenuation in a frequency band between 1 and 18 Hz. A comparison of their results with coda Q values previously obtained by Ibaiiez (1990) led them to conclude that, for the studied region, coda Q is close to intrinsic Q and that, in spite of the fact that scattering Q is more frequency-dependent than intrinsic Q, Ql also has a significant frequency dependence. Nevertheless, for their centre frequency of 1.5 Hz and for the distance range 0-170 km they obtained So = 0.62, L; = 0.044. From these values it is possible to obtain Ql= 156, Qs = 96 and QT = 59. At shorter distances (in the 0-80 km range) at the same centre frequency they obtained similar albedo and extinction length values, So = 0.66, L, I = 0.042, giving Ql = 165, Qs = 85 and QT = 56. These reported values are not in disagreement with those found in this study in the Almeria Basin (see Table 1). In addition, Ibaiiez (1990) obtained coda Q values in the same region investigated by Akinci et al. (1995) at the same frequency and distance ranges. By fitting his values, reported in Table 1 and Table 2 of Akinci et al. (1995) as Obs. Qc, using eq. (8) we obtain Q= 111.5f079 for the 0-170 km distance range and Q=86f08 for the 0-80 km distance range; in both cases an excellent correlation coefficient is obtained (p = 0.999). Both frequency-dependence laws compare well with previous determinations carried out by others in the same area and with the frequency dependence of coda Q found in the Almeria Basin. Despite the great amount of theoretical and observational work in this field, there are some continuing questions about the attenuative behaviour of the lithosphere. One of them is why coda Q sometimes reflects total attenuation and sometimes intrinsic absorption. The fact that different methods applied to the same data lead to consistent results means that we are dealing with the main attenuative properties of the lithosphere, but the simplicity of the theoretical models used to measure the attenuation parameters of the crust and upper mantle and the strong assumptions underlying them impede the resolution of fine features of the attenuation processes in the lithosphere. In fact, the basic assumption of the multiplelapse time-window analysis has a limit because of the isotropic scattering assumption and the spherical radiation assumption from the source. The former assumption leads to the concentration of coda energy near the source at long lapse times. The double-couple radiation pattern creates a large data scatter around the theoretical curve based on the latter assumption in the first time window. We hope that continuing studies on the depth-dependent attenuating structures (Hoshiba 1994), non- isotropic scattering (e.g. Sat0 1994b, 1995; Hoshiba 1995), the influence of the presence of direct surface waves on coda Q, the influence of the layered structure of the medium through which coda waves propagate, focal mechanism, geometrical spreading, etc. will help to detect and isolate anomalous behaviour of the waves and to construct and apply adequate corrections to each data point. In this way, the scatter in the data that can be seen in Fig. 3, and in most of the figures applying the multiplelapse time-window analysis in the literature (especially in the first time window), will decrease; the confidence intervals around the average values will also decrease and the increasing resolution of attenuation results will help to separate and resolve the fine structure of the quality factors of the lithosphere. The application of the actual available methods to small and homogeneous areas with good epicentre-station path coverage will also avoid the averaging of clearly different attenuative properties of different regions, sampled by coda waves, which could lead to Q models of the Earth with spurious features that conceal the true structure of the attenuation parameters. As pointed out by Mitchell (1995), we know almost nothing about the attenuation of high-frequency waves in oceanic lithosphere. Coda studies using data collected from the ocean bottom will be an exciting field of research that will permit the results obtained to be contrasted with the great amount of information about the seismic-energy attenuation in continental areas. The main result of this work is the estimation of net, intrinsic, scattering and coda-q values in the southeastern part of the Iberian Peninsula. In addition we have also tried to analyse the distribution of these attenuation parameters in the region and how they correlate with the main geotectonic features of the area. A first-order approach has lead to formulas relating the frequency dependence of the total quality factors to intrinsic and scattering attenuation. The application of these formulas to the results obtained in a small region has provided good results for the separation of the scattering and the intrinsic effects, showing a good correlation between these two parameters and the tectonic history and the actual geological characteristics of the area, with the Neogene and Quaternary having the lowest QI and QT. Therefore, the application of the relations found in this study to places and frequencies where total Q and its frequency dependence are known may provide a first approach to the separation of the attenuative part due to intrinsic absorption from that due to scattering attenuation. ACKNOWLEDGMENTS K. 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