Geophysical Journal zyxwvutsrqpon (1988) zyxwvutsrq 95, 391-396 zyxwvutsrq Anelastic Rayleigh wave attenuation in the Iberian Peninsula Jose A. Canas,* Fernando de Miguel,? Francisco Vidalt and Gerard0 Alguacil? Accepted 1988 June 3. Received 1988 May 18; in original zyxwvut form 1987 March 31 SUMMARY Fundamental mode attenuation coefficients of the vertical components of Rayleigh waves have been obtained for three different tectonic regions of the Iberian Peninsula. The western part of Iberia, which is tectonically most stable, has the lowest attenuation, and the largest quality factor Qa. Average QP at depths from 0 to 80 km is 126 zyx f 84 in the tectonically stable part of western Iberia, and 57 f 22 in the active part of eastern Iberia. Key words: attenuation, inversion, Iberian Peninsula. 1 INTRODUCTION Surface-wave attenuation studies have provided good knowledge of the anelastic attenuation under continental and oceanic regions (e.g. Tsai zyxwvutsrq & Aki 1969; Solomon 1973; Herrmann & Mitchell 1975; Souriau, Correig & Souriau. 1980; Mitchell 1981; Canas & Mitchell 1981). From the above studies it is possible to infer that the zyxwvuts Q structure under the oceans is better constrained than under continental regions. probably due to the difficulties in obtaining good amplitude data in continental regions. Amplitude data for oceanic paths are easier to obtain than for continental paths due to the fact that the structure under oceanic regions is much more homogeneous than in continental ones (e.g. Mueller et al. 1973; Yacoub &i Mitchell 1977; Souriau et al. 1980; Banda & Ansorge 1980). Therefore, large scatter in the attenuation data may be expected when working in continental regions. Most Qp values for depths to about 100 km in continental areas are near 100 (e.g. Mills & Hales 1978; Anderson & Hart 1978). Souriau et al. (1980) found a mean QP value of about 35 for the uppermost 100km under the French Central Massif, a value that was consistent with a Qa value of about 63 on the same region and for depths between 180 and 300 km (Dorel 1978). Cheng & Mitchell (1981) obtained Q,l structure under the United States. They found that their data were consistent with the models having Qa values between 80 and 300 for the uppermost 20 km and a value of about 2000 between 20 and 40 km depth. Singh (1982) determined average Q/, values under the Eurasian continent to be from about 50 to 200 down to about 100 km depth. Payo (1969) suggested that Q of Rayleigh waves (Q,) in the Iberian Peninsula decreased from about 60 at 10s period to about 20 at 50 s. The purpose of this study is to investigate possible lateral variations in QP in the upper part of the lithosphere of the Iberian Peninsula. In Section 2 we present the data used in this study. In Section 3 we use these data to derive anelastic attenuation coefficients. In Section 4 an inversion procedure 'Civil Engineering School. Poli~echnical University of Catalunya, 08034 Barcelona, Spain; t Geophysical Observatory of Cartuja. University of Granada. 18071 Granada, Spain. is used to determine anelastic quality factor Q-we use the definition of Q from e.g. Anderson, Ben-Menahem & Archambeau (1965) that considers that Q is the relative loss of energy per cycle-as a function of depth from these attenuation coefficients. Our results are summarized in Section 5. 2 DATA The two-station method has been applied to obtain the attenuation coefficients of the fundamental mode vertical components of Rayleigh waves. Records of eight earthquakes (Table 1, Fig. 1) were recorded at the seismograph stations in the Iberian Peninsula: TOL, MAL, €TO and EBR (Fig. 1). The stations TOL, PTO and MAL, belong to the WWSSN network, and the station EBR has similar characteristics to those of the long period WWSSN instruments. Departures from expected amplitude responses have been checked using the method of Mitchell & Landisman (1969). Seismograms have been digitized at 1 s interval, and multiple filter analysis (Dziewonski, Bloch & Landisman 1969; Hermann 1973) has been used to determine amplitudes at selected periods. In order to duplicate the original seismograms in detail, special care was taken when digitizing the peaks or the waveform or places where the slope changes rapidly. In such places a greater number of points must be used. Base-line corrections (Mitchell & Landismann 1969; James & Linde 1971) have been applied in an attempt to avoid scatter in the data. The method used in this study is to construct a straight line from the point where a trace terminates at one end of the record to the same point in time at the other end of the record. Unknown focal mechanisms of the earthquakes listed in Table 1 preclude us to obtain synthetic seismograms. Therefore, observed interstation group velocities of the fundamental mode of Rayleigh waves between pairs of seismograph stations have been selected such that the ob- served velocities were similar to the determined theoretical ones for the regions of this study (Fig. 2). The earth model to compute theoretical velocities for the path TOL-MAL is the one obtained by Sierra (1980). The earth model used to computed theoretical velocities for the path PTO-TOL and 391 Downloaded from https://academic.oup.com/gji/article/95/2/391/728439 by guest on 02 October 2022