Depth dependent seismic scattering attenuation in the Nuevo Cuyo region (southern central Andes)

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L24307, doi: /2009gl041081, 2009 Depth dependent seismic scattering attenuation in the Nuevo Cuyo region (southern central Andes) G. Badi, 1 E. Del Pezzo, 2 J. M. Ibanez, 3 F. Bianco, 2 N. Sabbione, 1 and M. Araujo 4 Received 22 September 2009; revised 13 November 2009; accepted 24 November 2009; published 29 December [1] In the present work we separated intrinsic from scattering attenuation coefficients both for the crust and the upper mantle in the tectonically highly active areas of the Southern-Central Andes - Nuevo Cuyo region, analyzing two groups of earthquakes, well separated in depth. This region is characterized by the presence of flat subduction. We apply MLTWA (Multiple Lapse Time Window Analysis), coda normalization and Q-coda techniques to measure the scattering and intrinsic attenuation coefficient and the total Q for S waves. We find that intrinsic attenuation does not decrease with depth whereas scattering attenuation is higher in the crust than in the upper mantle, and that intrinsic attenuation predominates over scattering attenuation. We interpret this observation in terms of the release of water and other fluids into the overlying lithosphere due to the dynamics of the subduction process, in agreement with most of the prevalent geodynamic models. Citation: Badi, G., E. Del Pezzo, J. M. Ibanez, F. Bianco, N. Sabbione, and M. Araujo (2009), Depth dependent seismic scattering attenuation in the Nuevo Cuyo region (southern central Andes), Geophys. Res. Lett., 36, L24307, doi: /2009gl Introduction [2] Subduction zones are regions characterized by complex geodynamics, both in surface and depth. This complex environment includes deep magma sources associated with dehydratation in the sub-ducting plate, which allow for water and other fluids to be released into the overlying mantle wedge [Abers et al., 2006]. The numerous attenuation studies of the subduction zones (for the central Andean plateau see Whitman et al. [1992] and Myers et al. [1998]), including attenuation tomography, are mainly focused on the P-wave attenuation measurements, despite the importance of the S-wave attenuation anomalies, that should be more directly associated with a thermally weakened crust than those for P waves (due to the presence of partial melts that increase S-wave attenuation). Moreover, there is another conceptual problem in the physical meaning of the attenuation along the path connecting source to receiver. The measured total-q inverse, Q T, is a function of intrinsic and scattering Q-inverse: Q T = Q i + Q s [Sato and Fehler, 1 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, La Plata, Argentina. 2 Osservatorio Vesuviano, Sezione di Napoli, Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy. 3 Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain. 4 Instituto Nacional de Prevención Sísmica, San Juan, Argentina. Copyright 2009 by the American Geophysical Union /09/2009GL , section 5.2]. Q s accounts for the interactions of the primary waves with heterogeneity along the path. Separation of the intrinsic and scattering part of the attenuation becomes crucial for a correct interpretation of the attenuation results, as high attenuation paths may be generated in highly heterogeneous zones due to scattering. This is the reason why the separation of total-q into intrinsic and scattering parts place constraints in the interpretation of the very nature of these zones. We study total, intrinsic and scattering attenuation in the subduction zone of Nuevo Cuyo, in the Southern Central Andes (located between 27 S and 33 S and 65 W and 70 W, Figure 1). Data from intermediate and shallow earthquakes allow for discrimination between upper mantle and crustal seismic attenuation properties. This region shows complex geodynamic features, with a flat subduction regime [Ramos et al., 2002], the presence of a high mountain range on the west where no active volcanism is present, fold and thrust belts in the centre, and several sedimentary basins on the East [e.g., Pujol et al., 1991]. Seismicity is located mainly in two clearly separated depth ranges, the first between 0 and 35 km of depth and the second between 100 and 200 km. The main fracture systems are oriented N-NW, S-SW and W-NW producing earthquakes up to magnitude 7.3 [e.g., Smalley et al., 1993]. We find the same S-wave intrinsic attenuation coefficient for both upper mantle and crust, whereas scattering attenuation is higher in the crust than in the upper mantle. Intrinsic attenuation is higher than scattering attenuation, confirming the presence of fluids released into the overlying lithosphere which govern the attenuation at depth. 2. Data [3] Selected data corresponds to 23 short period, vertical component seismic stations from the Red Sismologica Zonal Nuevo Cuyo belonging to Red Nacional de Estaciones Sismologicas of the INPRES, Argentina (Figure 1). We used data from the period with an initial data set composed of 456 earthquakes with local magnitudes between 3.4 and 4.8, focal depth between 0 and 300 km and epicentral distances between 5 and 350 km. Data was relocated using VELEST, JHD procedure [Kissling, 1995] and a subset was selected on the basis of an optimal signal to noise ratio. More than 4200 seismograms with a source depth range between 0 and 210 km (see Figure 1) were used for the attenuation study. Average S-wave velocities of 3.5 km/s and 4.6 km/s were derived by JHD procedure, respectively for the crust and the mantle, to be used in the MLTWA. 3. Methods [4] We apply three different methods to measure a) separated intrinsic and scattering Q-inverse; b) Q-coda inverse and c) direct S-wave total Q-inverse. L of5

2 [6] For b) we, applied the well known Aki and Chouet Single Back Scattering method [see Sato and Fehler, 1998, section 3.1]. [7] For c) we applied the Aki s Coda Normalization method [see Sato and Fehler, 1998, section 3.4] which gives an estimate of direct S-wave total attenuation. This last method has been applied for an easy comparison of the present results with those reported in literature. Figure 1. (top) Tectonic sketch map of Nuevo Cuyo Region. Shallow (dark circles) and intermediate (light circles) earthquakes. Open triangles represent stations. Dark gray solid lines define main geologic provinces and light gray dashed lines are sutures coinciding with known active master fracture systems (except the active faults southeast of Sierra Pie de Palo) [Ramos et al., 2002]. Black solid lines are contours of Benioff zone (depth in km). (bottom) Projection of hypocenters on AB line. Thick solid line represents the slab geometry and thick dashed line the lithospheric base. Thin solid line is the Mohorovicic discontinuity [Ramos et al., 2002]; its extrapolation to the West in dotted line. [5] For a) we apply the well known MLTWA (Multiple Lapse Time Window Analysis) [see Sato and Fehler, 1998, section 7.2], based on the analysis of the coda-normalized energy integrated in three successive time windows, as a function of the source-receiver distance. The energy is estimated by the m.s. of the filtered seismogram trace; the normalization is carried out with respect to a 15-seconds duration energy window, centered at 70 seconds lapse time from origin time. An S-wave velocity of 4.36 km/s is assumed, obtained as a weighted average of S-wave speeds in the whole earth volume encompassed by the scattered waves. The first of the three successive time windows starts at the S-wave arrival onset; each window has a duration of 15 seconds (see Figure 2a). The theoretical curves describing the pattern of integrated energy vs source-receiver distance are obtained from the hybrid single-scattering diffusion approximation of Zeng et al. s [1991] multiple scattering model. The theoretical curves (see Bianco et al. [2005] for the mathematics) are parameterized in terms of respectively the scattering and intrinsic attenuation coefficients (in turn expressed as a function of Q i and Q s by h s,i = 2pf VQ s;i where V is the wave velocity and f the frequency). 4. Data Analysis and Results [8] We analyze separately the crustal (0 40 km) and the mantle ( km) earthquakes. In order to investigate on the existence of possible lateral variations affecting the attenuation features in the crust, we initially grouped the 23 seismic stations in 7 zones based on considerations about their tectonic homogeneity. Then, we analyzed all the crustal earthquakes within a distance range of 90 km, separately for each group of stations. For the analysis of mantle earthquakes all station records were placed in a single group, as the mantle earthquake sources were clustered in a relatively smaller volume. Each trace was filtered in four frequency bands, centered respectively at frequencies f c = 1.5,3,6,12 Hz and with bandwidths in the interval [f c f c /2, f c + f c /2]. Then the envelope was estimated by calculating the r.m.s. of the filtered trace, and finally the coda normalization was carried out. In Figure 2a we plot a sample seismogram showing the three time windows and the coda window used for the application of the method. The integrated energy was finally corrected for geometrical spreading by multiplying by 4pR 2 where R is the source station distance. A sample plot of the corrected integrated energies as a function of distance at 6 Hz center frequency is shown in Figure 2b (representing the MLTWA application to shallow earthquakes). The fit of experimental data to the theoretical curves was carried out in the same way as described by Bianco et al. [2005], using a L2-norm-based misfit function calculated with a grid-search method using a grid of all the possible reasonable values of seismic albedo, B 0 and Extinction length, L e that in turn are dependent on h s and h i by the following relations: Le = h s + h i ; B 0 = h s Le. The minimum of the normalized (at minimum value) misfit function corresponds to the couple of B 0 and L e that best fit the experimental data. Error intervals for each estimate were calculated using an F-distribution at 70-percent confidence level. For this distribution we calculated the ratio of two random variables, each with a number of degrees of freedom, N df, given by N df = N data 2, as we invert for 2 parameters, B 0 and Le. In the case of the present data, all the solutions with normalized residuals greater than 1.1 are statistically different. We observed that all the analyzed groups of stations for crustal events shared the same L e and B 0 estimates within the error ranges, and thus we considered their average as a characteristic parameter for the crust. [9] In Table S1 of auxiliary material we report B 0 and L e (and the associated values of h s and h i, Q i and Q s ) together with their uncertainty for the two groups of data (crustal and mantle earthquakes). 1 The results are plotted in 1 Auxiliary materials are available in the HTML. doi: / 2009GL of5

3 Figure 2. (a) An example seismogram showing the coda shape. The segments marked by E1, E2 and E3 represent the time position and duration of the three windows utilized for MLTWA. S marks the position and duration of the S-wave time window utilized for coda normalization technique. coda marks the coda portion used for normalization in both MLTWA and coda normalization techniques. (b) Example of Log energy plot (corrected for geometrical spreading) as a function of Source-Receiver distance (crustal earthquakes, 6 Hz). Bold curves show the theoretical best fit, for Energy integrated in the windows E1, red circles; E2, blue triangles; and E3, green crosses. Figures 3a and 3b, representing the h i and h s pattern vs frequency (and the associated Q-inverse). [10] We analyzed Q c at the same frequency bands of the MLTWA for a fixed lapse time of 90 seconds to ensure that the sampled region by this coda envelope is the same as that analyzed in MLTWA, for which we used data in a distance range between 10 to 100 km. A lapse time of 90 seconds corresponds to a scattering ellipsoid with major semi-axis around 120 km. [11] Coda normalization method was applied using a 10- second long window over the S-wave onset and normalized by a 10-seconds long window centered at 90 seconds of lapse time. Q b values were obtained by a linear fit in the hypocentral distances range between 40 and 250 km. Figures 3c and 3d show the comparison among the Q values obtained with the different methods utilized. 5. Discussion [12] As can be observed from Figures 3c and 3d, Q i for crustal earthquakes is higher than Q i at 3 and 6 Hz for mantle earthquakes, showing an intrinsic attenuation increasing from mantle to crust. Even tough MLTWA implicitly includes the effects of energy leakage to the depth in addition to the actual intrinsic losses, we assume that the Figure 3. (a) h i and (b) h s (and corresponding Q i and Q s ) as a function of the frequency for crustal and mantle earthquakes. Comparison among Q c, Q b, Q i, Q s and Q T for (c) crustal earthquakes and (d) mantle earthquakes. 3of5

4 estimate of Q i is mainly associated with the inelastic properties of the complex geological structures of the region. On the other hand, Q s for mantle earthquakes is much smaller than Q s for crustal events, showing a decrease of scattering when passing from crust to mantle, probably due to the more homogeneous structure of the lithospheric mantle crossed by the seismic waves. At shallow depth, Q i is similar to Q s, indicating a similar contribution of both mechanisms in the attenuation process (Figure 3). On the contrary, Q s is much smaller than Q i for mantle earthquakes, which can be interpreted in terms of a major contribution of anelastic absorption with respect to the scattering attenuation. Finally, we compared (Figures 3c and 3d) the average values of Q i, Q s and Q T derived from MLTWA with the Q b and Q c. It is remarkable that similar values of Q c were found for the data at the two focal depth ranges, and that they are really close to the Q i estimated by MLTWA. This result is in agreement with many other observations reported in literature [e.g., Sato and Fehler, 1998, sections 5.1 and 7.2]. Q b is similar to Q T obtained summing Q i and Q s for both crustal and mantle events, as expected. Remarkable are the differences between the scattering attenuation between crustal and mantle earthquakes, indicating that the scattering phenomena are mainly concentrated in the crust. Interesting is also the anomaly (at 3 Hz) in the pattern of Q s with frequency, possibly depending on the ratio between the predominant scatterer size and wavelength. [13] The present results have been compared with the majority of the applications of the same approach in other areas of the world (see Figure 4). Considering the values of inverse Q obtained for shallow earthquakes, Nuevo Cuyo region belongs to the zones characterized by high attenuation, as, e.g., Central California and Southern Spain (Andalucia). Interestingly, scattering attenuation measured for mantle earthquakes shows, on the contrary, that Nuevo Cuyo region behaves much more similarly to low scattering attenuation zones, like Southern Apennines and Friuli (Italy). Figure 4. Several tectonically active zones in the world investigated with MLTWA (values taken from Bianco et al. [2005] and from E. Del Pezzo and F. Bianco, Earth Print Repository, 2007, have been compared with Nuevo Cuyo region data for both deep and shallow earthquakes. (top) Scattering attenuation; (bottom) intrinsic attenuation. 6. Conclusions [14] In the present work we separated intrinsic and scattering attenuation coefficients both for the crust and the upper mantle in the tectonically active area of the Southern-Central Andes - Nuevo Cuyo region, analyzing two groups of earthquakes, well separated in depth. In terms of absolute values of Q i, Nuevo Cuyo shows values that are approximately in the middle of the interval spanned by those calculated for other regions of the world. We observed at Nuevo Cuyo clear differences in the scattering attenuation for crustal events compared to mantle events. We interpret this observation in terms of the different earth volumes encompassed by the scattered waves respectively generated by the shallow (crust) and intermediate (mantle) sources. The scattered waves generated by the mantle earthquakes encompass an earth volume comprising the lithosphere situated above the flat subduction zone which characterizes the area under investigation, whereas the scattered waves generated by the shallower crustal earthquakes are mainly trapped inside a highly heterogeneous crust. MLTWA estimates of separated intrinsic and scattering attenuation parameters are effectively averages in these different volumes and their difference reflects the differences in the scattering and inelastic properties of these volumes. [15] The predominance of intrinsic attenuation in the upper mantle can thus be explained by the presence of water and other fluids released into the overlying lithosphere by the dynamics of the subduction process, in agreement with most current interpretations. Finally, the high heterogeneity deduced by the attenuation estimates in the crust is in agreement with the tectonic complexity of the upper plate in subduction in the Nuevo Cuyo region: the rough topography of the Andes (6959 m), presence of valleys, thin skinned tectonics and inverse faults over crystalline basement. [16] Acknowledgments. Work partly supported by the Italy s DPC- INGV projects SPEED and UNREST ; by Italy s PRIN Project 2007LL5AA3 004 and by the Spanish Government Project HISS. References Abers, G. A., P. E. van Keken, E. A. Kneller, A. Ferris, and J. C. Stachnik (2006), The thermal structure of subduction zones constrained by seismic imaging: Implications for slab dehydration and wedge flow, Earth Planet. Sci. Lett., 241, Bianco, F., E. Del Pezzo, L. Malagnini, F. Di Luccio, and A. Akinci (2005), Separation of depth dependent intrinsic and scattering seismic attenuation in the northeastern sector of the Italian Peninsula, Geophys. J. Int., 161, , doi: /j x x. Kissling, E. (1995), Velest user s guide Short introduction, report, Inst. of Geophys. and Swiss Seismol. Serv., Zurich, Switzerland. Myers, S. C., S. Beck, G. Zandt, and T. Wallace (1998), Lithospheric-scale structure across the Bolivian Andes from tomographic images of velocity and attenuation for P and S waves, J. Geophys. Res., 103, 21,233 21,252. Pujol, J., J. M. Chiu, R. Smalley Jr., M. Regnier, B. Isacks, J. L. Chatelain, J. Vlasity, J. Castano, and N. Puebla (1991), Lateral velocity variations in the Andean foreland in Argentina determined with the JHD method, Bull. Seismol. Soc. Am., 81, of5

5 Ramos, V. A., E. O. Cristallini, and D. J. Pérez (2002), The Pampean flatslab of the central Andes, J. South Am. Earth Sci., 15, Sato, H., and M. C. Fehler (1998), Seismic Wave Propagation and Scattering in the Heterogeneous Earth, 308 pp., Springer, New York. Smalley, R., Jr., J. Pujol, M. Regnier, J.-M. Chiu, J.-L. Chatelain, B. L. Isacks, M. Araujo, and N. Puebla (1993), Basement seismicity beneath the Andean precordillera thin-skinned thrust belt and implications for crustal and lithospheric behavior, Tectonics, 12, Whitman, D., B. L. Isacks, J.-L. Chatelain, J.-M. Chiu, and A. Perez (1992), Attenuation of high-frequency seismic waves beneath the central Andean plateau, J. Geophys. Res., 97, 19,929 19,947. Zeng, Y., F. Su, and K. Aki (1991), Scattering wave energy propagation in a random isotropic scattering medium: 1. Theory, J. Geophys. Res., 96, M. Araujo, Instituto Nacional de Prevención Sísmica, Roger Balet 47, San Juan 5400, Argentina. G. Badi and N. Sabbione, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, La Plata 1900, Argentina. F. Bianco and E. Del Pezzo, Osservatorio Vesuviano, Sezione di Napoli, Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano, 328, I Napoli, Italy. (delpezzo@ov.ingv.it) J. M. Ibanez, Instituto Andaluz de Geofísica, Universidad de Granada, Campus Universitario de Cartuja s/n, E Granada, Spain. 5of5

Estimation of S-wave scattering coefficient in the mantle from envelope characteristics before and after the ScS arrival

GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 24, 2248, doi:10.1029/2003gl018413, 2003 Estimation of S-wave scattering coefficient in the mantle from envelope characteristics before and after the ScS arrival