ISTerre, Université de Grenoble I, CNRS, BP 53, F Grenoble Cedex 9, France

Transcription

1 Body wave imaging of the Earth s mantle discontinuities from ambient seismic noise Authors: P. Poli 1, M. Campillo 1, H. Pedersen 1, Lapnet Working Group 1 ISTerre, Université de Grenoble I, CNRS, BP 53, F Grenoble Cedex 9, France Ambient seismic noise correlations are now widely used for high-resolution surface wave imaging of the Earth s lithosphere. Similar observations of the seismic body waves that propagate through the interior of the Earth would provide a window into the deep Earth. We report the observation of the mantle transition zone through noise correlations of P-waves as they are reflected by the discontinuities associated with the top (410 km) and the bottom (660 km) of this zone. Our data demonstrate that high-resolution mapping of the mantle transition zone is possible without using earthquake sources. One Sentence Summary: We extract seismic body waves reflected at deep mantle interfaces from seismic noise, and show for the first time that they can be used to finely characterize the upper and lower limits of the mantle transition zone (410km and 660 km interfaces). Main Text: The Earth s upper and lower mantle are separated by the transition zone, where the mantle mineralogy changes. At the top (~410km depth) and bottom (~660km depth) of the transition zone, phase changes introduce a rapid increase in seismic velocities over narrow depth intervals. This transition zone has a major role in Earth dynamics, particularly as it influences the convection within the mantle, slowing the subduction of slabs and the ascent of plumes (1,2). Information from rock physics and the seismic character of the 410 km and 660 km discontinuities can constrain the mineralogy and temperatures of the mantle (3). However, mapping the depths and lateral variations of these discontinuities (4,5) remains difficult, as seismic studies based on the analysis of waves emitted by earthquakes are limited by the geographical distribution of the earthquakes and by the uncertainties in our knowledge of the location and the rupture processes. Following promising results from correlations of the coda of seismic waves (6), it was recently proposed that correlations of the continuous records of seismic ambient noise recorded at two distant points can provide an estimation of the Earth s impulse response between these two points (7). This impulse response contains information on seismic wave speeds (7) and amplitude decays (8) without the need for the use of active sources or earthquakes. As these seismic noise sources are located at Earth s surface, the noise correlations are dominated by surface waves, and the technique has become useful for seismic imaging at different scales (9-13). Seismic noise propagates continuously through the Earth, and is mainly created by oceanic swells and atmospheric disturbances (14-19). Surface waves, however, are not sufficient to explore the deep structure of the Earth, as they have limited depth resolution. Recent studies have detected high-frequency body waves within noise correlations at both the crustal scale (20-22) and at a very local scale (23). Here, we describe the extraction of body-wave reflections from the 410-km and 660-km discontinuities of the Earth s transition zone using seismic-noise correlations. It is now known that seismic noise includes body waves that propagate through the whole planet (19,24,25), just as surface waves propagate through the upper layers of the Earth (Fig. 1A). As for the surface wave component of the wave field, we do not expect the noise field to be under the exact mathematical conditions for retrieval of the complete Earth impulse responses (26,27). We show in the following that conditions are nevertheless favorable enough to extract deep body wave phases by correlating ambient seismic noise. As the signals we track have small amplitudes, we have applied specific processing techniques that are designed to improve the signal-to-noise ratio. We used data from the temporary POLENET/ LAPNET experiments in northern Finland (28) (Fig. 1B), complemented with data from permanent broadband stations. We previously extracted P-waves and S-waves (22) that are reflected on the Moho (the lower limit of the Earth s crust). These data showed that the ancient crust in the study area (29-30) is relatively transparent to seismic waves. Here, we used data from 42 stations that were continually operating from January to December For each of the 861 station pairs (Fig. S1), we calculated the noise correlation of the vertical records in the frequency range of 0.1 Hz to 0.5 Hz using the same processing as that implemented to extract Mohoreflected waves (22). We subsequently used the station pairs for which potential body waves reflected at the 410 km and 660 km interfaces did not arrive within the surface-wave train. For each of these traces, we folded the positive and negative time lags of the correlations and zeroed out the time window corresponding to the surface-wave arrivals. To observe small amplitude waves (Fig. S2), we used stacking techniques that allow us to enhance the coherent body waves. Prior to the stack, we aligned the traces along the predicted arrival times of the 410-km P-wave reflection using the AK135 standard Earth model (31) that was adapted to take into account the local crustal structure. Once aligned, the whole dataset was stacked in the slowness-time domain. The results of the slowness-time stack (Fig. 1C) show two peaks of energy at arrival times of approximately 100 and 150 s. Both peaks are located at the zero slowness, which means that no significant velocity correction is needed in addition to the initial alignment using the AK135 model, and that these are vertically propagating waves. We tentatively interpreted these two peaks as P-waves reflected by the 410-km discontinuity (P410P) and the 660-km discontinuity (P660P). To support this interpretation, we calculated the synthetic seismograms for each station pair and applied the same processing as that used for the field data, to compare the two types of data. Prior to the stacking, we normalized the

2 spectral amplitude of the synthetic seismograms and multiplied it by the average spectrum of the noise correlations. The comparison of the synthetics and the data stack confirms our interpretation that the observed peaks are the P410P and P660P reflections, despite slightly different waveforms (Fig. 1C). The agreement of data and synthetic stacks is a good indication of the quality of the retrieved Green s function. We attribute the minor differences between field data and synthetics to structural differences between the global reference model (31) and the Earth s mantle beneath the study area. To test this hypothesis, we calculated the synthetic seismogram stacks for a series of models (Table S1), and we qualitatively evaluated the fit between the stack of synthetic seismograms and the correlations (Fig. S3). With the AK135 model as reference, we modified the depths of the two discontinuities and used gradients over narrow depth intervals, rather than first-order discontinuities. The stack of synthetics associated with the best model (Fig. 2A) is in good agreement with the data: the overall fit of the observations was drastically improved compared to that obtained using the AK135 model. With this refined model, the arrival times and the relative P410P and P660P amplitudes are similar. The final model (Fig. 2B) of our study shows a 410-km discontinuity that is 15 km thick and ranges from 405 to 420 km in depth. The 660-km discontinuity is 4 km thick, at depths of 650 to 654 km. The depths of these two discontinuities are within the variations observed at a global scale (4,5) and in good agreement with a receiver function study in the same area (32). Our additional constraints on the fine structure of the discontinuities corresponded to those predicted by the thermodynamic modeling of the phase transitions (33,34) and to constraints provided by seismological studies (35,36). We have shown that it is possible to identify and characterize deep body waves that propagate through the Earth. Our study used a dense seismic network that is located above a relatively transparent Earth crust. Using seismic noise to image mantle discontinuities has several advantages. First, the correlation technique is independent of earthquake occurrence, and therefore independent of the uncertainties that are associated with source location, origin time, and detailed slip history. Secondly, the amount of noise correlation scales according to N 2, where N is the number of stations, so it is relatively easy to obtain a large amount of data. Finally, the body waves that we have extracted are relatively high frequency ( Hz) and they are sufficiently broadband to finely resolve the structure of the discontinuities. Reference and notes: 1. D. Zhao, Global tomographic image of mantle plumes and subducting slabs: insight into deep Earth dynamics. Phys. Earth Planet. Inter. 146, 3-34 (2004). 2. C. Li, R. D. van der Hilst, E. R. Ehgdahl, S. Burdick, A new global model for P-wave speed variation in Earth s mantle. Geochem. Geophys. Geosyst. 9, Q05018 (2008), doi: /2007gc Q. Cao, R. D. Van der Hilst, M. V. De Hoop, S. H. Shim, Seismic imaging of transition zone discontinuities suggests hot mantle west of Hawaii. Science 332, (2011). 4. Y. J. Gu, A. M. Dziewonski, C. B. Agee, Simultaneous inversion for mantle shear velocity and topography of transition zone discontinuities. Earth Planet. Sci. Lett. 157, (2003). 5. P. Shearer, G. Masters, Global mapping of topography on the 660-km discontinuity. Nature 355, (1992). 6. M. Campillo, A. Paul, Long-range correlations in the diffuse seismic coda. Science 299, (2003). 7. N. M. Shapiro, M. Campillo, Emergence of broadband Rayleigh waves from correlations of the ambient seismic noise. Geophys. Res. Lett. 31, L07614 (2004), doi: /2004gl G. A. Prieto, M. Denolle, J. F. Lawrence, G. C. Beroza, On the amplitude information carried by ambient seismic field. Compte Rend. Geosci. 3, (2011). 9. N. M, Shapiro, M. Campillo, L. Stehly, M. H. Ritzwoller, High-resolution surface-wave tomography from ambient seismic noise. Science 307, (2005). 10. K. G. Sabra, P. Gerstoft, P. Roux, W. A. Kuperman, M. C. Fehler, Surface-wave tomography from microseisms in Southern California. Geophys. Res. Lett. 32, L14311 (2005) doi: /2005gl H. Yao, R. D. van der Hilst, Analysis of ambient noise energy distribution and phase velocity bias in ambient seismic noise tomography, with application to SE Tibet. Geophys. J. Intl. 179, (2009). 12. L. Stehly, B. Fry, M. Campillo, N. Shapiro, J. Guilbert, L. Boschi, D. Giardini, Tomography of the Alpine region from observations of seismic ambient noise. Geophys. J. Intl. 178, (2009). 13. F. C. Lin, M. H. Ritzwoller, R. Snieder, Eikonal tomography: surface-wave tomography by phase-front tracking across a regional broad-band seismic array. Geophys. J. Intl. 177, (2009). 14. M. S. Longuet-Higgins, A theory of the origin of microseisms; Philos. Trans. R. Soc. London, Ser. A, 243, 1-35 (1950). 15. A. F. Friedrich Krüger, K. Klinge, Ocean-generated microseismic noise located with the Gräfenberg array. J. Seismol. 2, (1998). 16. T. Tanimoto, Excitation of normal modes by atmospheric turbulence: source of long-period seismic noise. Geophys. J. Intl. 136, (1999). 17. G. Ekström, Time domain analysis of Earth's long-period background seismic radiation. J. Geophys. Res. 106, (2001). 18. J. Rhie, B. Romanowicz, Excitation of Earth's continuous free oscillations by atmosphere ocean seafloor coupling.

3 Nature 431, 552 (2004). 19. G. Hillers, N. Graham, M. Campillo, S. Kedar, M. Landès, N. Shapiro, Global oceanic microseism sources as seen by seismic arrays and predicted by wave action models. Geochem. Geophys. Geosyst. 13, Q01021 (2012) doi: /2011gc Z. Zhan, S. Ni, D. V. Helmberger, R. W. Clayton, Retrieval of Moho-reflected shear-wave arrivals from ambient seismic noise. Geophys. J. Intl. 1, (2010). 21. E., Ruigrok, X. Campman, K. Wapenaar, Extraction of P-wave reflections from microseisms, C. R. Geoscience, 343, (2011). 22. P. Poli, H. A. Pedersen, M. Campillo, POLENET LAPNET working group. Emergence of body waves from crosscorrelation of seismic noise. Geophys. J. Intl. 188, (2012). 23. P. Roux, P-waves from cross-correlation of seismic noise. Geophys. Res. Lett. 32, L19303 (2005) doi: /2005gl P. Gerstoft, P. M. Shearer, N. Harmon, J. Zhang, Global P, PP, and PKP wave microseisms observed from distant storms. Geophys. Res. Lett. 35, L23306 (2008) doi: /2008gl M. Landès, F. Hubans, N. M. Shapiro, A. Paul, M. Campillo, Origin of deep ocean microseisms by using teleseismic body waves. J. Geophys. Res. 115, B05302 (2010) doi: /2009jb P. Gouédard, L. Stehly, F. Brenguier, M. Campillo, Y. C. de Verdiére, E. Larose, L. Margerin, P Roux, F. J. Sànchez- Sesma, N. Shapiro, R. L. Weaver, Cross-correlation of random fields: mathematical approach and applications, Geophys. Prospect., 56, (2008). 27. Tutorial on seismic interferometry: Part 2 Underlying theory and new advances, K. Wapenaar, E. Slob, R. Snieder, A. Curtis, Geophysics, 75, 75A211-75A227, doi: / (2010). 28. E. Kozlovskaya, M. Poutanen, POLENET/ LAPNET Working Group, POLENET/LAPNET- a multidisciplinary geophysical experiment in northern Fennoscandia during IPY Eos Trans. AGU 87, Fall Meet. Suppl., Abstract S41A-1311 (2006). 29. T. Janik, E. Kozlovskaya, J. Yliniemi, Crust-mantle boundary in the central Fennoscandian shield: constraints from wide-angle P- and S-wave velocity models and new results of reflection profiling in Finland. J. Geophys. Res., 112, B04302 (2009) doi: /2006jb P. Poli et al., Geophys. J. Int. in press (available at B. L. N. Kennett, E. R. Engdahl, R. Buland, Constraints on seismic velocities in the Earth from travel times. Geophys. J. Intl. 122, (1995). 32. A. Alinaghi, G. Bock, R. Kind, W/ Hanka, K. Wylegalla, TOR, SVEKALAPKP, Working Groups. Receiver function analysis of the crust and upper mantle from the North German Basin to the Archean Baltic Shield. Geophys. J. Intl. 155, (2012). 33. E. Onthani, T. Sakai, Recent advances in the study of mantle phase transitions. Phys. Earth Planet. Intl. 170, (2008). 34. L. Stixrude, C. Lithgow-Bertelloni, Thermodynamics of mantle minerals II. Phase equilibria. Geophys. J. Intl. 184, (2011). 35. T. Melbourne, D. Helmberger, Fine structure of the 410-km discontinuity. J. Geophys. Res. 103, (1998). 36. J. Lawrence, P. Shearer, Constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveform. Geochem. Geophys. Geosyst. 7, 1-19 (2006). 37. M. Bouchon. A simple method to calculate green s functions for elastic layered media. Bull. Seismol. Soc. of Am., 71, (1981). Acknowledgments: We thank the QUEST Initial Training network funded within the EU Marie Curie Programme for support, as well as from the ANR BegDy project, the Institut Paul Emil Victor, and European Research Council through the advanced grant Whisper Continuous data from the POLENET-LAPNET seismic experiment are available at the RESIF data center ( under network code XK. The POLENET/LAPNET project is a part of the International Polar Year and a part of the POLENET consortium, and is supported by the Academy of Finland (grant No ) and University of Oulu, International Lithosphere Programme task force VIII, grant No. IAA of the Grant Agency of the Czech Academy of Sciences, and the Russian Federation: Russian Academy of Sciences (Programmes No 5 and No 9). The Equipment for the temporary deployment was provided by: RESIF SISMOB (France), EOST-IPG Strasbourg (France), Seismic pool (MOBNET) of the Geophysical Institute of the Czech Academy of Sciences (Czech Republic), Sodankyla Geophysical Observatory (Finland), Institute of Geosphere Dynamics of RAS (Russia), Institute of Geophysics ETH Zürich (Switzerland), Institute of Geodesy and Geophysics, Vienna University of Technology (Austria), University of Leeds (UK). POLENET/ LAPNET data were prepared and distributed by the RESIF Data Centre. The POLENET/ LAPNET working group consists of: E. Kozlovskaya, T. Jämsen, H. Silvennoinen, R.

4 Hurskainen, H. Pedersen, C. Pequegnat, U. Achauer, J. Plomerova, E. Kissling, I. Sanina, R. Bodvarsson, I. Aleshin, E. Bourova, E. Brückl, T. Eken, R. Guiguet, H. Hausmann, P. Heikkinen, G. Houseman, P. Jedlicka, H. Johnsen, H. Kremenetskaya, K. Komminaho, Helena M., Roland Roberts, B. Ruzek, H. Shomali, J. Schweitzer, A. Shaumyan, L. Vecsey, S. Volosov. A P P P K P P S t a t i o n 1 S t a t i o n 2 U p p e r M a n t l e T r a n s i t i o n z o n e L o w e r M a n t l e 72 B 68 C D a t a 0 s l o w [ s / k m ] M o d e l t i m e [ s ] Figure 1: Reflected body waves from the mantle transition-zone discontinues beneath Finland. (A) The Earth model in which noise is generated from oceanic sources (stars) and propagates partly as body waves (dashed blue lines) to the seismic sensors (pentagons). Correlation of the seismic noise recorded at the two sensors in theory yields the complete set of waves that would be recorded at one sensor if a seismic source had been active at the other sensor. The inset (right) shows the two body wave reflections that this report focused on: P-waves reflected on the 410-km and 660-km discontinuities (red dashed lines). (B) Map showing the stations of the seismic array (red triangles) in northern Finland. (C) Time slowness stacks of the noise correlations (top) and the synthetic seismograms calculated using the AK135 standard Earth model (bottom). Note the significant wave arrivals at the same times as predicted by the AK135 model.

5 Figure 2: Data fit and final model obtained from forward modeling. (A) Comparison of the zero slowness trace extracted from the time-slowness stack of the noise correlations (red) and two zero slowness traces of the synthetic seismogram stacks (as processed for the noise correlations): the AK135 standard Earth model (blue, left) and our final model (blue, right). The predicted vertical travel times for the P410P and P660P reflections in the AK135 model are shown (dashed lines). The stack traces are normalized using the peak amplitude. (B) Final model (red line) for the mantle transition zone beneath northern Finland as compared to the AK135 model (dashed blue line). Insets: detailed structure of the two discontinuities.