Seismic waveform inversion at the regional scale: application to southeastAsia

1. Seismic waveform inversion at the regional scale: application to southeastAsia Barbara Romanowicz1, Aimin Cao2, M.Panning3, F. Marone4 ,Yann Capdeville5, Laurent Stehly1 and Paul Cupillard1 1Univ. of California, Berkeley 2Rice Univ. 3Princeton U. 4P. Scherrer Institute,Switz.erland 5 Institut de Physique du Globe, Paris, France

2. I- Background:In the context of global S velocity, long period tomography

3. Standard tomographic ingredients: Parametric data • Body wave travel times • well separated phases • Ray theory or, more recently finite frequency (“Banana-doughnut”) kernels • Surface waves • Group/phase velocities • Path average approximation (PAVA)

5. Waveform Tomography observed synthetic • Need framwork for computation of 3D synthetics • (2) Framework needs to be appropriate for body waves

6. Waveform Inversion Methodology: • Non-linear Asymptotic Coupling Theory (NACT); 3 component waveforms • extension to anisotropic inversion • iterative inversion for elastic and anelastic structure SS Sdiff PAVA NACT

7. Elastic structure- SAW24B16 (SH) Anelastic structure QRLW8 Mégnin and Romanowicz, 2000 Gung and Romanowicz, 2004

8. II. Beyond PAVA and NACT (in the context of normal mode summation)

9. M S R M S R Born approximation: Single scattering Integration over the Whole sphere PAVA approximation (1D in the Vertical plane) NACT (2D in the vertical plane) Both include multiple forward scattering

10. “N-BORN“ • Add PAVA term (multiple forward scattering) into BORN. • Application to South East Asia • Comparison of NACT and N-Born inversion

11. Spherical spline parametrization StartingNACT radially aniso- tropic model Level 4 Splines l~800km Level 6 Splines l~200km Red: NACT region Blue: NBORN region

12. We include both fundamental mode and overtone waveforms By including overtones, we improve depth resolution into the transition zone after Ritsema et al, 2004

13. Isotropic S velocity NACT NBORN 80km 150km 250 km

15. NACT A B A B Friederich, 2003 NBORN A B

16. NACT NBORN Vs Radial anisotropy: x

17. III. Beyond N-Born:Towards numerical methods

18. Waveform Tomography observed synthetic • Normal mode perturbation theory (generally to 1st order) • (2) Numerical methods (e.g. SEM)

19. “Hybrid” Approach u(m) ∂u(m)/ ∂m Use coupled spectral element method of Capdeville et al. (2002) to accurately forward model wave propagation through a 3D medium. Use NACT, with the hope that the derivatives are the correct sign. Much faster than cSEM! NACT G1= Normal modes in 1D G2 = Spectral element method Li and Romanowicz, 1995 Capdeville et al., 2002

20. Preliminary “Level 4” radially anisotropic upper mantle model Obtained using SEM, starting from 1D model – courtesy of Ved Lekic

21. IV-exploratory: summed event inversion • SEM is accurate but very time consuming: the wavefield computation for a single event can take a couple of hours (depending on the computer and the maximum frequency, and distance) not very practical for tomography • Can we speed up the computation by computing SEM synthetics for many events simultaneously (e.g. Capdeville et al., 2005, GJI)?

22. Summed seismogram inversion • Start with N-Born model • Restrict study to smaller region • Collect a dataset of waveforms in the distance range 4 to 40 degrees (~100 events) • period range 60-400 sec • Data: summed waveforms for all events at one or a subset of stations • Synthetics: RegSEM for summed events in the N-Born model

24. Summed seismograms at station XAN XAN Time (seconds) Black: observed trace (filtered between 60 and 250 s) Red: RegSEM synthetic in the 3D N-Born starting model

25. NBorn starting model Inversion usingRegSEM – Individual seismograms

26. “Summed seismogram” inversion “Individual seismogram” inversion

27. Current developments: • larger dataset, • Extended tests • -progressively reach shorter periods and higher resolution

28. Thank You!

30. Seismic tomography - Linearized inverse problem: dd = A dm Step 1: forward mi+1 = mi + dm dd = u(t)obs- u(t)pred A: Fréchet Derivatives Wave propagation theory

31. Seismic tomography - Linearized inverse problem: Step 2: inverse dd = A dm Step 1: forward mi+1 = mi + dm dd = u(t)obs- u(t)pred A: Fréchet Derivatives Wave propagation theory

32. P-wavespeed S-wavespeed Surface wave tomography (Lebedev et al., 2006) PRI-P05 (Montelli et al.) MIT-P06 (Li et al.) Courtesy of Rob van der Hilst

34. NBORN model predictions: NBORN/NACT Residual variance Damping Factor

35. Marone and Romanowicz, 2007

36. Radial anisotropy Gung et al., Nature, 2003

37. AB CD Hawaii Pacific Superplume

38. Ritzwoller and Shapiro, 2002 Kustowski et al. 2007 NACT NBORN

39. Level 6 Spherical splines (preliminary) Version cargese

40. Panning et al., 2008

41. Panning et al., 2007

42. N-Born inversion • 152 events • One iteration only • Starting model: 3D “NACT” model • Forward model: N-BORN • Partial derivatives: BORN (Capdeville,2006) • Elastic and radially aniosotropic structure

43. Upper mantle:Q - lower mantle: Vsh Degree 2 only