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, Berkeley2Rice Univ.

3Princeton U.4P. Scherrer Institute,Switz.erland

5 Institut de Physique du Globe, Paris, France

I- Background:

In the context of global S velocity, long period

tomography

• 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)

Standard tomographic ingredients:Parametric data

observed

synthetic

Waveform Tomography

(1)Need framwork for computation of 3D synthetics(2) Framework needs to be appropriate for body waves

– Non-linear Asymptotic Coupling Theory (NACT); 3 component waveforms

– extension to anisotropic inversion – iterative inversion for elastic and

anelastic structure

Waveform Inversion Methodology:

NACT

PAVASS Sdiff

Elastic structure- SAW24B16 (SH)

Mégnin and Romanowicz, 2000

Anelastic structureQRLW8

Gung and Romanowicz, 2004

II. Beyond PAVA and NACT

(in the context of normal mode summation)

S R

M

S R

M

PAVA approximation (1D in theVertical plane)

NACT (2D in the vertical plane)

Both include multiple forwardscattering

Born approximation:

Single scatteringIntegration over theWhole sphere

• “N-BORN“– Add PAVA term (multiple forward

scattering) into BORN.

• Application to South East Asia– Comparison of NACT and N-Born

inversion

Red: NACT regionBlue: NBORN region

Level 6Splines~200km

Spherical splineparametrization

Level 4Splines~800km

Starting NACTradially aniso-tropic model

We include both fundamental mode and overtone waveforms

By including overtones, we improve depth resolution into the transition zone

after Ritsema et al, 2004

NACT

NBORN

80km 150km 250 km

Isotropic S velocity

Friederich, 2003

A

A

B

B

NACT

NBORN

BA

NACT NBORN

Radial anisotropy:

Vs

III. Beyond N-Born:Towards numerical methods

observed

synthetic

Waveform Tomography

(1)Normal mode perturbation theory (generally to 1st order)(2) Numerical methods (e.g. SEM)

“Hybrid” ApproachUse 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!

1= Normal modes in 1D2 = Spectral element methodCapdeville et al., 2002

NACT

Li and Romanowicz,1995

u(m) ∂u(m)/ ∂m

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

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)?

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

Summed seismograms at station XAN

Black: observed trace (filtered between 60 and 250 s)Red: RegSEM synthetic in the 3D N-Born starting model

Time (seconds)

XAN

NBorn starting model Inversion usingRegSEM –Individual seismograms

“Summed seismogram” inversion“Individual seismogram” inversion

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

Thank You!

d = A m

- Linearized inverse problem:

Seismic tomography

mi+1 = mi + md = u(t)obs- u(t)pred A: FréchetDerivatives

Wave propagationtheory

Step 1: forward

d = A m

- Linearized inverse problem:

Seismic tomography

mi+1 = mi + md = u(t)obs- u(t)pred A: FréchetDerivatives

Wave propagationtheory

Step 2: inverse

Step 1: forward

PRI-P05 (Montelli et al.)Surface wave tomography

(Lebedev et al., 2006)

P-wavespeed

S-wavespeed

MIT-P06 (Li et al.)

Courtesy of Rob van der Hilst

Damping Factor

Resid

ual vari

an

ce

NBORN model predictions: NBORN/NACT

Marone and Romanowicz,2007

Radial anisotropy

Gung et al., Nature, 2003

AB CD

Pacific Superplume

Hawaii

Kustowski et al.2007

Ritzwoller and Shapiro, 2002

NACT NBORN

(preliminary)Level 6Sphericalsplines

Versioncargese

Panning et al., 2008

Panning et al., 2007

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

Upper mantle:Q - lower mantle: Vsh

Degree 2 only