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
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