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Seismic waves and 3D structure. Barbara Romanowicz UC Berkeley CIDER’04/KITP 07/21/04. Body waves : ray theory Surface waves : path average approximation (PAVA) “Infinite frequency approximations”. Vasco and Johnson, 1998. Van der Hilst et al., 1998. Montelli et al., 2004.
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Seismic waves and 3D structure Barbara Romanowicz UC Berkeley CIDER’04/KITP 07/21/04
Body waves: ray theory Surface waves: path average approximation (PAVA) “Infinite frequency approximations”
Vasco and Johnson, 1998
Surface waves P S
observed Synthetic (PREM)
“Fresnel zone” l = wavelength Df < p 2(r-z)<l/2
How can we model full waveforms, i.e. body waves (including diffracted waves), AND surface waves (fundamental mode and overtones), in a realistic 3D earth • with “accurate” broadband kernels • and reasonable computation time for global tomographic inversions ?
Path AVerage Approximation • 1D kernels • Along branch coupling • zeroth order asymptotics • NACT • 2D kernels (in vertical plane) • across branch coupling • zeroth order asymptotics • NACT + focusing • 2.5D kernels (off plane effects included) • order 1/l asymptotics • Full 1st order Born Approximation • 3D kernels • single scattering
Synthetic seismogram comparison • Reference : “Exact numerical ”: • Spectral Element Method (SEM) • Compare differences between synthetics computed in PREM (1D) and synthetics computed in simple 3D models
SS Sdiff PAVA NACT Li and Romanowicz , 1995
PAVA NACT
Top trace: observed (80-300sec); Bottom trace: PREM predictions
Motivation for seismic Q tomography: Faul and Jackson, in prep.
Long period waveforms Upper Mantle Q models Depth = 160 km Rayleigh waves Spectral ratios, P,PP
Hotspot distribution QRLW8 Weighted by buoyancy flux
Depth = 450 km Depth = 600 km Degree 8 no focusing Degree 12 with foc. Degree 16 with foc. -50 0 dln(1/Q)(%) 50
QRLW8 QRLf12 SAW24B16 Pacific “superplume”
Sharp boundary of the African Superplume Ni et al., 2002
Toh et al. 2004
CSEM Synthetics Toh et al. 2004