IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS013 Oral Presentation 2222
High-Resolution 3D Anisotropic Structure of the North American Upper
Mantle from Inversion of Body and Surface Waveform Data
Prof. Barbara Romanowicz
Berkeley Seismological Laboratory University of California at Berkeley IASPEI
Federica Marone
Seismic anisotropy provides insight into paleo and recent deformation processes and therefore mantle
dynamics. Our knowledge of the upper mantle anisotropic structure under North America is based
mainly on global tomographic models or SKS splitting studies which lack horizontal and vertical
resolution respectively. In particular, the azimuthal anisotropy derived from local SKS splitting
measurements and that predicted from surface wave inversions shows a well documented discrepancy
especially under continents. We present here the first 3D regional tomographic model of the North
American upper mantle, which simultaneously includes both radial and azimuthal anisotropy and
reconciles this discrepancy. The novelty of our approach consists in the joint inversion of fundamental
and higher mode surface waveforms together with constraints on azimuthal anisotropy derived from
SKS splitting measurements, producing a 3D anisotropic model with enhanced depth resolution down to
the transition zone. In a first step, we inverted long period waveform data simultaneously for
perturbations in the isotropic S-velocity structure, the anisotropic parameter x= (Vsh/Vsv)2 and the
depth to the Moho, in the framework of normal mode asymptotic theory (NACT) (Li and Romanowicz,
1995), correcting for 3D structure outside of the region of study using a global radially anisotropic
model of the upper mantle, and correcting for crustal structure using a non-linear approach adapted to
the large lateral variations of Moho depth. The resulting 2D broad-band sensitivity kernels allow us to
exploit the information contained in long-period seismograms for body, fundamental and higher-mode
surface waves at the same time. We have adapted the NACT algorithm for the regional case by
implementing a lateral parametrization in terms of spherical splines on an inhomogeneous triangular
grid of nodes, with the finest mesh for North America. The inverted dataset consists of more than
100,000 high quality 3-component body, fundamental and overtone surface waveforms, recorded at
broad-band seismic stations in North America from teleseismic events. Notable features of the radially
anisotropic part of the model are a positive ξ anomaly down to 300 km under cratons, indicating the
presence of horizontal shear in the asthenosphere and a negative ξ anomaly beneath the Basin and
Range province, which suggests the presence of mantle upwelling. Our 3D azimuthal anisotropic model
indicates the presence of two layers of anisotropy with distinct fast axis directions under the stable part
of the North American continent: a deeper layer with the fast axis direction aligned with the absolute
plate motion direction suggesting lattice preferred orientation of anisotropic minerals in a present day
asthenospheric flow and a shallower lithospheric layer likely showing records of past tectonic events.
Under the tectonically active western US, where the lithosphere is thin, the direction of tomographically
inferred anisotropy is stable with depth and compatible with the absolute plate motion direction.
Synthetic SKS splitting parameters computed for this new 3D azimuthal anisotropic model are in good
agreement with observations throughout North America. We believe that previous surface wave based
models had rather low resolution power at depths greater than 200 km, and strongly underestimated
the strength of the anisotropy at large depths beneath cratons. Still, the distribution of data, particularly
SKS splitting data, across the US is very uneven, with large holes in the central and eastern US.
Keywords: lithosphere, asthenosphere, anisotropy