HR: 0830h
AN: S41E-0135    [PDF]
TI: Structure of the African Superplume from Approximate and Numerical 3D Waveform Modeling
AU: * Ni, S
EM: stone@gps.caltech.edu
AF: seismological laboratory, caltech 252-21, pasadena, ca 91125 United States
AU: Helmberger, D
EM: helm@gps.caltech.edu
AF: seismological laboratory, caltech 252-21, pasadena, ca 91125 United States
AU: Tromp, J
EM: jtromp@gps.caltech.edu
AF: seismological laboratory, caltech 252-21, pasadena, ca 91125 United States
AB: We present a new method of generating analytical synthetics for tomographic-style models. These models are perturbations to a 1D layered model involving changes in block velocities producing 3D images. The procedure is broken into four steps: (1) construction of ray paths for the reference 1D model, (2) generation of perturbed paths, (3) construction of 2D synthetics in the plane containing the source and receiver, and (4) addition of out-of-place contributions from virtual receivers weighted by diffraction operators. In step one, the ray paths reflecting from the various interfaces $(i)$ are established with ray parameter ($p_i$) and travel time ($t_i$). Next, these values are corrected after adding the velocity perturbations where ray segments in faster blocks grow relative to slower blocks. This new set of ray parameters can be used to generate 2D WKM synthetics or Cagniard-deHoop synthetics. Contributions from virtual receivers at neighboring azimuths are added by convolving with diffraction operators which are defined by the source duration and travel time to the 3D structure. We suggest a particularly simple approximation based on 4 virtual receivers which produces synthetics in agreement with 3D Spectral Element Method (SEM) synthetics . We applied both methods to study 3D structure of the African Superplume, which is modeled as a large scale ridge-like velocity anomaly beneath Africa. The structure starts from the southern Indian Ocean and extends northwestern to the Atlantic ocean, covering a space of 7000~km long, 1000~km across and extending 1200~km upward from the core mantle boundary (CMB). The structure seems to have sharp boundaries with shear velocity inside 3% lower than the ambient mantle while P velocity is quite normal. The 3D structure can be best illustrated by seismic waveforms recorded by South Africa array from earthquakes ocurring in western Pacific Ocean. Great circle paths for the Sdiff phase from these events travel along the axis of this structure for over 60 degrees with the northern and southernmost paths missing the structure. The Sdiff waveforms are simple but delayed up to 20 seconds for paths sampling the middle of the structure. Sdiff waveforms display two arrivals for paths along both boundaries. This complexity of waveforms can be explained by 3D multi-pathing due to the rapid lateral variation of shear wave velocity. Modeling of SS travel times for some events requires that the structure must be high ($>$1200~km), combined with previous analysis of S, SKS and ScS waveforms around 85-95 degree, we exclude any thin model of 300~km or less with very slow basal layer (up to 10% lower S velocity) at the bottom of the superplume. This large scale anomalous structure with sharp boundaries can be interpreted as a thermal-chemical plume.
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 7260 Theory and modeling
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Seismology [S]
MN: 2003 Fall Meeting