Investigating the Seismic Structure of the Atlantic Upper Mantle with Surface Waves

The seismic properties attributed to the oceanic upper mantle are strongly influenced by the measured seismic properties of the Pacific upper mantle, which has been the focus of several regional studies. Less is known about the seismic structure of the mantle beneath the Atlantic, and differences in the spreading rates and plate velocities of the Atlantic and Pacific oceans suggest that different dynamical processes are occurring beneath the two basins. This research seeks to address three key issues: (1) What controls the seismic structure as a function of depth and seafloor age? Can these properties be attributed solely to temperature, or must composition and melt also be considered? (2) What controls along-axis and off-axis seismic anomalies? Do they reflect a variable mantle source, and what roles do temperature and composition play? (3) Is the mantle fabric associated with slower plate velocities weaker and more variable than that produced by faster spreading rates? Each of these questions invites a comparison with the faster-spreading Pacific upper mantle, and together they present an opportunity to investigate possible spreading-rate dependence of seismic structure as well as the mechanisms that might produce it. We are investigating 3-D variations in seismic velocity, attenuation, and anisotropy within the Atlantic upper mantle. We are collecting waveforms from earthquakes with magnitude > 6.0 that occurred within or on the margins of the Atlantic basin during the period 1993 to present. To avoid contamination of mostly-oceanic paths by continental lithosphere, we consider only seismic stations located within or on the margins of the basin. The phase delay and amplitude of fundamental- and higher-mode Rayleigh and Love waves is measured using the generalized seismological data functional approach of Gee and Jordan (1992). The phase-delay observations are inverted for frequency-dependent phase-velocity maps, which show how velocity anomalies vary as a function of seafloor age and proximity to hotspots.