Mapping Chemical Plumes from Bottom to Top

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Helmberger EAR-0229885 Considerable progress has been made in mapping out lateral variation in the Earth, demonstrating the active dynamic processes going on beneath us. Advances in seismic tomography in terms of travel time and waveform methodology has allowed major structures in the mantle to be determined. Relatively fast velocities ring the Pacific derived from Global long-period studies and further resolution into sharper sheet-like features with shorter wavelength bodywaves and higher-mode efforts. The velocities beneath the slowest lower mantle structures, mid-Pacific and South Africa can be modeled by a D" (roughly 300 km thick) with about 3% drop in shear velocity in basic agreement with some recent tomography studies. Beneath southern Africa, a large structure about 1200 km across extends upward about 1500 kms with about the same S-velocity reduction of 3%. The boundaries of this anomaly appear sharp enough to produce multipathing in SKS. Recently released SKS data from the South African Array reveals jumps in SKS travel time of up to 8 secs, which can be contoured, delimiting the sharpness of this huge structure as it turns from a northwest direction to an east west direction beneath the Indian Ocean. To make accelerated progress in developing 3D images of this structure and a possible sister structure beneath the Pacific, the investigators plan a concentrated effort to construct whole mantle 2D cross sections by modeling waveform data along selected corridors. Using their new waveform modeling tools in combination with the ever-expanding IRIS networks of PASSCAL arrays and fixed stations will allow them to address the following questions: How sharp are the sides of these huge structures and what is their 3D geometry; how uniform is the roof structure and does the Rayleigh number decrease drastically to form fast moving smaller plumes interacting with the 660 transition zone and can this be seen in distortions of the 670 km triplications; what is the fine-scale structure beneath the Pacific (SV and SH) and how is it related to the mid-mantle structure? The investigators purpose to address these issues with a broadband data gathering exercise followed by detailed modeling, including all body wave phases sampling selected corridors.