抄録
We conducted broadband ocean bottom seismic observations on the French Polynesian seafloor from 2003 to 2005 to image the mantle structure beneath the South Pacific superswell. The superswell region has many hot spots and low seismic velocity anomalies in the lower mantle and may be a site of major mantle plumes. To obtain a lower mantle image, we measured approximately 1500 relative times of long-period P-waves by using a waveform cross-correlation from the broadband seismic waveforms data at the ocean floor and islands in the South Pacific superswell region. We also collected 600 first arrival times by manual picking from the same waveform data. And then we inverted them with the first arrival times in the Bulletin of International Seismological Center to a tree-dimensional P-wave velocity structure of the whole mantle.To obtain an image of the shallower mantle, we measured the dispersions of fundamental mode Rayleigh waves at periods between 40 and 140 seconds by the two-station method. We determined three-dimensional shear wave speed structure beneath the South Pacific superswell down to a depth of 200 km from the dispersion data.The obtained seismic image indicates: (1) narrow mantle plumes in the upper mantle beneath most of the hot spots in the superswell region; (2) narrow plumes exist only beneath some of the hot spots (the Society and possibly the Pitcairn) in the mantle transition zone and the top of the lower mantle; (3) broad upwelling in the mid-lower mantle beneath the superswell region. We compared the seismic image with analog experiments of plumes to discuss nature and evolution of mantle plumes beneath the South Pacific.
