Seismic observations of splitting of the mid-transition zone discontinuityin Earth's mantle

The transition zone of Earth's mantle is delineated by globally observed di scontinuities in seismic properties at depths of about 410 and 660 kilomete rs. Here, we investigate the detailed structure between 410 and 660 kilomet ers depth, by making use of regional stacks of precursors to the SS phase. The previously observed discontinuity at about 520 kilometers depth is conf irmed in many regions, but is found to be absent in others. There are a num ber of regions in which we find two discontinuities at about 500 and 560 ki lometers depth, an effect which can be interpreted as a "splitting" of the 520 kilometer discontinuity. These observations provide seismic constraints on the sharpness and observability of mineralogical phase transitions in t he mantle transition zone.