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We perform a global survey for the presence of the Lehmann discontinuity using ∼20,000 long-period SS precursors. This data set is highly sensitive to upper mantle reflectors and the coverage is more complete than in previous studies. Our survey indicates that the Lehmann discontinuity is a local feature that is observed under continents more than twice as often as it is observed under oceans. We observe significant variations in travel times and waveforms associated with this shallow mantle reflector, indicating its complexity and lateral depth variations. Little signal is detected on the continent-scale stacks of the SS precursors, which provides further evidence for the intermittent and variable nature of the Lehmann discontinuity. |
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