title: Seismic evidence for an inner core transition zone
abstract: Seismic waves that traverse Earth's inner core along north-south paths produce unusually broad pulse shapes at long periods (compared with waves along east-west paths) and reflections from below the inner core boundary at short periods. The observations provide compelling evidence for a seismic velocity discontinuity along north-south paths about 200 kilometers below the inner core boundary separating an isotropic upper inner core from an anisotropic lower inner core. The triplication associated with such a structure might be responsible for reported waveform complexity of short-period inner core arrivals along north-south paths and, if the depth of the boundary is laterally variable, their large travel-time variation.
authors.affiliation: X. D. Song, Lamont-Doherty Earth Observatory, Palisades, NY 10964, and Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA. D. V. Helmberger, Seismological Laboratory, California Institute of Technology.
authors.initials: X
authors.lastname: Song
authors.initials: DV
authors.lastname: Helmberger