Seismic waves that traverse Earth's inner core along north-south paths
produce unusually broad pulse shapes at long periods (compared with w
aves along east-west paths) and reflections from below;the inner core
boundary at short periods. The observations provide compelling evidenc
e for a seismic velocity discontinuity along north-south paths about 2
00 kilometers below the inner core boundary separating an isotropic up
per inner core from an anisotropic Lower inner core. The triplication
associated with such a structure might be responsible for reported wav
eform 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.