Fine structure of the 410‐km discontinuity

The April 14, 1995, earthquake in western Texas ( M w 5.7) produced a strong topside reflection off the 410‐km discontinuity which was recorded on a multitude of seismic arrays throughout the southwestern United States. Data from 394 vertical short‐period and 24 broadband instruments provide dense coverage of this event from distances of 11° to 19° and provide a detailed look at the subcontinental 410‐km structure. The salient features of this data set are (1) the strong dependence on wavelength of the 410‐km triplication range, (2) the uniform amplitude ratio of the direct P and reflected P 410 phases on both short‐period and broadband recordings throughout the triplication, and (3) the abrupt termination of the short‐period P 410 phase at 13.3°. These features are best modeled by a composite discontinuity in which a sharp velocity jump of 3% is overlain by a linear velocity jump of 3.5% spread over 14 km. The interference of energy turning in the diffuse and sharp portions of this discontinuity structure reproduces both the long‐ and short‐period triplication range and the step‐like behavior of the P 410 short‐period amplitude, which cannot be reproduced with either a simple linearly diffuse or a purely sharp discontinuity. This composite structure produces a triplication range which depends on source frequency and has an apparent depth which depends on observation frequency. Additionally, this is the structure expected from mineralogical arguments for the α to β olivine phase transition.