Fine structure of the 410-km discontinuity

Abstract

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.