FINE-STRUCTURE OF THE 410-KM DISCONTINUITY

The April 14, 1995, earthquake in western Texas (M-w 5.7) produced a s trong topside reflection off the 410-km discontinuity which was record ed on a multitude of seismic arrays throughout the southwestern United States. Data from 394 vertical short-period and 24 broadband instrume nts provide dense coverage of this event from distances of 11 degrees to 19 degrees and provide a detailed look at the subcontinental 410-km structure. The salient features of this data set are (1) the strong d ependence on wavelength of the 410-km triplication range, (2) the unif orm amplitude ratio of the direct P and reflected P-410 phases on both short-period and broadband recordings throughout the triplication, an d (3) the abrupt termination of the short-period P-410 phase at 13.3 d egrees. These features are best modeled by a composite discontinuity i n which a sharp velocity jump of 3% is overlain by a linear velocity j ump of 3.5% spread over 14 km. The interference of energy turning in t he diffuse and sharp portions of this discontinuity structure reproduc es both the long- and short-period triplication range and the step-lik e behavior of the P-410 short-period amplitude, which cannot be reprod uced with either a simple Linearly diffuse or a purely sharp discontin uity. This composite structure produces a triplication range which dep ends on source frequency and has an apparent depth which depends on ob servation frequency. Additionally, this is the structure expected from mineralogical arguments for the alpha to beta olivine phase transitio n.