Global‐scale analysis of the mantle Pds phases

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Abstract
A global‐scale analysis of travel times of the mantle Pds phases (converted from P to S from the discontinuity at a depth d beneath the receiver) is presented. Most of the data are related to P′410′s and P′660′s, recorded by the broadband stations of Geoscope, Incorporated Research Institutions for Seismology (IRIS) and other networks. In order to provide a good global‐scale coverage, published measurements at 37 stations are complemented by our measurements at 45 stations. Lateral resolution of our data is many times higher than that of SS and its precursors, previously used for similar purposes, but we have fewer data in the oceans. Lateral variations of travel times of the major Pds phases are strongly correlated with the lateral P and S velocity variations in the uppermost mantle, as revealed by tomographic studies. The scaling coefficient between the teleseismic S and P residuals, as found from the variations of travel times of the major Pds phases, is around 3.7 for North America and around 3.3 for the rest of the world. Almost all the differential travel times t(P′660′s) – t(P′410′s) are in the range of ±1 s, relative to the average value of 24.0 s. If the lateral variations of the velocities within the mantle transition zone (MTZ) can be neglected, the corresponding variations of thickness of the MTZ are in the range of ±10 km, relative to the standard depth of 250 km, and do not show significant correlation with either velocities in the upper mantle above the MTZ or with the variations of thickness of the MTZ, as found in some recent studies of precursors to SS. There is a weak but marginally significant positive correlation between the variations of thickness of the MTZ inferred from our data and the S velocity variations in the MTZ, as given by Su et al. [1994]. This correlation suggests that both variables contain temperature‐dependent components. Lateral variability of the MTZ is also manifested by extreme weakness of P′410′s at several locations. This may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity. At some stations there are arrivals that can be interpreted as P′520′s. This interpretation implies that the strengh and/or sharpness of 520 km discontinuity, as well as its depth, are laterally variable in a broad range.

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