Evidence for a sharp 670‐km discontinuity as inferred from P‐to‐S converted waves

Abstract

The fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short‐period and broadband seismograms. Using a stacking technique to analyze the P wave coda of teleseismic records, evidence is found for coherent near‐receiver P‐to‐S converted phases generated by the 400‐ and 670‐km discontinuity beneath a number of the stations used in this study. Variations in travel time, slowness, and amplitude of these phases as observed among the stacks for stations of the Regional Seismic Test Network in the United States and of the Network of Autonomously Recording Seismographs in western Europe are very likely the expression of upper mantle heterogeneity. Observations of coherent converted phases from the 400‐km discontinuity are fewer in number than of phases converted at the 670‐km discontinuity, suggesting that the latter is more pronounced. Some of the seismograms, especially of station RSCP, show extremely high‐amplitude P‐to‐S converted phases from the 670‐km discontinuity. These seismograms allow a detailed waveform comparison of the converted phase with the direct P phase and present evidence for a sharp 670‐km discontinuity.

Mantle discontinuity structure from midpoint stacks of converted P to S waves across the Yellowstone hotspot track

“…To avoid this source of error, seismograms with visible PP or PcP arrivals in these distance ranges are not used. While such a lengthy source function is not an optimal estimate of the convertedS wave waveform [Paulssen, 1988], this approximation only results in a minor degradation of the ability of the deconvolution to "spike" the converted S waves. This degradation is more than compensated for by the increased stacking resolution provided by the large increase in useable data.…”

“…The most robust characteristic used in P ds phase identification is observation of the correct moveout associated with a Pds wave [e.g., Vinnik et al, 1996;Paulssen, 1988;Bostock, 1996]. This moveout can be directly observed by plotting all of our seismograms with respect to ray parameter ( Figure 5).…”

Complex Structure of Mantle Discontinuities at the Tip of the Subducting Slab beneath Northeast China. A Preliminary Investigation of Broadband Receiver Functions.

“…Previous studies on the P660s phase show that there are a great variety of amplitudes observed. Even in the same NARS array, Paulssen (1985Paulssen ( , 1988 observed an anomalously high-amplitude P660s phase in both individual seismograms and stacked waveforms, while van der Lee et al, (1994) found no clear P660s signals in any individual seismogram and just weak energy in the stacked waveforms. Motivated by these apparently inconsistent observations and the recent availability of high-quality broadband data, we have carried out this study of the P660s phase using broadband data recorded at stations in the western Pacific region.…”

“…Although studies of mantle discontinuities using long-period SS precursors by Shearer and Masters (1992) and Shearer (1993) provide a well-covered global observation of the "410-km" and "660-km" discontinuities, it is insufficient to resolve "local" velocity structures near subducting slabs. Recent studies (e.g., Paulssen, 1988;Bock and Ha, 1984;Richards and Wicks, 1990;Mizuta et al, 1991;Vidale and Benz, 1992;Wicks and Richards, 1993;Niu and Kawakatsu, 1995) show that analysis of short-period reflected and/or converted waves at upper-mantle discontinuities is an effective method to obtain detailed information about these discontinuities. A serious drawback of reflected-and converted-wave observations is that these phases are generally of low amplitude, which makes it difficult to identify them.…”

Upper mantle compressional velocity structure beneath the West Mediterranean Basin

“….. [Paulssen, 1988;Nakanishi, 1988Nakanishi, , 1989Benz and Vidale, 1993] or even absence of evidence [Wajeman, 1988] for such an interface. Whether the "410-km" discontinuity involves a phase transition of an olivine-rich [Bina and Wood, 1987] …”