Seismic velocity and density jumps across the 410- and 660-kilometer discontinuities

Science

Volumn 285, Issue 5433, 1999, Pages 1545-1548

  Shearer, Peter M ^a   Flanagan, Megan P ^b  

^a UNIVERSITY OF CALIFORNIA   (United States)

^b LAWRENCE LIVERMORE NATIONAL LABORATORY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

DISCONTINUITY; MANTLE; SEISMIC VELOCITY;

AMPLITUDE MODULATION; ANALYTICAL ERROR; ARTICLE; CALCULATION; DENSITY; EARTHQUAKE; NONBIOLOGICAL MODEL; PARAMETER; PRESSURE; PRIORITY JOURNAL; TEMPERATURE; VELOCITY;

EID: 0033520370     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.285.5433.1545     Document Type: Article

References (41)

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19. The reflection coefficient is the ratio of the reflected wave amplitude to the incident wave amplitude.
20. SS and PP are the multiples of the direct S and P phases that contain a single reflection off Earth's surface midway between source and receiver. The precursors to SS and PP resulting from underside reflections off a discontinuity at depth d are termed SdS and PdP, respectively. For example. S410S indicates the S reflection off the bottom of the 410-km discontinuity.
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25. We obtained this factor by computing the effect of time shifts on the amplitude of the SS and PP reference pulses in the stacks, assuming that the time shifts have a Gaussian distribution characterized by a standard deviation, σ. The time shifts result from differences in the two-way surface-to-discontinuity travel times caused by both discontinuity topography and upper mantle velocity heterogeneity. In general, σ will vary depending on the wave type (for example, P or S) and the geographic diversity of ray geometries in the stack. For ScS waves, σ = 4 s was obtained for mixed continental and oceanic paths (3), whereas σ = 2.5 s has been estimated for SdS waves in purely oceanic regions (11). The limited distribution of reflection points contained within each of our source-receiver range bins is likely to decrease σ, whereas the mixture of continental and oceanic points is likely to increase σ. For σ values of 2, 3, and 4 s, SdS amplitudes in our stacks are reduced by 8, 16, and 28%, respectively. The time shifts for PdP phases are likely to be smaller than those for SdS. For σ values of 1.0, 1.7, and 2.5 s, PdP amplitudes are reduced by 4, 12, and 23%, respectively. Our amplitude corrections are based on σ = 2.5 s for SdS and σ = 1.7 s for PdP.
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41. This work was supported by NSF grants EAR93-15060, EAR95-07994, EAR96-14350, and EAR96-28020. M.P.F. was supported by an NSF Postdoctoral Fellowship and the Cecil H. and Ida M. Green Foundation. J. Gaherty and J. Vidale provided constructive reviews of an earlier version of this report.