TY - JOUR
T1 - Is there a first-order discontinuity in the lowermost mantle?
AU - Liu, Xian Feng
AU - Tromp, Jeroen
AU - Dziewonski, Adam M.
N1 - Funding Information:
This research has been supported by grants from the National Science Foundation and the David and Lucile Packard Foundation. We thank Rick O'Connell and two anonymous reviewers for their comments and suggestions. [RO]
PY - 1998/8
Y1 - 1998/8
N2 - In 1983, Lay and Helmberger [Geophys. J. R. Astron. Soc. 75 (1983) 799-837] reported the detection of a precursor to the seismic phase ScS. They attributed this precursor to a sharp seismic discontinuity located several hundred kilometers above the core-mantle boundary. Such a lowermost mantle discontinuity implies the existence of a sharp phase change or a chemical boundary. Precursors to ScS and, less frequently, PcP have since been observed in numerous locations, but are not a global phenomenon. Frequently, PcP precursors are weak or absent when ScS precursors are observed in the same location, and vice versa. There can be significant variations in the amplitude and arrival time of the precursor relative to the main phase. The presence or absence of these precursors has led to speculations about the nature of the lowermost mantle. Here we demonstrate that ScS or PcP precursors may be produced by gradients in seismic wave speed associated with large-scale lowermost mantle heterogeneity. Rather than a phase or chemical boundary with substantial topography, such gradients require lateral variations in temperature and, close to the core-mantle boundary, composition.
AB - In 1983, Lay and Helmberger [Geophys. J. R. Astron. Soc. 75 (1983) 799-837] reported the detection of a precursor to the seismic phase ScS. They attributed this precursor to a sharp seismic discontinuity located several hundred kilometers above the core-mantle boundary. Such a lowermost mantle discontinuity implies the existence of a sharp phase change or a chemical boundary. Precursors to ScS and, less frequently, PcP have since been observed in numerous locations, but are not a global phenomenon. Frequently, PcP precursors are weak or absent when ScS precursors are observed in the same location, and vice versa. There can be significant variations in the amplitude and arrival time of the precursor relative to the main phase. The presence or absence of these precursors has led to speculations about the nature of the lowermost mantle. Here we demonstrate that ScS or PcP precursors may be produced by gradients in seismic wave speed associated with large-scale lowermost mantle heterogeneity. Rather than a phase or chemical boundary with substantial topography, such gradients require lateral variations in temperature and, close to the core-mantle boundary, composition.
KW - Core-mantle boundary
KW - D″ layer
KW - Global
KW - Lower mantle
KW - Mantle
KW - Mantle discontinuity
KW - Seismology
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U2 - 10.1016/S0012-821X(98)00095-8
DO - 10.1016/S0012-821X(98)00095-8
M3 - Article
AN - SCOPUS:0032456429
SN - 0012-821X
VL - 160
SP - 343
EP - 351
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 3-4
ER -