Mid-Mantle
Reflectors/Discontinuities
The nature of heterogeneities in the lower mantle is
important in understanding Earth's evolution. The seismic signatures of the
lower mantle heterogeneities generally appear to be more subtle than those of
the upper mantle except for the lowermost several hundreds of kilometers. We
have observed clear later arrivals 40-90 seconds after P wave at seismic
stations in Japan (J-array) from deep earthquakes that occurred in the Mariana
and the western end of the Indonesia subduction zones and found that these
later arrivals are generated at underneath velocity reflectors
(discontinuities) at depths around 1000 kilometers (Figure 1). We are currently
working on the details of the lateral extension and seismic properties of the
mid-mantle reflectors in order to understanding the possible mechanisms capable
of producing these seismic structures.
Figure 1. Seismograms recorded by the J-array stations from two
deep earthquakes occurred at the western end of the
RELATED PUBLICATIONS
F. Niu, H. Kawakatsu, and Y. Fukao, A slightly dipping and strong seismic
reflector at mid-mantle depth beneath the Mariana subduction zone, J.
Geophys. Res., submitted,
2002.
L. Vinnik, F. Niu, and H. Kawakatsu, Broadband converted phases from mid-mantle discontinuities, Earth, Planets and Space, 50, 987-997, 1998. [PDF]
F. Niu, and H. Kawakatsu, Depth variation of the mid-mantle seismic discontinuity, Geophys. Res. Lett., 24, 429-432, 1997. [PDF]
H. Kawakatsu, and F. Niu, Seismic evidence for a 920-km discontinuity in the mantle, Nature, 371, 301-305, 1994.