Geoscience Reference
In-Depth Information
on the ocean floor where no permanent stations
are available at the moment.
discontinuity is much more difficult to observe
in PP precursors (Figure 10.4b), but the '410' and
'520' are seen as often as in the SS precursor
data. The Pds receiver functions show a clear
'410' and '660' (Figure 10.4c), but here the 520 km
discontinuity has almost disappeared. The Pds
receiver functions do show the Lehmann discon-
tinuity and additional upper mantle reflectors.
Regional variations in the observation of mantle
discontinuities will be discussed in the following
sections.
(b) S-to-P
Just as with the P-to-S receiver
functions, we can also study S-to-P receiver
functions (Figure 10.3b). One has to be careful,
as these waves travel most of the mantle as shear
wave, and problems may arise due to anisotropy.
The S receiver function technique is extensively
described by Yuan
et al
. (2006) and has mainly
been used to image lithosphere structure. From
deep events we also have near-source S-to-P
conversions (e.g. Vidale & Benz, 1992), and a
range of other reflected and converted phases
(Figure 10.3c). As deep events mainly occur in
slabs, this is a good method to investigate the
detailed structure of subduction zones. S-to-P
receiver functions have also provided some
observations of lower mantle reflectors (see Niu
& Kawakatsu, 1997; Castle & Creager, 1999;
Kaneshima &Helffrich, 1999; Vinnik
et al
., 2001).
10.3.1 410 km discontinuity
The top of the transition zone is marked by the
410-km discontinuity, which is observed in most
data types and is generally characterized by a sim-
ple single peak in the seismic observations (Figure
10.5). It was first discovered in the 1960s using
regional arrays studies (Johnson, 1967), and has
been seen consistently ever since in a range of
data types (e.g. Fukao, 1977; Vinnik
et al
., 1983;
Grand & Helmberger, 1984; Revenaugh & Jordan,
1989; Bowman & Kennett, 1990; Shearer, 1991;
Vidale & Benz, 1992; Flanagan & Shearer, 1998b).
PP and SS precursors both detect the 410 km dis-
continuity, and joined global topography maps
have beenmade (Flanagan&Shearer, 1998a, 1999;
Chambers
et al
., 2005b). These maps show that
at the largest sale, the topography of the '410'
may be correlated with local velocity perturba-
tions, suggesting the expected behavior for a posi-
tive Clapeyron slope. However, the smaller-scale
structure does not follow any correlation, suggest-
ing the existence of small-scale compositional
heterogeneity around 410 km depth (Chambers
et al
., 2005b). Chambers
et al
. (2005a) studied
the impedance contrast at the 410 km disconti-
nuity by combining SS and PP precursors, and
found strong regional variations but was not able
to determine any specific correlation with tec-
tonic features. Schmerr and Garnero (2007) found
a deepened '410' associated with decreased reflec-
tion S410S amplitudes in a region near the South
America subduction zone.
When you look in more detail, it is seen that
the 410 km discontinuity is strongly frequency
10.3 The Transition Zone
Following on from the histogram showing the
numbers of papers citing a discontinuity at a cer-
tain depth in the mantle (Figure 10.1), let us first
investigate histograms of depths at which discon-
tinuities are observed using global seismological
data sets (Figure 10.4). Themost useful seismolog-
ical data types for such an exercise are SS and PP
precursors, and Pds receiver functions. These data
all have reasonable global coverage, and will be
the main data types used to discuss the different
seismic observations in this chapter. The SS and
PP precursor data are taken fromDeuss (2009) and
the Pds receiver functions data are from Andrews
and Deuss (2008). The SS precursor histogram
(Figure 10.4a) looks most similar to Figure 10.1,
suggesting that this is a good data type for ob-
serving a range of discontinuities, including both
strong and weak reflectors. The SS precursors
show the transition zone discontinuities at 410,
520 and 660 km depth, and also the Lehmann dis-
continuity at 220 km depth as well as additional
upper and lower mantle reflectors. The 660 km
