Seismic Observations of Mantle Discontinuities and Their Mineralogical and Dynamical Interpretation - Physics and Chemistry of the Deep Earth - page 299

phase (such as S, P, SS, PP or ScS), which is easily
observed in single seismograms. Waves which
are reflected by discontinuities in the mantle
share a very similar mantle path to the refer-
ence phase. These reflected waves can arrive
either as precursors or postcursors to the man-
tle reference phase. Stacking of large numbers of
seismograms makes the discontinuity reflections
visible (Shearer, 1991). Postcursors mainly occur
as near source or near receiver top-side reflec-
tions before or after a reference phase such as S
or P. It is difficult to distinguish if the reflection
occurred near the source or the receiver. P P ,SS
and PP precursors occur at the midpoint between
the source and receiver, so the reflection loca-
tion is easily determined making these useful
data types to determine discontinuity topogra-
phy maps.

Schmerr & Garnero, 2007; Deuss, 2009). Just
like P P precursors, these precursors arrive before
the major SS (or PP) wave because they are not
reflected from the Earth's surface but from a dis-
continuity below the surface bounce point of the
SS (or PP). Precursors are named SdS (or PdP),
where d is the depth of the discontinuity, thus
S660S (or P660P) are the underside reflections of
the 660 km discontinuity in the middle of the mi-
nor arc (80-180 ◦ epicentral distance). These data
are unique in probing the mantle discontinuities
with good global coverage, beneath both conti-
nents and oceans, and they can be used to draw
inferences in many different regions. Long period
SS waves are suitable for searching for discontinu-
ities as they are sensitive to weak discontinuities
and steep gradients in the velocity profile (e.g.
Deuss & Woodhouse, 2002). The rays have a rel-
atively large Fresnel zone (Chaljub & Tarantola,
1997; Deuss, 2009), so it is possible to resolve
long wavelength characteristics and topography
of the discontinuities (Shearer & Masters, 1992;
Flanagan&Shearer, 1998a; Gu et al ., 1998; Cham-
bers et al ., 2005b; Ritsema et al ., 2009; Houser
& Williams, 2010). SS precursors have been stud-
ied using global data sets, and therefore provide
a means to investigate global characteristics and
maps of topography on transition zone disconti-
nuities.
More recently, PP precursors have also been
studied (Rost & Weber, 2001; Chambers et al .,
2005b; Deuss et al ., 2006; Thomas & Billen, 2009)
and the combination of both SS and PP can be used
to determine velocity and density jumps at the
transition zone discontinuities (Shearer & Flana-
gan, 1999; Chambers et al ., 2005a). The Fresnel
zone of the SS and PP waves is very complicated
with a minimax characteristic saddle shape, and
Chaljub and Tarantola (1997) and Neele et al .
(1997) suggested that this can cause problems
when determining topography on transition zone
discontinuities. They argue that deepening of the
660 km discontinuity in subduction zones will be
underestimated in studies of SS and PP precur-
sors. Migration techniques can be used to correct
for the complicated Fresnel zone (Shearer et al .,
1999; Thomas &Billen, 2009; Cao et al ., 2011). All

(a) P P Precursors Early studies using subsur-
face reflections used precursors to P P (Figure
10.2a) (Adams, 1968; Whitcomb & Anderson,
1970; Adams, 1971; Engdahl & Flinn, 1969). This
is in fact a short name for the phases PKPPKP,
or PKIKPPKIKP. These precursors are underside
reflections which arrive at the station before the
major P P phase as the waves do not travel all
the way up to the Earth's surface but are instead
reflected by a discontinuity. P P approaches dis-
continuities in the mantle transition zone with a
steep angle of incidence and is a high frequency
phase, making it very valuable in determining the
sharpness of discontinuities (Benz &Vidale, 1993;
LeStunff et al. , 1995; Nakanishi, 1988; Xu et al. ,
1998, 2003). As this phase has such a unique path
geometry between the source and receiver, it pro-
vides a completely different global surface data
coverage compared to most other techniques.

(b) SS and PP precursors More recently, precur-
sors to the SS and PP waves have been used
extensively to study the global seismic charac-
teristics of the transition zone discontinuities
(Figure 10.2b). It has mainly been applied to
SS waves (see for example Shearer, 1993, 1996;
Lee & Grand, 1996; Flanagan & Shearer, 1998a;
Gu et al. , 1998, 2001; Deuss &Woodhouse, 2001;