Up: Seismic investigation of upper
For PP and SS phases precursors exist, which are generated in a similar way as
for P'P'. The ray path of the mother phase (PP or SS) and the underside
reflection at the discontinuity (PP or SS) is displayed in figure 3.3 b).
Arrivals preceeding PP and SS have been studied intensively (Bolt et
al. 1968; Bolt, 1970; King et al., 1975), but early array studies showed
that these precursors have slownesses significantly different than predicted
for PP (Wright and Muirhead, 1969; Wright, 1972). Therefore,
these phases are interpreted as asymmetric reflections of PP (Wright
1972), as the result of scattering (King et al., 1975) or can be
seen as upper side reflections from the discontinuities at the receiver side. The ray
path of an upper side reflection (Ppp) is shown in Figure 3.3 b). The wave
is reflected at the free surface near the receiver and, between this surface
reflection point and the receiver, once more at the upper side of the
discontinuity. Ppp phases show slownesses and waveforms similar to P and
However, phases from deeper discontinuities have been found with correct travel times
and slownesses to be interpreted as PP.
These precursors have been used to resolve the structure of the upper mantle
beneath the reflection point (Shearer, 1990; Wajeman, 1988; Neele and
Snieder, 1992; Vasco et al., 1995; Flanagan and Shearer, 1998 and 1999).
Results for stacks of a huge number of seismograms stacked with
respect to the source receiver distance, IASP91 theoretical travel times for
the PP and SS time windows and number of seismograms used to compute the
stacks (after Flanagan and Shearer, 1998 and 1999).
The top panels show travel time curves calculated for IASP91. The major phases
and the reflections at the upper mantle discontinuities are labelled.
The middle panel on the left hand side shows the stacks for SS and precursors,
the right hand side for PP. The same time window as for the theoretical
travel times was chosen. The bottom panels show the number of stacked
seismograms for each distance. The strong horizontal lines at 0 min
travel time mark the SS and PP phase. The precursor of the 410 and the 660
form approximately parallels to PP and SS and are marked. The phase PP
is not visible, as discussed in chapter 2.2.3. Due to the long period nature of
the stacked seismograms, shallow discontinuities (H, G and L) and the fine
structure of the discontinuities can not be resolved.
The stacking of a huge number of globally recorded long period seismograms
impressively shows the existence of PP and SS phases. Stacks for PP
and SS depending on epicentral distance are shown in the mid panel of Figure
3.4 (Flanagan and Shearer, 1998 and 1999). The stacked seismograms
are aligned on the PP (SS) arrival at a travel time of 0 s. The PP
(SS) phases are approximately parallel to PP (SS) indicating similar
slownesses. Due to the longer differential travel times SS - SS, the
SS phases are easier to detect. As described in section 2.2.3, the stacks
for PP lack the reflection from the 660, although the theoretical travel time
for the reflection from the 660, computed for IASP91, is marked. The top
panels show the theoretical travel times for IASP91. The bottom panels show
the number of seismograms used to compute the stacks.
The globally recorded seismograms and the stacking methods used to generate
the stacks of Figure 3.4 can be used to generate maps of the 410 and 660 to
infer regional differences. For this purpose events with reflection points in
a similar region are stacked. These ''topography'' maps of the discontinuities may
reflect the movements of cold and hot material in the mantle (Shearer,
In the stacks, the shallower discontinuities (H, G and L) discussed in chapter 2 are
not detected, partly due to the long period data used.
The inversion of PP - PP (SS - SS) differential travel times to discontinuity
depth may contain gross errors, because of the use of geometrical optics for
the inversion (Neele et al., 1997).
The global studies of the underside reflections give a good overview on the
structure of the discontinuities. However, as a result of the use of long period waves
and the correlated huge Fresnel zones (PP: 10 x 15,
SS 18 x 23 ), the resolution is poor. But these
studies are well suited to constrain velocity and density jumps across the
discontinuities (Shearer and Flanagan, 1999).
Up: Seismic investigation of upper