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depths found using receiver functions. For North
America, they find a depth of over 200 km in
some regions, which they call 'continental cores'.
Physically, the surface wave and temperature
method is more directly related to the thermal
and mechanical mechanism that is thought to
cause the lithosphere boundary. These mecha-
nisms would lead to quite a broad discontinuity
or gradient in velocity structure, which can be
picked up by surface waves. It can also be used
in both continental and oceanic regions and thus
provide global maps and it is not limited to find-
ing sharp discontinuities only. Receiver function
can only observe sharp velocity contrasts, which
might actually not exist at the LAB. Also, receiver
functions cannot be used in oceanic regions.
In yet another approach, Yuan and Romanowicz
(2010) studied azimuthal anisotropy in the litho-
sphere, again under North America, using long
period seismic waveforms and SKS wave splitting
data. They defined the LAB as the depth at which
the fast axis of the anisotropic changes towards
the direction of absolute plate motion. They find
depths varying from 180 to 240 km, much closer
to the results of Priestley and McKenzie (2006)
than to the receiver functions studies. It seems
most likely that receiver functions are observing
a mid lithospheric reflector instead of the LAB
and that the LAB can only be reliably observed
using long period surface and body wave methods.
10.4.2 Lehmann discontinuity
A discontinuity at 220 km depth was first dis-
covered by Lehmann (1959, 1961) using S and P
wave triplications (Figure 10.8a). There is a promi-
nent discontinuity at 220 km depth in PREM
(Dziewonski & Anderson, 1981), but its global
existence has long been debated and its nature
was unknown. In PREM, the Lehmann disconti-
nuity marks the transition from radial anisotropy
in the top 220 km of the Earth, to isotropic struc-
ture below. One might argue that the Lehmann
discontinuity is, at least in some regions, the
same as the LAB. For example in North America
and Canada, the Lehmann discontinuity has often
been observed (Lehmann, 1961; Bostock, 1997;
Deuss &Woodhouse, 2002) at depths very similar
Earlier array studies
SS precursors
180-240 km depth
180-240 km depth
2%
4%
6%
published observation
S220S/SS amplitude
(a)
(b)
Fig. 10.8
Maps showing locations where the Lehmann discontinuity has been seen (a) in regional seismic array
studies (Lehmann, 1959, 1961; Adams, 1968; Fukao, 1977; Niazi, 1969; Given & Helmberger, 1980; Bowmann &
Kennett, 1990; Revenaugh & Jordan, 1991b; Vidale & Benz, 1992; Revenaugh & Sipkin, 1994; Gaherty & Jordan,
1995; Bostock, 1997; Thybo & Perchuc, 1997; Shen
et al
., 1998; Liu
et al
., 2003; Tkal ci c
et al
., 2006) and (b) in a
global study of SS precursors. Adapted from Deuss and Woodhouse (2004). Reproduced with permission of Elsevier.
