170 Seismology and Earth Structure
Source
PKP-DF
(PKIKP)
PcPPKP
180°
90°
12
13
6
21
20
175°
17
6
7 8
9 10 11
PP
PKP-AB
PKP-AB
PP
PKP-DF
PP
PPP
PKP-AB
PKP-AB
PP
PcPPKP
PPP
PKP-DF
New Zealand
May 23, 1968
PTO
∆ = 179.3°
N
MAL
∆ = 175°
1 cm 1 min
Antipodal focusing
Fig. 3.5-11 Focusing of P waves at the antipode, 180° from an earthquake. Left: Seismic rays PKP-AB and PP focus at the antipode. Dashed lines
represent wave fronts, whose propagation time in minutes is given. Right: Seismograms showing antipodal focusing of core phases. (Rial and Cormier,
1980. J. Geophys. Res., 85, 2661–8, copyright by the American Geophysical Union.)
waves sample physical properties over a period of seconds,
whereas the lithosphere and asthenosphere are inferred from
data sampling periods of thousands and millions of years
(Section 5.7).
The depth and magnitude of the LVZ vary regionally. In
tectonically active regions like western North America, the
LVZ is well developed and relatively shallow. In stable continental regions that have not experienced tectonism for a long
time, the LVZ is deeper and less pronounced, and may not even
be present. The thick, high-velocity layer under continents has
led to the suggestion that it may reflect a chemically distinct
tectosphere. For this hypothesis, continents behave differently
from the oceanic lithosphere, where surface wave dispersion
shows a pronounced LVZ for all ages (Fig. 2.8-7). This persistence may reflect the fact that oceanic lithosphere is never older
than 180 Ma, tectonically young by continental standards,
because older oceanic lithosphere is subducted away.
We will show in Section 5.7 that the contrast between the
high-velocity seismic lithosphere and the asthenosphere LVZ
is probably related to variations in material strength between
the cold lithosphere and the warmer asthenosphere. There may
also be some effects of partial melting. This situation differs
from the velocity differences between the crust and the mantle,
which result from their different compositions. Beneath the
LVZ, which extends to an average depth of about 200 km,
temperatures increase only slowly, but velocities increase significantly in response to the increasing pressure.
The transition zone between the upper and lower mantles is
marked by the velocity discontinuities at depths of about 410
explore its mineralogical causes in Section 3.8, and consider the
effects of subducting lithosphere in Section 5.4.
We have already discussed the velocity structure of the uppermost mantle shown by surface wave dispersion (Section 2.8).
Body wave analyses reveal a similar structure. The sub-crustal
lithosphere shows generally fast P- and S-wave velocities of
about 8.1 and 4.5 km/s. This high-velocity layer provides a
way of defining the lithosphere, termed the seismic lithosphere
or lid, from seismological observations. The thickness of the
seismic lithosphere varies with location. At mid-ocean ridges,
where oceanic plates are created, its thickness approaches zero.
Beneath stable cratons, the fast lithospheric velocities extend
to about 200 km. As a global average, the seismic lithosphere
extends to about 80–100 km depth.
In most regions of the world, we find a seismic low-velocity
zone (LVZ) beneath the seismic lithosphere. The LVZ approximately coincides with the expected mechanically weak
asthenosphere underlying the stronger lithosphere. The lithosphere and asthenosphere are defined by their mechanical
properties, such that plates of strong lithosphere slide over
weaker asthenosphere. This contrast, as we will see, results
from the fact that the lithosphere is the cold outer thermal
boundary layer of the solid earth (Sections 3.8, 5.1). By contrast, the high-velocity seismic lithosphere and underlying LVZ
are seismologically defined entities. The rough correspondence
between the seismological and mechanical layers indicates that
the two are closely related, and that seismic observations can be
used to map mechanical structure. The two sets of layers are
not identical for several reasons including the fact that seismic
Source
PKP-DF
(PKIKP)
PcPPKP
180°
90°
12
13
6
21
20
175°
17
6
7 8
9 10 11
PP
PKP-AB
PKP-AB
PP
PKP-DF
PP
PPP
PKP-AB
PKP-AB
PP
PcPPKP
PPP
PKP-DF
New Zealand
May 23, 1968
PTO
∆ = 179.3°
N
MAL
∆ = 175°
1 cm 1 min
Antipodal focusing
Fig. 3.5-11 Focusing of P waves at the antipode, 180° from an earthquake. Left: Seismic rays PKP-AB and PP focus at the antipode. Dashed lines
represent wave fronts, whose propagation time in minutes is given. Right: Seismograms showing antipodal focusing of core phases. (Rial and Cormier,
1980. J. Geophys. Res., 85, 2661–8, copyright by the American Geophysical Union.)
waves sample physical properties over a period of seconds,
whereas the lithosphere and asthenosphere are inferred from
data sampling periods of thousands and millions of years
(Section 5.7).
The depth and magnitude of the LVZ vary regionally. In
tectonically active regions like western North America, the
LVZ is well developed and relatively shallow. In stable continental regions that have not experienced tectonism for a long
time, the LVZ is deeper and less pronounced, and may not even
be present. The thick, high-velocity layer under continents has
led to the suggestion that it may reflect a chemically distinct
tectosphere. For this hypothesis, continents behave differently
from the oceanic lithosphere, where surface wave dispersion
shows a pronounced LVZ for all ages (Fig. 2.8-7). This persistence may reflect the fact that oceanic lithosphere is never older
than 180 Ma, tectonically young by continental standards,
because older oceanic lithosphere is subducted away.
We will show in Section 5.7 that the contrast between the
high-velocity seismic lithosphere and the asthenosphere LVZ
is probably related to variations in material strength between
the cold lithosphere and the warmer asthenosphere. There may
also be some effects of partial melting. This situation differs
from the velocity differences between the crust and the mantle,
which result from their different compositions. Beneath the
LVZ, which extends to an average depth of about 200 km,
temperatures increase only slowly, but velocities increase significantly in response to the increasing pressure.
The transition zone between the upper and lower mantles is
marked by the velocity discontinuities at depths of about 410
explore its mineralogical causes in Section 3.8, and consider the
effects of subducting lithosphere in Section 5.4.
We have already discussed the velocity structure of the uppermost mantle shown by surface wave dispersion (Section 2.8).
Body wave analyses reveal a similar structure. The sub-crustal
lithosphere shows generally fast P- and S-wave velocities of
about 8.1 and 4.5 km/s. This high-velocity layer provides a
way of defining the lithosphere, termed the seismic lithosphere
or lid, from seismological observations. The thickness of the
seismic lithosphere varies with location. At mid-ocean ridges,
where oceanic plates are created, its thickness approaches zero.
Beneath stable cratons, the fast lithospheric velocities extend
to about 200 km. As a global average, the seismic lithosphere
extends to about 80–100 km depth.
In most regions of the world, we find a seismic low-velocity
zone (LVZ) beneath the seismic lithosphere. The LVZ approximately coincides with the expected mechanically weak
asthenosphere underlying the stronger lithosphere. The lithosphere and asthenosphere are defined by their mechanical
properties, such that plates of strong lithosphere slide over
weaker asthenosphere. This contrast, as we will see, results
from the fact that the lithosphere is the cold outer thermal
boundary layer of the solid earth (Sections 3.8, 5.1). By contrast, the high-velocity seismic lithosphere and underlying LVZ
are seismologically defined entities. The rough correspondence
between the seismological and mechanical layers indicates that
the two are closely related, and that seismic observations can be
used to map mechanical structure. The two sets of layers are
not identical for several reasons including the fact that seismic