Geology Reference
In-Depth Information
Fig. 9.16
A low-velocity zone at planetary scale (
gray region
). Seismic rays penetrating the LVZ are bent downwards,
determining the formation of a shadow zone (
A
,
B
) and a duplication zone (
B
,
C
) at the Earth's surface
Fig. 9.17
Travel-time
curve (
left
) and delay time
curve (
right
)inpresenceof
an LVZ
appearance of continuous wave trains with slowly
decreasing amplitude, following the direct arrival
of
S
waves or
P
waves. Aki (
1969
) named these
wave trains
coda
and proposed that they were
formed by composition of incoherent waves scat-
tered by distributed heterogeneities in the litho-
sphere. In particular, he observed that while the
main phase amplitude decreased with increasing
epicentral distance, the coda amplitude was al-
most independent from the range. An in-depth
treatment of this interesting topic is beyond the
scope of this topic and can be found in Sato
et al. (
2012
) and in Shearer (
2007
). Here we
are concerned only with the observation that
seismic waves are strongly scattered by the LVZ,
which requires the existence of smallscale het-
erogeneities within this layer. The scattering is
observed as a pronounced coda behind the first
arrivals in the offset interval of
500-1,400 km
(Thybo
2006
), with more than 7 s duration at
short offset and more than 2-3 s at far offsets.
The gap in the travel time curve associated
with the upper mantle LVZ is observed between
800 and 1,000 km (
8
ı
from the epicenter).
Seismologists, petrologists, and the general geo-
dynamics community have wondered about the
origin of this region since the end of the 1950s.
Some of these scientists argued that it originates
Fig. 9.18
Deflection of seismic rays from a random dis-
tribution of scatterers may determine multiple arrivals at a
location
(Thybo
2006
). In general,
S
-waves are strongly
attenuated within the LVZ, especially under
oceanic regions. The approximately constant
depth of the top boundary suggests that the upper
mantle LVZ arises from metamorphic or other
pressure-driven transformations, because none of
the known phase transformations of peridotite
occurs at the lithostatic pressure of 100 km depth
(
3.1 GPa). Another feature of this region is the
presence
seismic wave scatterers
onascaleof
7
3 km (Thybo
2006
).
Scattering
of seismic
waves consists into the deviation of seismic rays
associated with high-frequency waves from small
random heterogeneities (Fig.
9.18
).
The most evident phenomenon associated with
scattering from small scale heterogeneities is the
