Geoscience Reference
In-Depth Information
We can use Eqs. (4.27) and (4.24) with Fig. 4.10 to estimate the energy released
each year by earthquakes of various magnitudes and depth ranges. About three-
quarters of the energy is released by shallow-focus events and the earthquakes
with magnitude greater than 7 account for almost all the energy loss.
4.2.7 The observation of earthquakes: seismic phases
The various seismic-ray paths within the Earth are coded as follows:
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P
a P-wave in the mantle
S
an S-wave in the mantle
K
a P-wave through the outer core
I
a P-wave through the inner core
J
an S-wave through the inner core
c
a reflection from the mantle-outer-core boundary
i
a reflection from the outer-core-inner-core boundary
p
a P-wave reflected from the surface of the Earth close to the earthquake focus
s
an S-wave reflected from the surface of the Earth close to the earthquake focus
LR
aRayleigh wave
LQ
aLovewave.
The first arrival at a seismometer is always P, closely followed by pP, the
P-wave reflected once near the focus (Fig. 4.9). The S-waves arrive next and
finally the surface waves.
When a body wave is incident on a discontinuity in the Earth, some of the
energy is reflected as a P-wave, some is reflected as an S-wave, some is trans-
mitted as a P-wave and some is transmitted as an S-wave. This is termed
mode
Figure 4.15.
Some of the
possible ray paths for
seismic waves
penetrating the Earth. In
the mantle and core, the
velocities increase with
depth, so the ray paths
bend away from the
normal. The decrease in
velocity at the
mantle-core boundary
causes those rays
refracted into the core to
bend towards the normal.
PP
PKP
P
PKIKP
Inner Core
Outer Core
PKJKP
sS
PKIKPPKIKP
Mantle
sSP
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The terminology P (primary), S (secondary) and L was introduced by von dem Borne (1904).
