Geoscience Reference
In-Depth Information
Time 1
PKJKP
awave that passes through the mantle and outer core as a P-wave, then through
the inner core as an S-wave and up through the outer core and mantle as a
P-wave
PKiKP
a P-wave that travelled down through the mantle and outer core, was reflected
at the outer-core-inner-core boundary and travelled back up through the outer
core and mantle
sSP
awave that travelled from the focus as an S-wave, was reflected at the Earth's
surface close to the focus, then travelled through the mantle as an S-wave, was
reflected for a second time at the Earth's surface, converted to a P-wave and
travelled as a P-wave through the mantle.
Time 2
Figure 4.15 shows some ray paths (see also Section 8.1.1 and Figs. 8.1-8.3).
Figure 4.16 is a travel-time plot for all the main earthquake phases. Identifying
and recognizing the phases on an earthquake record is a skilled task (Fig. 4.17).
Combining earthquake records, to produce a record section greatly facilitates
identification of phases (Fig. 4.18). In Fig. 4.18(a) the main body-wave phases
from the mantle and also some from the core are immediately apparent even to
the untrained eye. The surface waves, which have greater amplitude and arrive
later than the main body-wave phases, do not show up well on Fig. 4.18(a) but
are very clear on the longer-period section shown in Fig. 4.18(b).
Time 3
4.2.8 Earthquake mechanisms
The relative motions between plates on the Earth's surface, although regular over
geological time, are not continuous over a daily or yearly period. Stress builds up
along a fault, or in a region, over a period of years before it reaches some critical
level; then an earthquake and, perhaps, aftershocks relieve it (Fig. 4.21). The
length of the
fault plane
, along which the rocks are displaced, varies from metres,
for a very small earthquake, to about 1000 km, for a very large earthquake. The
1960 Chile earthquake had a fault plane 1000 km in length, and the aftershock
zone of the 1957 Aleutian earthquake (
M
w
=
Figure 4.21.
A simple
model of the elastic-
rebound theory of
earthquake source
mechanisms. Over time,
one side of the fault is
displaced relative to the
other side (times 1 and 2).
The deformation continues
until the stresses on the
fault are large enough to
overcome the friction
between the two blocks of
material; then an
earthquake (sudden
displacement or rupture)
occurs, and the strain is
released (time 3). Thus, the
size of the earthquake is
directly related to the
friction in the fault.
9.1) was some 1200 km long, the
longest aftershock zone known. Figure 4.22 illustrates the basic types of faults
and the various names by which they are known. Most earthquakes occur along
plate boundaries as a direct result of plate motions; these are
interplate
(between-
plates) earthquakes.
Intraplate
(within-plate) earthquakes comprise only a small
proportion of the total number occurring, but they can be large and can produce
considerable damage. Examples of these include the sequence of great 1811 and
1812 earthquakes in New Madrid, Missouri, U.S.A., two of which must have
had magnitudes of 7.3-7.5. These earthquakes, which have repeated about every
500 yr, are thought to be due to a heterogeneous zone in the lower crust that
focusses deformation.
To determine the location of any particular earthquake, the seismic waves it
produces must be observed at a number of recording stations around the world.
