Geoscience Reference
In-Depth Information
(a)
(b)
1000
10.0
100
9.0
10
8.0
1
7.0
0.1
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
Depth (km)
Figure 4.10.
The variation of (a) earthquake occurrence and (b) maximum
magnitude, with focal depth for earthquakes with
M
w
>
5.45 recorded during the
period 1977-1997. Most of the earthquakes were shallow and resulted from brittle
fracture and frictional sliding along fault planes. The number of earthquakes
decreases exponentially with depth down to about 300 km. Seismic activity levels
then stay low at just over one per year down to 500 km depth. There the activity
increases, reaching almost six per year before ceasing completely at
670 km. The
largest earthquakes are shallow: the maximum magnitude decreases with depth
down to
∼
∼
300 km; it then remains at
∼
7.5 until 550 km, where it increases, reaching
∼
8.2 by 600 km. (After Frohlich (1998).)
hours of the main shock, usually reaches a magnitude about 1.2 units lower
than that of the main shock (Bath 1979). (Earthquake magnitude is discussed in
Section 4.2.3.)
Sometimes the main earthquake is preceded by a small
foreshock
. Unfortu-
nately, it is not usually possible to identify it as a foreshock until after the main
event has occurred. Earthquake
swarms
are yet another sequence of activity. In a
swarm there is no main earthquake but instead a large number of small shocks,
often with many occurring every day. The numbers of shocks build up slowly to
a maximum and then die down again. Most earthquake swarms occur in volcanic
areas, especially along the mid-ocean ridges, and these events usually have very
shallow foci.
Earthquakes do not account for all the motion that the plate-velocity mod-
els predict (Section 2.4).
Afterslip
,aterm used to describe motion that takes
place over some time after an earthquake, is so gradual that no seismic event
is detected: it is
aseismic
. The recent establishment of the GPS satellite net-
work (Section 2.4.1) means that continuously operating GPS receivers can now
be deployed to investigate the occurrence of aseimic slip and afterslip along
particular regions of interest. A 1994
M
w
=
7.6 earthquake beneath Honshu,
