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(b) Which types of seismic wave can be detected by a vertical-component seismo-
meter?
5. During a microearthquake survey in central Turkey, an earthquake was recorded by
three seismometers (Fig. 4.53). Calculate the origin time for this earthquake and locate
the epicentre relative to the seismometers by using information in the table that follows.
Hours
Minutes
Seconds
Seismometer 1
P-wave arrival time
13
19
58.9
S-wave arrival time
13
20
4.7
Seismometer 2
P-wave arrival time
13
20
2.6
S-wave arrival time
13
20
10.8
Seismometer 3
P-wave arrival time
13
19
54.5
S-wave arrival time
13
19
57.4
Figure 4.53.
A map
showing the locations of
seismometers 1, 2 and 3.
Assume that the focus was at the surface and use P- and S-wave velocities of 5.6 and
3.4 km s
−
1
, respectively.
6. Calculate the amount by which the seismic energy released by an earthquake increases
when the surface-wave magnitude increases by one unit. Repeat the calculation for
an increase of one unit in the body-wave magnitude.
7. The daily electrical consumption of the U.S.A. in 2001 was about 9.9
×
10
9
kilowatt
hours. If this energy were released by an earthquake, what would its magnitude be?
8. It is sometimes suggested that small earthquakes can act as safety valves by releas-
ing energy in small amounts and so averting a damaging large earthquake. Assum-
ing that an area (perhaps California) can statistically expect a large earthquake
(
M
S
>
8) once every 50-100 years, calculate how many smaller earthquakes with
(a)
M
S
=
6, (b)
M
S
=
5or(c)
M
S
=
4would be required during these years in order
to release the same amount of energy. If it were possible to trigger these smaller
earthquakes, would it help?
9. (a) Calculate the total amount of energy released each year by earthquakes. Suggest
an estimate of the amount by which this figure may be in error.
(b) Compare the total amount of energy released each year by earthquakes with the
energy lost each year by heat flow (Chapter 7).
10. Using the earthquake travel-time curves in Fig. 4.16 and the two earthquake records
in Fig. 4.17, determine for each earthquake (a) the epicentral distance (in degrees)
and (b) the origin time.
11. Explain how the epicentre, focal mechanism and slip vector of an earthquake are
determined. (Cambridge University Natural Sciences Tripos IB, 1974.)
