Causes and Prediction of Earthquakes and Volcanoes - Natural Hazards - page 180

seismometers at any distance from the epicenter and
surface (Rayleigh) waves at a period of 18-22 seconds
and a wavelength of 60 km. The characteristics of
seismic waves will be described in more detail in the
following chapter. This magnitude scale is called
surface magnitude or M s . Alternatively, the amplitude
of the initial seismic wave (P-wave), m b , having a
period of 1-10 seconds can be used, because it is not
affected by the focal depth of the source of the earth-
quake. This measure is called the body-wave magni-
tude. The body-wave magnitude can be related to the
M s scale as follows:

Annual frequency of earthquakes by magnitude on the
M s

Table 9.1

scale.

Magnitude Frequency

>8.0 1

7.0-7.9 18

6.0-6.9 120

5.0-5.9 800

4.0-4.9 6200

3.0-3.9 49000

2.0-2.9 365000

m b = 2.9 + 0.56 M s (9.1)

Some of the largest earthquakes, by magnitude, on the
M s

Table 9.2

scale.

Overall the m b value is less representative than the M s
one of earthquake magnitude. The energy released by
an earthquake can be related to these scales as follows:

Magnitude, Death-

Date Location M s toll

log 10 E = 4.8 + 1.5 M s (9.2)

1 November Lisbon, Portugal 8.7 70 000

1755
31 January

log 10 E = 5.0 + 2.6 M b (9.3)

Andes (Columbia) 8.6 500

1906
16 August

The energy values are defined in ergs. For each unit
increase in surface magnitude, M s , energy increases by
31.6 times. None of these scales is appropriate for large
seismic events. Here, the seismic moment, M w , is now
used. This scale is based upon the surface area of the
fault displacement, the average length of movement
and the rigidity of the rocks fractured.
Table 9.1 lists the annual frequency of earthquakes
over the twentieth century on the M s scale, while Table
9.2 lists the largest events over the same period. No
historical earthquake has exceeded a magnitude of 9.0
on the M s scale. An earthquake larger than M s = 8.0
occurs about once a year. Its impact is catastrophic.
About 18 earthquakes occur each year in the 7.0-7.9
range. These earthquakes are deadly near population
centers. About 120 and 800 earthquakes occur
annually in the 6.0-6.9 and 5.0-5.9 ranges, respec-
tively. The former earthquakes are deadly near popula-
tion centers with inferior construction standards, while
the latter can still cause significant property losses. The
largest earthquakes appear to occur along the western
edges of the North and South American Plates, and in
China (Figure 9.1).
The M s scale requires instrumentation. It cannot be
used to assess the magnitude of prehistoric earthquakes
or ones occurring in countries without a network of

Valparaiso, Chile 8.6 20 000

1906
18 April 1906 San Francisco, US 8.3 3 000

3 January 1911 Tienshan,
Kazakhstan 8.4 452

16 December Kansu, China 8.6 200 000

1920
1 September Tokyo, Japan 8.3 143 000

1923
3 March 1933 Japanese Trench 8.1 3 064

15 August Assam, 8.6 1 526

1950 India/China

22 May 1960 Chile 8.5 2 231

27 March Alaska 8.6 100

1964
26 January Gujarat, India 7.6 20 000

2001

seismic stations. Instead, the Mercalli scale (or its
modified form) is used. This scale, summarized in
Table 9.3, can be related approximately to the M s scale.
The Mercalli scale qualitatively estimates the strength
of an earthquake using descriptions of the type of
damage that has occurred close to the origin or epi-
center of the earthquake in built-up areas. Determining