Earthquakes - Geotechnical Engineering Investigation - page 914

greater than 5.0 generate ground motions sufficiently severe to cause significant damage
to poorly designed and constructed structures, and M

4 is generally considered as the

damage threshold.
Magnitude is not, however, a measure of the damage that may be caused by an earth-
quake, the effect of which is influenced by many variables. These include natural condi-
tions of geology (soil type, rock depth and structure, water-table depth), focal depth,
epicentral distance, shaking duration, population density, and construction quality.

Correlations with Magnitude
Empirical correlations between M , I o , and g are given in Figure 11.12, where I o signifies epi-
central intensity. Near the source there is no strong correlation with g (Bolt, 1978).
Acceleration has also been related to magnitude by the expression given by Esteva and
Rosenblueth (1969) as follows:

g 2000 e 0.8 M R 2 (11.10)

where R is the focal distance in kilometers and g is in cm/sec 2 per 1000 cm/sec 2 .

Seismic Risk Maps
An early seismic risk map is given in Figure 11.3 with correlations between zones, maxi-
mum accelerations, and magnitude M . Zone 3 was considered as running a high risk of
damaging earthquakes; zone 2, moderate risk; zone 1, low risk; and zone 0, essentially no
risk. It was subsequently updated by Algermissen and others in 1969 and the boundary

Epicentral
acceleration
a o

Magnitude
M

Energy
E

I o V o

Col 1 Col 2

Ergs

Col 3 Col 4 Col 5

cm
sec 2

a g

cm
sec

I

10 14

2

II

M = 3

4
6
8
10

10 16

0.005 g

III

CLASS E

M = 4 0.01 g

IV

10 18

1

20

V

M = 5

40
60
80
100

VI

0.05 g

5

CLASS D

10 20

10
20

VII

0.1 g

M = 6

200

CLASS C

VIII

500

FIGURE 11.12
A summary of rough relationships between
magnitude, energy, and epicentral acceleration
and between acceleration, intensity, and
ground velocity. Approximations are for an
order of magnitude. (From Faccioli, E. and
Resendiz, D., Seismic Risk and Engineering
Decisions , Lomnitz, C. and Rosenblueth, Eds.,
Elsevier, New York, 1976, pp. 71-140, Chap. 4.)

400
600
800
1000

IX 100

M = 7

0.5 g

10 22

CLASS B

1.0 g

X

500

M = 8

2000

XI

CLASS A

3 g

4000

10 24