Environmental Engineering Reference
In-Depth Information
within the ground more quickly than low frequencies, so at greater distances the propor-
tion of seismic vibration energy at high frequencies will decrease.
12.2.2
Earthquake magnitude, and intensity
There are a number of different measures of the magnitude of an earthquake.
(a)
The Local or Richter magnitude
M
log
A
(12.1)
L
10
where A
maximum seismic wave amplitude (in thousandths of a millimetre)
recorded from a standard seismograph at a distance 100 km from the earthquake epi-
centre or
Ml g
A ( )
k
(12.2)
L
10
scaling constant.
This allows calculation of M
L
at different distances from the earthquake.
(b)
The Body Wave magnitude
where F(
)
distance correction; k
Ml g
V .3 log
(12.3)
b
10
where V
maximum ground velocity in microns/sec recorded by the seismograph;
distance from epicentre in kilometres.
(c)
The Moment Magnitude
(
M
W
) which assigns a magnitude to the earthquake in accor-
dance with its seismic moment M
O
, which is directly related to the energy released by
the earthquake:
M(log
M/1.5)
10.7
(12.4)
W
10
O
where M
O
is the seismic moment in dyn-cm.
Moment magnitude is the scale most commonly used for engineering applications.
Figure 12.3
shows the relationship between the different scales.
Earthquakes with magnitude of less than 3 or 4 will usually not cause any felt effect,
and earthquakes with a magnitude less than about 5 will usually not cause any damage
unless they are shallow, near the structure and/or ground conditions cause amplification.
The maximum recorded magnitude is approximately 8.9. The scale is not linear. Each step
in the magnitude scale represents a thirty-fold increase in energy released by the earthquake
e.g. a Magnitude 5 earthquake represents 900 times the energy of a Magnitude 3, and a
Magnitude 8 represents about 10
6
times the energy of a Magnitude 4.
Another measure of earthquake size is the fault area, or the area of the fault surface
which is ruptured. The fault area ruptured in an earthquake depends on the magnitude
and stress drop in the earthquake. For a given magnitude, a higher stress drop will give a
small rupture area. Typically, a Magnitude 4 earthquake ruptures a fault area of about 1
square kilometre, Magnitude 5 about 10 square kilometres, and Magnitude 6 about 100
square kilometres.
Earthquake intensity is a qualitative value based on the response of people and objects
to the earthquake. The intensity depends on distance from the earthquake, ground condi-
tions and topography, so there will be a range of intensity values for any earthquake. The
