Environmental Engineering Reference
In-Depth Information
The seismicity can be evaluated by a modified Richter relation (Gibson, 1994)
bM
bM
(12.9)
log
(P)
log
[10(10
10
)]
log
(N
o
)
max
10
10
10
where P is the return period in years for an earthquake of magnitude M or greater and N
o
is the rate of occurrence of earthquakes, given as the number of earthquakes of
Magnitude zero or greater per year per unit volume or per unit area. An area of
100
100 kilometres is commonly used. This must be converted to per square kilometre
or per cubic kilometre for ground motion calibrations.
“b” is the Richter “b” value, which gives the relative number of small earthquakes to
large. It is the logarithm to the base 10 of the ratio of the number of events exceeding
M
1 to the number exceeding M. A value of 1.0 would correspond to ten times as many
earthquakes exceeding magnitude M as would exceed magnitude M
1.
M
max
is the magnitude of the maximum credible earthquake for the area. Because of the
low probability of very large earthquakes, M
max
does not critically affect ground motion
recurrence estimates for return periods up to hundreds of years, especially when the “b”
value is high. However, it is more important for low AEP events such as may be important
for design of high hazard dams. That maximum credible magnitude causes the magnitude
recurrence plot to flatten out and asymptote to that value.
The seismicity parameters N
o
and b are determined using available earthquake data. In
most places there are insufficient earthquake data to determine M
max
from historical
records, but values can be estimated by considering the tectonic situation and local fault
dimensions. The “b” value is much more critical than N
o
, and a small adjustment to “b”
will give a large change in predictions.
Hazard evaluation depends on the extrapolation of data from small earthquakes to
larger magnitudes. The lower the “b” value, the greater the resulting hazard estimate. A
catalogue with missing small earthquakes will give an invalid low “b” value, and an esti-
mated hazard that will be too high. A catalogue with smaller quarry blasts incorrectly
identified as earthquakes will give a high “b” value and an extrapolation that is non-
conservative.
Figure 12.5
shows the output of a probabilistic assessment of seismic hazard expressed
in peak ground (bedrock) acceleration - as would be required for analysis of liquefaction.
Note that the contribution of earthquake magnitude is separated, because the magnitude,
as well as peak ground acceleration is important.
12.3.2
Seismic hazard from known active faults
This method is used where faults in the vicinity of the dam can be identified. The proce-
dure will usually involve:
(a) Identification of major faults within the vicinity of the dam. This may involve an area
up to several hundred kilometres from the site.
Figure 12.6
shows an example;
(b) Assessment of whether the faults are active or potentially active, by consideration of
whether modern (including small) earthquakes have been recorded along the fault.
This may also involve geomorphological studies, e.g. of displaced river terraces
and/or trenching across faults to identify past displacements and determine their ages;
(c) Assessment of the Maximum Credible Earthquake magnitude on each identified fault.
This will usually be determined by considering the length and/or area of the fault and
the type of fault. The likely focal depth and, hence, focal distance are also estimated;
(d) For a deterministic approach, assess the peak ground acceleration (pga) at the project site
resulting from the MCE at each of the faults and determine the most critical earthquake.
