Environmental Engineering Reference
In-Depth Information
ICOLD (1983) conclude that:
- Earthquakes of magnitude 5 to 6.5 were induced in 11 of 64 recorded events;
- The greatest seismic events have been associated with very large reservoirs (but there is
insufficient data to show any definite correlation between reservoir size and depth and
seismic activity);
- In view of the above, a study of possible induced seismic activity should be made at
least in cases where the reservoir exceeds 10
9
m
3
in volume, or 100 m in depth;
- The load of the reservoir is not the significant factor, rather it is the increased pore
water pressure in faults, leading to a reduction in shear strength over already stressed
faults.
ICOLD (1983) and (1989c) give more details and references on this issue.
ANCOLD (1998), as advised by G. Gibson, indicates that reservoir induced seismicity
events usually occur initially at shallow depth under or immediately alongside a reservoir.
As years pass after first filling and groundwater pore pressure increases permeate to
greater depths and distances, the events may occur further from the reservoir. This occurs
at a rate of something like one kilometre per year.
Reservoir induced seismicity is experienced under new reservoirs, usually starting
within a few months or years of commencement of filling and usually not lasting for more
than about twenty years. Once the stress field and the pore pressure fields under a reser-
voir have stablised, then the probability of future earthquakes reverts to a value similar to
that which would have existed if the reservoir had not been built. Most of the earthquake
energy does not come from the reservoir, but from normal tectonic processes. The reser-
voir simply acts as a trigger.
If there is a major fault near the reservoir, reservoir induced seismicity can produce
earthquakes exceeding Magnitude 6.0 (Xinfengjiang, China, 1962, M6.1; Koyna, India,
1967, M6.3). Such events will occur only if the fault is already under high stress. A number
of Australian reservoirs have triggered earthquakes equalling or exceeding Magnitude 5.0
(Eucumbene, 1959, M5.0; Warragamba, 1973, M5.0; Thomson, 1996, M5.2).
It is more common for a reservoir to trigger a large number of small shallow earth-
quakes, especially if the underlying rock consists of jointed crystalline rock like granite
(Talbingo, 1973 to 1975; Thomson, 1986 to 1995). These events possibly occur on joints
or local minor faults rather than major faults, so are limited in size and only give magni-
tudes up to 3 or 4. There is no hazard to the dam from such low magnitude reservoir
induced earthquakes, even if they occur regularly. Their shallow depth means that they
may often be felt or heard.
12.3
EVALUATION OF SEISMIC HAZARD
12.3.1
Probabilistic approach
This method is generally used where active faults cannot be identified, or to assess
the hazard from earthquakes in areas away from active faults. The probabilistic estima-
tion of ground motion requires the following seismicity information about the surround-
ing area:
- The rate of occurrence and magnitude of earthquakes;
- The relative proportion of small to large events (b value);
- The maximum earthquake size expected (maximum credible magnitude);
- The spatial distribution of earthquake epicenters including delineation of faults.
