Earthquakes
caused by the filling of a huge reservoir in the southern
African country of Lesotho have terrified local people for more
than a year. Houses in seven villages beside the reservoir of
Katse Dam have
been damaged by tremors, and in the village of
Mapeleng, 11 houses were made uninhabitable by the quakes. In
late January 1997, twelve families left Mapeleng, abandoning homes
which were damaged more
than a year ago by earthquakes. Tremors
continue to strike the area, according to the World Bank, a project
funder.
Reservoir–induced seismicity (RIS) is a widely recognized but
little
understood phenomenon that has occurred near at least 200
reservoirs; 32 of those cases had quakes larger than 4.0 magnitude
on the Richter scale. The biggest quake from RIS is thought to
be a 6.3
quake from Koyna Dam in India in 1967, which killed 180
people.
The 182–meter–high Katse Dam is one of five dams in the World
Bank–sponsored Lesotho Highlands Water Project (LHWP), which will
divert water from Lesotho's rivers to South Africa's Gauteng Province.
Katse began to experience minor earthquakes in November 1995,
just one month after it began to fill. According to project
authorities,
the dam is built to withstand a magnitude 6.5 earthquake within
a 20–kilometer range; the region has experienced earthquakes as
high as 6.0 without the dam.
After the first quakes
in Lesotho, project authorities installed
temporary replacement housing for the eleven Mapeleng families
whose homes were destroyed, and promised to move all frightened
villagers to a new location. The
temporary housing was made of
uninsulated metal, a poor choice for the area's harsh climate.
Six feet of snow fell in the Highlands last winter.
In August 1996, project authorities said they
hoped to have seriously
damaged houses rebuilt "within six months." In late
January 1997, the Highlands Church Action Group (HCAG), a Lesotho
nongovernmental organization monitoring the
project, wrote that
the 12 families left Mapeleng to "seek refuge in other villages"
because to date "absolutely no progress" had been made
on resettling the village or rebuilding
houses. "We would
like [project authorities] to comply with the letter and spirit
of the treaty to not worsen the lives of the communities in the
project area," HCAG says."We request
that [project authorities]
take action to accommodate these people as soon as possible."
The project treaty requires that people affected by the project
not be left worse off.
When asked
what they thought about the temporary replacement housing,
Mapeleng residents told International Rivers in August 1996, "They're no good.
The walls are like paper, and they're very cold."
- Lori
Pottinger, Director of the Southern Africa Program for
International Rivers, says:
- "Mapeleng residents whose homes were dangerously damaged
have been living in these inadequate, uninsulated sheds for
more
than a year now. Given that the project's highly complex engineering
works are proceeding on schedule, there is no reason why the
social and environmental aspects of the project are lagging
so
far behind stated benchmarks."
In addition to the destroyed houses, another 50 that were cracked
by the tremors remain mostly unrepaired to this day. Four springs
also dried up
because of the tremors, although one has started
flowing again. In April 1996, a quake caused a 1.5–kilometer–long
crack in the village.
Earthquake damage is only the latest in a long list of
problems
the dam has brought local people problems which project authorities
have been mostly unable to resolve in the ten years of construction.
– 30 –
BACKGROUND ON RESERVOIR–INDUCED SEISMICITY
Excerpted from Silenced
Rivers: The Ecology and Politics of Large Dams
by Patrick
McCully (Zed Books, UK, 1996)
As with most aspects of seismology, the actual mechanisms of RIS
are not well understood, and it is impossible to predict accurately
which dams
will induce earthquakes or how strong the tremors are
likely to be. Most of the strongest cases of RIS have been observed
for dams over 100 metres high –– but dams just half this height
are also
believed to have induced quakes. Reservoirs can both
increase the frequency of earthquakes in areas of already high
seismic activity and cause earthquakes to happen in areas previously
thought to be
seismically inactive. The latter effect is the most
dangerous as structures in areas thought to be quiescent are not
built to withstand even minor earthquakes.
The most widely accepted
explanation of how dams cause earthquakes
is related to the extra water pressure created in the microcracks
and fissures in the ground under and near a reservoir. When the
pressure of the water in the
rocks increases, it acts to lubricate
faults which are already under tectonic strain, but are prevented
from slipping by the friction of the rock surfaces.
For most well–studied cases of RIS,
the intensity of seismic activity
increased within around 25 kilometres of the reservoir as it was
filled. The strongest shocks normally occured relatively soon
–– often within days but sometimes
within several years –– after
the reservoir reached its greatest depth. After the initial filling
of the reservoir, RIS events normally continued as the water level
rose and fell but usually with less
frequency and strength than
before. The pattern of RIS is, however, unique for every reservoir.
Also available: Excerpts from "Seismic Phenomena at Mapeleng
Village – Katse Reservoir –
Report by Experts," February
1996
