ABSTRACT
Reservoir induced siesmicity is areal problem in highly agriculture and densely
populated areas .this phenomena occur either immediately after filling of the reservoir,
or after a delay of a few years. many characteristics typify those sites where triggered
events have occurred by different mechanisms .
Earthquakes are multiple hazards in the case of a dam ,so this presentation summarises
research concerns and application procedures to mitigate hazards and maintain the safety
of dams .
Introduction
Dams are earth or concrete barriers built across a drainage course to impound
water. The lakes they create are called reservoirs. Dams are among the largest
and most important projects in civil engineering. Although They are expensive
structures, they provide flood control water storage, hydroelectnc power, and many
other benefits.
Dams are affected by normal regional earthquakes, but there are certain types of human
Activity , particularly the impoundment of reservoirs can trigger earthquakes, so
The likelihood of damaging earthquakes must be considered during planning and after
construction to ensure continual safety of downstream habitation , agriculture and
economy
Earthquakes
An earthquake is a series of vibrations induced in the earth's crust by the abrupt rupture
and rebound of rocks in which elastic strain has been slowly accumulating. It is caused by
a slip on a fault, which is a thin zone of crushed rock between two blocks of rock. The
fault can be any length, from a few millimeters to thousands of kilometers. It is a fracture
in the crust of the earth along which rocks on one side have moved relative to those on
the other side.
An earthquake occurs when stresses in the earth's outer layer push the sides of the fault
together. Stress builds up and the rocks slips suddenly, releasing energy in waves that
cause the shaking. Earthquakes can cause severe damage to property and extensive loss
of life.
An earthquake will not kill by itself. However, an earthquake may cause extensive
structural damage and collapse that could cause fatalities and injury.
Earthquakes can occur at any time of the day and are not related to weather patterns. An
earthquake cannot be prevented, although mitigation measures can be taken, particularly
structure measures, to reduce the impact that an earthquake has on the surface
What causes an earthquake
*Tectonic causes
Most earthquakes are caused by movements of the tectonic (tiktoo = generate) plates.
These plates form the earth's crust. Most of these movements we can't feel, they are much
to small to notice.
Only seismographs can registrate them. Each day the crust is moving, so is Africa
moving towards from Europe and it is likely a bay will occur between Germany and the
Netherlands.
Most of these plates are next to each other and won't move. Some plates stick together and
form large plates, which sometimes abruptly move, causing vibrations in the earth's crust
which we might feel as an earthquake. Plates can collide (compression-zone; and may
form a new mountain range or a big canyon) or slip below the other plate (subduction-
zone; and may also form a new mountain range).The place where this happens, many
kilometers below the earth's surface, we call the
hypocentre.
Figure 1 : a) oceanic- continental convergence b) – continental- continental convergence
Why do these plates move? This is due to the flows of hot and less hot rock and material
inside the earth. The warm material wants to move up (warm air rises, cold will drop)
pushing it's way through the other material, causing these vibrations.
Tectonic earthquakes are much more powerful than the earthquakes caused by volcanic
activity. From magnitude 4,8 all earthquakes reported have been tectonic earthquakes
*Volcanic causes.
Smaller earthquakes are quite often caused by moving magma below the earth's surface,
pushing it's way through. Near by active volcanoes these minor earthquakes are regular
and won't cause much damage. Eruptions like the one of the Krakatau (1883), which
could be heard 5000 kilometers away, won't cause seismic waves of any importance.
* human causes
Some earthquakes are caused by human activity such as industrial or military explosions
and various types of cultural noises (traffic, industry, construction works ), which are
examples of controlled seismic sources , where place time of occurrence and source
intensity are determined in advance or , at least, highly predictable . Other types of man –
made seismic sources are induced or triggered events , . To some degree, hypocentral
locations of induced earthquakes are predictable. However, their size and time of
occurrence are not Many man-made events such as underground nuclear explosions and
some of the reservoir- induced earthquakes, are large enough to be received
teleseismically at seismograph stations throughout the world A brief overview of major seismic source types is sketched in figure.. 2
figure2
reservoir triggered earthquakes
over 75 dam reservoirs have been found to trigger seismicity after their filling
(Gupta and Rastogi , 1976 ; Gupta , 1992).
There are many known cases of reservoir induced seismicity, and they seem to fall into
tow broad categories as far as the response of the crust is concerned see table 1 .
Table1
DELAYED RAPID CASES
Seismic activity does not occur until some years after the reservoir has been filled . Increase in seismicity almost immediately upon reservoir filling. TIMING
Larger earthquakes Small earthquakes MAGNITUDE
Greater depth Shallow DEPTH
Often at some distance ~10 Km from the deep part of the reservoir . In the immediate vicinity or just below the reservoir . FOCUS
KOYONA DAM (INDIA)*
ASSWAN (EGYPT)*
OROVILLE (CALIFORNIA)
*NUREK RESERVOITADJIKISTAN).
*MONTICELLO(CAROLAINA)
*KARIBA(ZIMBABWE)
EXAMPLES
Characteristics of reservoir induced seismicity
Numerous studies have been carried out in the attempt to determine what unique
physical or geological characteristics typify those sites where triggered events have
occurred, only four significant correlations seem to emerge:-
(1) Triggered events are more likely beneath large and, particularly, deep reservoirs than
beneath those of smaller size.
(2)Triggered events are more likely, at least during reservoir filling, in areas of nor¬mal
and strike-slip faulting than in areas of reverse or thrust faulting .
(3)The largest triggered events have occurred predominantly in areas of late
Quaternary faulting (Packer et a!., 1981).
(4)Triggered events are more likely during periods of rapid changes in wa¬ter level than
at other times, as was documented at Nurek Dam, Tadjikistan, by Simpson and
Negmatul¬ laev( 1981) . Some of the more significant earthquakes thought to be triggered
by reservoir filling are listed in Table 2 .
DAM MAX MAGNI-TUDE year Depth Reference
Koyna, India 6.5 1967 100 Gupta and Rastogi (1976)
Kremasta, Greece 6.3 1966 120 Comminakis and others (1968)
Xinfengjiang, China 6.1 1962 100 Sheng and others (1973)
Kariba, Zambia-Zimbabwe 5.8 1963 123 Gough and Gough (1970a; 1970b)
Srinagarind, Thailand 5.8 1983 140 Ghose and Oike (1987)
Marathon, Greece 5.7 1938 60 Galanopoulos (1967)
Oroville, USA 5.7 1975 204 Toppozada and Morrison (1982)
Varragamba, Australia 5.4 1973 104 Guha and Patil (199C)
Aswan, Egypt 5.3 1981 110 Kebeasy and others (1987)
Coyote Valley, USA 5.3 1962 22 Toppozada and Cramer (1978)
Akasombo, Ghana 5.3 1964 109 Guha and Patil (1990)
Kinnersani, India 5.3 1969 62 Guha and Patil (1990)
Volte Grande, Brazil 5.1 1974 32 Guha and Patil (1990)
Benmore, New Zealand 5.0 1966 86 Adams (1974)
The largest generally accepted induced event to date is of magnitude 6.5 (Koyna, 1967),
and there is no particular reason to assume that this is the largest event that is physically
possible. Nevertheless, in view of the uniqueness of the Koyna event---and the thousands
of dams that have not experienced similar earthquakes---it would appear unduly
conservative at this time to assume that still larger triggered earthquakes should be
considered in the siting and design of most structures. In areas close to major active
faults, larger design earthquakes are often stipulated, of course, as representative of
possible naturally occurring events.
Prediction of Reservoir Induced Seismicity
It is not easy to predict whether a new reservoir will experience reservoir induced
seismicity, because the two most important factors - the state of stress and the rock
strength at earthquake depths - cannot be measured directly.
This is the same reason why prediction of normal (non-induced) earthquakes is normally
Unsuccessful
