What are Earthquakes?
Earthquakes are powerful manifestations of sudden releases of strain energy accumulated during extensive time intervals. They radiate seismic waves of various types which propagates in all directions through the Earth's interior. The passage of seismic waves through rocks cause shaking that we feel as earthquakes.
What is a fault?
A fault is a fracture having appreciable movement parallel to the plane of the fracture. Faults are of practical importance because they generate earthquakes. It is important to understand faults for designing the ling-term stability of dams, bridges, buildings, power plans etc. We need to understand the basic anatomy of faults, to appreciate their behavior. The most obvious feature related to faulting is the displacement of marker layers along the actual movement surface called the fault plane.
Can earthquakes be predicted?
No. at least not their time of occurrence. However earthquake prone areas can be identified and one can estimate the intensity of the earthquake, as and when they would occur, in advance, to a fair degree of accuracy.
How does an earthquake damage buildings?
Ground Shaking
This is the most common and the principal cause of earthquake – induced damage. As the earth vibrates the building on the ground starts responding to the vibration in varying degrees depending upon how these have been designed and constructed.
Ground Failure
There are four types of ground failures i.e., fault, landslides, settlement and soil liquefaction.
In case of fault, the ground ruptures along a fault zone. This rapture may be very limited or may extend over hundreds of kilometers, and can be horizontal, vertical or both. A building, which was directly standing on this fault will be severely damaged. Landslides can destroy a building whereas settlement would only damage the building.
Soil Liquefaction is a phenomenon where low density saturated sands of relatively uniform size starts behaving like a jelly with no strength to hold a building up, and the building just sinks in or gets tilted on one side. The phenomenon of liquefaction is particularly important for dams, bridges, underground pipelines and buildings close to river banks, sea shore or large lakes.
Tsunamis
These are seismic sea waves and are generally produced by a sudden movement of the ocean floor. The water waves rush towards land suddenly and with great velocity causing havoc on coastal areas.
Fire
Earthquake does not itself cause fire, however structures can catch fire as a consequence of damages caused by earthquake. In such cases often it is difficult to control fire as earthquake can also cause damage to water supply as well as cause traffic jams making access by fire fighting personnel and equipment difficult.
Can structures be made earthquake proof?
Yes. Structures can be designed and constructed to withstand a particular intensity of earthquake. The cost of making structures earthquake proof is high. Therefore, only some specific types of structures such as atomic power stations, dams, refineries where consequences of damage to the structures are serious need be designed to make them earthquake proof, normal residential building can be made earthquake resistant.
What is an earthquake resistant structure?
It is a structure which does not collapse during an earthquake though, at times, it may suffer damage. The idea is to prevent the structure from collapsing so that lives and valuable kept in the structure are saved. The damaged part can be repaired at a fraction of the cost that one would have to incur in making the structures earthquake proof.
How does one make a structure earthquake resistant?
The exact method depends upon the type of the building being designed; however there are some general principles, which are followed. These are:
Structures should not be brittle. It should not collapse suddenly. Rather it should be tough and should be able to deflect or deform considerably.
Resisting elements such as bracing or shear walls must be provided evenly throughout the building, in both directions, side-to-side as well as top to bottom
All elements such as walls and the roof are tied together to retain integrity of the structure during shaking of the structure because of the earthquake. Pathways are provided in the building to enable forces to get transmitted across connections and thereby separation of parts is prevented.
Special care is taken of the foundation. It is tied together well as well tied firmly with the walls. Material used during construction should be of good quality and should be protected from debilitating effects of rain, sun, insects etc.
Can an existing structure be made earthquake resistant?
Yes and the process is called retrofitting.
Are there any B.I.S. Standards on making structures earthquakes resistant?
Yes, there are 9 nos. of such standards and one companion special publication issued by Bureau of Indian Standards.
Are high-rise buildings unsafe?
Not necessarily. Earthquake resistant high-rise buildings are common in Japan and USA. In India too the requisite expertise is available; there are several earthquake resistant structures. The catch is in the design and construction.
What should the occupants of a building do during earthquake?
Earthquakes do not cause death – buildings do. Falling heavy objects or collapsing of walls and roofs hurt people. The collapsing walls and the vibrations can cause short circuiting of live electric wires and cause electric fires. A burning gas stove left to itself too can cause fire. Hence the following steps are advised:
Keep calm
Keep away from windows, doors, almirahs, mirrors, balconies etc.
Stay away from falling bricks or stones and try to get under a study table or a cot to avoid getting hurt by falling objects.
Switch off electric connections and gas connections.
If in open space avoid going near a tall building, street light lamppost, chimney, hoardings etc. These may fall upon you.
What important Variables need to be considered while designing structure?
Variables need to be considered while designing structures are such as:
Shape of the building. Different shaped buildings behave differently. Geometric shapes such as a square or rectangle usually perform better than buildings in the shape of an LT.U., H, +, O, or a combination of these.
Various materials used to construct the buildings can be used. Each material behaves differently. Ductile materials perform better than brittle ones. Examples of ductile materials include steel and aluminium. Examples of brittle materials include brick, stone and unstrengthened
Height of the building. Different heights shake at different frequencies
Soil beneath the building
Regional topography
Magnitude/duration of the quake
Direction and frequency of shaking
The number of earthquakes the building has previously had and the kinds of damage suffered, if any
Proximity to other buildings
Intended function of the building (e.g. hospital, fire station, office building).
Why are Earthquakes measured differently by different people?
The intensity of the earthquake is valued according to the Richter Scale (Charles Francis Richter scale (Charles Francis Richter, 1900-1985) or the modified Mercalli scale (Giuseppe Mercalli scale (Giuseppe Mercalli, 1850-1914). The first scale furnishes an evaluation (magnitude) of the quantity of freed energy, while the seconds scale assigns a degree to the effects on the environment. Thus although each earthquake has a unique Magnitude, its effect will vary greatly according to distance, ground conditions, construction standards and other factors which are expressed in Mercalli Intensity Scale.
Rating the Intensity of an earthquake’s effects does not require any instrumental measurements. Thus seismologists can use newspaper accounts, diaries and other historical records to make intensity ratings of past earthquakes, for which there are no instrumental recordings. This form of rating also helps communicate the risks to habitations more clearly.
Such research helps promote our understanding of the earthquake history of a religion, and estimate future hazards.
