Earthquakes: How do we defend ourselves

Strong Shock - overthrow of valuable objects from cupboards, fall of plaster, swinging of bulbs and ceiling fans, general panic, and minor but no real damage to the building.
Earthquakes:  How do we defend ourselves
Representational Picture

In accordance with the intensity of seismic activities, observed over vast period of time in different parts of the country, India has been divided in to five seismic zones, 1 to 5, with intensity of earthquake least in zone 1 and maximum intensity in zone. J &K falls in to 4 and 5, where plate tectonic disturbances are prominent. For buildings, utility services, and vital infrastructure, intensity ratings of earthquakes from 7 to 10 on richter scale are important and potentiality of damage likely to be caused in case of severe earthquake is indicated here under

Intensity 7:  Strong Shock – overthrow of valuable objects from cupboards, fall of plaster, swinging of bulbs and ceiling fans, general panic, and minor but no real damage to the building.

Intensity 8: Very Strong Shock – cracks in the walls, shatters old buildings and producing slight cracks in the ground.

Intensity 9: Extremely Strong Shock – uproots buildings, causing huge cracks in the ground and  triggers landslides.

Intensity 10: Extreme Intensity Shocks – total destruction in the region

Small earthquakes are caused by volcanic eruptions, these may cause considerable damage in small areas, but by far the most important earthquakes from engineering stand point are of tectonic origin, which are associated with large scale strains created in the crust of the earth.

Plate tectonic disturbance is a phenomenon,which could be explained by a theory, called Elastic Rebound Theory, implying that strain energy that accumulates due to deformation in the earth's mass gets released through rupture, when it exceeds the resilience of storing material (earth's crust).

The energy thus released in the form of P, S, R and L waves called primary, secondary, rayleigh and love waves, which impart energy to the media (earth's surface) through which they pass and vibrate the structures standing on the earth's surface. It holds that surface of earth consists of about a dozen giant plates of rock, 100 km thick, which float on the earth's semi molten mantle and propelled by an undetermined force.

The plates are in constant motion and where they meet are fault lines. Friction temporarily locks them in place, causing strains and stresses to build up near their edges. Eventually when the stresses in the crust of the earth fall short of standing the strains, the rock fractures, allowing these plates to resume their motion. The energy released causes major earthquakes and the dangers imposed are nonpareil in many ways.

The public desires that earthquakes may be predicted like weather. Research efforts are on to analyse precursor effects to make predictions, as indicated above, when fracture takes place in plate tectonic disturbance, energy is released in the form of different energy waves consisting of primary waves, which travel fastest, having highest frequency and small amplitude with the least destructive impact, secondary waves, travelling slower having less frequency and higher amplitude and most destructive impact and rayleigh/love waves having least frequency too and even higher amplitude travel slowest, having very destructive value. Using this time difference in the arrival of P, S and R/L waves and due to the highly sensitive censors installed deep inside earthquake prone areas, where the sensitive censors pick up the motion of P-waves, which are first to arrive in the earthquake prone area with the least destructive impact and the people in the area could be warned well in time to move out of their houses in to the open before the arrival of S, R, L waves, which are the most destructive,thereby thousands of lives could be saved.

Most of the casualties take place due to collapse of the structures during major earthquakes and in order to avoid or minimise the loss to the lives of the people, the only course open to engineers is to design the structures that are earthquake resistant, not earthquake proof, which in the event of major earthquakes undergo large deformations and damage but do not collapse, thereby avoiding major loss to property life.

The earthquake resistance could be achieved by conceiving of sound structural form and configuration by the architect and the structural engineer proportioning the member sizes.

If the configuration is not sound, the structure becomes vulnerable for attack easily by the earthquake forces, any amount of strengthening the selection will not be of any use and poor structural form can not be made to behave satisfactorily in an earthquake.

It is of paramount importance that structural engineer be familiar with performance characteristic of different types of materials as well as different configurations and framing systems. There is of course no universal ideal form and configuration for particular type of structure yet there are certain guiding principles to be borne in mind.

The structure should be simple, symmetrical not too elongated in plan or elevation, light in weight; the seismic force is directly proportional to the mass of the system as it is advantageous to use construction materials with a high strength to weight ratio to minimise the inertial forces.

Usually the heavy structures resist the external forces and other imposed forces, other than seismic forces due to their own weight ( gravity structures) such as retaining walls, Dams etc., but in case of earthquakes, plate tectonic disturbance sets up waves, creating a quick and periodic displacement of foundation upon which the structure rests, consequently the structure is subjected to forces created by its own mass known as inertial forces acting against their own structures, greater the mass of structure greater the magnitude of inertial forces and worst the seismic effects.

It does not mean that structures elongated in plan can not be constructed, but elongated plans can be re-configured in to forms of T, L, and U shapes with seismic joints introduced at meeting junctions, thereby breaking the bigger mass into smaller masses to withstand seismic onslaught.

It is also to be asserted that here are limitations in countering the seismic forces under certain conditions, engineers may be able to design a earthquake proof building at a huge cost, which may not fail structurally, but then the important special feature of earthquake phenomenon is that in the soils, consisting of soft clay, sand or silt, where water table is comparatively higher or in the water logged areas, having penetration resistance (N-value) less than 20, the clayey soil is turned in to a jelly called liquefaction under severe earthquakes and founding of earthquake proof structure on a jelly is impossible and will go deep down in to jelly type soil ,suffering tremendous settlement. 

In Kashmir there are structures constructed in stone masonry in mud mortar in the far flung and mountainous areas, using locally available stones which are cheap, where people can hardly afford to buy materials which are comparatively lighter in weight, flexible and ductile in nature but very costlier.

These stone walled structures are not provided with earthquake resistant features such as vertical and horizontal belts at plinth, lintel, roof levels and in the corners and around door/ window openings of the structure. Moreover, wall length and height of the structure has to be restricted to 6 and 2.7 meters respectively. These stone masonry structures crumble down during severe earthquakes as observed during oct 2005 earthquakes that hit J & K.  

Srinagar city in general, and downtown area in particular are full of old and aged buildings, long due for repairs, restoration and retrofitting, separated by narrow alleys, and God forbid, if an earthquake of intensity rating 8 or more on rector scale hits Kashmir, the consequences will be catastrophic in terms of loss to life and property. 

reyaz1@live.co.uk

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