an earth quake is a shaking of the ground. it occurs when masses of rocks changes position below the earth surfaces. the shifting masses send out shock waves that may be powerfull enough to wresk buildings, roads and bridges. they may even alter the surface of the earth,thrusting up cliffs and opening fissures in the ground.
earthquakes go on almost continuosly and it can be detected by only sensitive intrument called seismographs.earthquake produce such tragic and dramatic effects as destroyed cities, broken dams, earth slides, giant sea waves and volcanic erruptions.
HOW EARTHQUAKE TAKE PLACE
earth quakes may be caused by volacnic eruptions or by man made blasts of dynamite or atomic energy. a more frequent and powerfull type of earth quake is the ‘tectomic temblor’ pressure from within the earth put strain upon great rock masses beneath the earth’s surface. the strain builds upuntil suddenly the masses are displayed along some line of weakness known as fault. the masses slip and slide in opposite directions along thsi fracture in the rock,shaking and ground above. the masses may move up and down, sideways or both verticaly or horizontally.
on the earth’s surface, displacement fo the ground may vary from a few inches to many feet somtimes a fault line appears on the surface of the earth, showing the location of the fault beneath.
the shock (seismic) waves caused by the shifting rock move out in all directions, in a great eatth quake shocks may be felt by people a thousand miles away from the centre. seismographs on the other side of the world pick up the waves,this is because the shock waves of a largeearth quake move through the entire structure of he earth and also travel all over its surface.
there are two classes of seismic waves, interior and surface which travelthrough the earth, in turn are of two type. the primary wave, or p wave,is the faster of the two and reaches the earth’s surface first. it alternatively compresses and expands the rock through which it passes. the secondary wave or S wave is a ‘ shake’ wave. it it moves partical from side to side, at right angles top its direction of travel. by comparing the arrival times of p waves and s waves qar seismological observing stations scientists can determine the centre of earth quake thousand of miles away.
the most powerfull shock waves, however travel on the earth’s surface. surface waves of which there are atleast two types travel more slowly then interior waves.
measuring earth quakes
A seismograph records the pattern of shock waves on a revolving drum paper, thin wavy lines show the strength of the various waves and the times at which occured. the tracing is called a seismogram. the study of earth quakes is seismology.
JHON MILNE, an english scietist was an early inventor of seismograph. basically, the instrument is a heavy pendulum with a stylus, or needle suspended above in front of a revolving drum. during an earthquake the pendulum and needle remain stable while the drum on the base moves, recording the waves patterns,. in some seismographs a ray of light traces the pattern of photographic pape.
the strength of an earth quake can also be determined by the amount of damage done or by instrument readingsthe modified meracali intensity scale is commonly use to indicate the amount if destruction caused by an earthquake. it defines 12 levels of earthquake strength. the richter magnitude scale grades earthquake on a 1 to 10 scale. it is based upon the amount of energy released by the rock movements rather than upon surface damage.
where earth quakes occur
most earthquakes take place in one of two great ‘earth quake belts’ that gridle the world. the belts coincide with the more recently formed mountain ranges and with the volcanic belts. one circles the pacific ocean along the mountainous west coasts of north and south america and truns through the island areas of asia. more earthquakes occur in the pacific belt than in the pacific belt than in the second belt.
the rock movements that causes earthquakes occur at varying depths beneath the surface. the point at which rock movement is known as the focus of an earthquake. most earthquake are shallow, with focusses no more than 37 miles below the surface. intermediate earthquakes have focusses between 37 and 93 miles below the surface. a deep earthquake has a focus between 93 and 435 miles down. none has an ever been recorded below 477 miles.
the point of greatest destructions is an earth quake is the epicenter. this is the point on the surface directly above the ground focus.
factors other than the earthquake intensity influence the amount of destructions caused,. soft ground for example magnifies the shocks. an area underlaid rock will not severally shaken.
the structure of buildings is also important. building with steel frames eveloped by reinforced concrete withstand earthwuakes very well.solid foundations are also needed for buildings in earth quake zones. balconies, parapets and similar ornamentation should be avoided since these may break off during earth quake, creating another hazard.
some of the side efffects of earthquakes cause the most damage. fires break out and rage unchecked over wide areas, since water mains are often broken. seismic sea waves are another danger. these are known by the japanese name of tsunamis, though they are are often called inpatly called tidal waves. they are not caused by the tides but are probably of vertical fault movements or landslides under the sea. in the deep ocean the waves travel at speeds of over 600 miles an hour but remain low in height. as the tsunamis approach shores they build up into powerfull walls of water 90 feet or more in height .
while seismograph can record the shocks that sometimes preecede an earthquake, scientist havent yet learned how topredict when a earthquake wil occur. certain signs however do give advance warning of destructive tsunamis.