EarthQuakes

Earthquakes
   

         Suddenly shaking and trembling of Ground/ Earth surface is known as Earthquakes. The numerous shocks which continually take place are due to sharp movements along fractures (called faults) which relieve stress in the crustal rocks. Stress accumulates locally from various causes until it exceeds the strength of the rocks, when failure and slip along fractures occur, followed usually by a smaller rebound. A small movement on a fault, perhaps a few centimeters or less, can produce a considerable shock because of the amount of energy involved and the fault may 'grow' by successive movements of this kind. Earthquakes range from slight tremors which do little damage, to severe shocks which can open fissures in the ground, initiate fault scarps and landslides, break and overthrow buildings, and sever supply mains and lines of transport.

 The worst effects are produced in weak ground, especially young deposits of sand, silt and clay. These sediments may shake violently if their module of elasticity and rigidity are insufficient to attenuate adequately the acceleration imparted to their particles by an earthquake. The bedrock beneath them may be little affected by reason of its strength. Lives and property may be saved if earthquake resisting structures are built ( Rosen blueth, 1980). These have frames of steel or wood that is founded directly onto rock whenever possible, and will remain intact when shaken. Dams, embankments, slopes and underground excavations can be designed so as to function whilst shaking (Newmark and Rosenblueth, 1971).

Prior to a major earthquake, strain in the crust accumulates to the extent that small changes may be noticed in the shape of the land surface, in water levels, in the flow, temperature and chemistry of springs, in the magnetic properties of the strained crust and the velocity with which it transmits vibrations, and in the frequency and location of very small (micro-) earthquakes. These precursors are studied in an attempt to predict location and time of major earthquakes. When a major earthquake at sea rapidly changes the elevation of the ocean floor, a volume is created that has to be filled by sea-water. Sea-level drops, sometimes causing beaches in the region to be exposed, and large waves, called tsunamis, may be generated as sea-level reestablishes itself  these can devastate coastal areas when they strike a shore-line.

Most of the active earthquake centers at the present day are located along two belts at the Earth's surface: one belt extends around the coastal regions of the Pacific, from the East Indies through the Philippines, Japan, the Aleutian Isles, and thence down the western coasts of North and South America; the other runs from Europe (the Alpine ranges) through the eastern Mediterranean to the Himalayas and East Indies, where it joins the first belt. These belts are mainly parallel to the younger mountain chains, where much faulting is associated with crumpled rocks, numerous volcanoes are also situated along the earthquake belts. It is estimated that 75 per cent of all earthquake activity occurs in the circum-Pacific belt and about 22 per cent in the Alpine area. Many smaller shocks also occur in zones of submarine fault activity associated with the oceanic ridges, such as the mid-Atlantic Ridge and others in fault-zones on the continents, e.g. the Rift Valley system of Africa. In areas remote from these earthquake zones only small tremors and shocks of moderate intensity are normally recorded. for example, earthquakes in Britain include those at Colchester (1884), Inverness (1901,` 1934), Nottingham (1957), Dent (1970), and Lleyn (1984). All earthquakes are generated in the outer 700 km of the Earth and all destructive earthquakes wherever they occur originate at depths less than 70 km.

The intensity of an earthquake can be estimated from the effects felt or seen by an observer, and such observations are collected and used to determine the centre of the disturbance. They are graded according to a Scale of Intensity such as the Mercalli Scale, which has twelve grades.
         

How to Detect.

        I.            Detected only by instruments.

     II.            Felt by some persons at rest; suspended objects may swing.

  III.            Felt noticeably indoors; vibration like the passing of a truck.

  IV.             Felt indoors by many, outdoors by some; windows and doors rattle.

    V.             Felt by nearly everyone; some windows broken; pendulum clocks stop.

  VI.            Felt by all, many frightened; some heavy furniture moved, some fallen plaster; general damage slight.

VII. Everyone runs outdoors; damage to poorly constructed buildings; weak chimneys fall.

VIII. Much damage to buildings, except those specially designed. Tall chimneys, columns fall; sand and mud flow from cracks in ground.

IX. Damage considerable in substantial buildings; ground cracked, buried pipes broken.

X. Disastrous; framed buildings destroyed, rails bent, small landslides.

XI. Few structures left standing; wide fissures opened in ground, with slumps and landslides.

XII. Damage total; ground warped, waves seen moving through ground, objects thrown upwards.

·       Movement after the breaking of Rock in Earth subsurface is called Aftershocks.

   The observed intensity at points in the area affected can be marked on a map, and lines of equal intensity (isoseismal lines) then drawn to enclose those points where damage of a certain degree is done giving an isoseismal map.

 A more accurate measure of earthquake activity is provided by the amount of seismic energy released in an earthquake; this defines its magnitude, for which the symbol M is used. The Scale of Magnitudes due to C. F. Richter (1952) and now in general use is based on the maximum amplitudes shown on records made with a standard seismometer. The scale is logarithmic and is related to the elastic wave energy (E), measured in joules (1 erg= 10"7joules), an approximate relationship being log E«4.8 -I-1.5 M, M ranges from magnitude 0 to magnitude 9. The smallest felt shocks have M= 2 to 2\. Damaging shocks have Af =5 or more; and any earthquake greater than M= 7 is a major disaster. The Richter Scale of Magnitudes and the Mercalli Scale of Intensities are not strictly comparable; but M= 5 corresponds roughly with Grade VI (damage to chimneys, plaster, etc.) on the Mercalli Scale. The historic record of earthquakes reveals that shocks of large magnitude occur less frequently than those of lesser magnitude. A relationship exists between the magnitude of an earthquake that is likely to occur at a location and its return period, and this relationship is used to select the accelerations that must be resisted by the earthquake resisting structures for the locality.

              When an earthquake occurs elastic vibrations (or waves) are propagated in all directions from its centre of origin, or focus; the point on the Earth's surface immediately above the earthquake focus is called the epicenter,Here the effects are usually most intense. 
        There are three types of waves.

1.     body waves, comprising of compressional vibrations, called primary or P waves, which are the fastest and the first to arrive at a recording station, and transverse or shear vibrations, called S waves, a little slower than the P waves.These P waves are able to travel through both solid rock, such as granite mountains, and liquid material, such as volcanic magma or the water of the oceans.    

2. surface waves, (or L-waves) similar to the ripples seen expanding from
      the point where a stone is dropped into water, and created by Love wave (LQ) and Rayleigh-wave (LR) ground motions. 
     Surface waves are of long period that follow the periphery of the Earth; they are the slowest but have a large amplitude and do the greatest damage at the surface. M is calculated from their amplitude. The vibrations are detected and recorded by a seismograph, an instrument consisting essentially of a lightly suspended beam which is pivoted to a frame fixed to the ground, and which carries a heavy mass. Owing to the inertia of the heavy mass a movement is imparted to the beam when vibrations reach the instrument, and the movement is recorded on a chart on a rotating drum. On this record, or seismogram, time intervals are marked, from which the times of arrival of the vibrations can be read off.

3. The slower wave through the body of rock is called the secondary or S wave. As an S wave propagates, it shears the rock sideways at right angles to the direction of travel. If a liquid is sheared sideways or twisted, it will not spring back, hence S waves cannot propagate in the liquid parts of the earth, such as oceans and lakes.