While they inspired dread and superstitious awe since ancient times, little was understood about them until the 20 th century. Seismology, which involves the scientific study of all aspects of earthquakes, has yielded plausible answers to such long-standing questions as why and how earthquakes occur. Cause of Earthquake According to the Elastic Rebound Theory (Reid 1906), earthquakes are caused by pieces of the crust of the earth that suddenly shift relative to each other. The most common cause of earthquakes is faulting; i.e., a break in the earth ’ s crust along which movement occurs. Most earthquakes occur in narrow belts along the boundaries of crustal plates, particularly where the plates push together or slide past each other. At times, the plates are locked together, unable to release the accumulating energy. When this energy grows strong enough, the plates break free. When two pieces that are next to each other get pushed in different directions, they will stick together for many years, but eventually the forces pushing on them will cause them to break apart and move. This sudden shift in the rock shakes the ground around it. Earthquake Terminology The point beneath the earth ’ s surface where the rocks break and move is called the focus of the earthquake. The focus is the underground point of origin of an earthquake. Directly above the focus, on earth ’ s surface, is the epicenter . Earthquake waves reach the epicenter first. During an earthquake, the most violent shaking is found at the epicenter. Earthquake s release the strain energy stored within the crustal plates through ‘seismic waves’ . There are three main types of seismic waves . Primary or P-waves vibrate particles along the direction of wave, Secondary or S-waves that vibrate particles perpendicular to the direction of wave while Raleigh or R-waves and Love or L-waves move along the surface. Earthquake Magnitude A number of measures of earthquake ‘size’ are used for different purposes. From a seismologic point of view, the most important measure of size is the amount of strain energy released at the source, indicated quantitatively as the earthquake magnitude . Charles F. Richter introduced the concept of magnitude, which is the logarithm of the maximum amplitude measured in micrometers (10 -6 m) of the earthquake record obtained by a standard short-period seismograph, corrected to a distance of 100 km; i.e., M L = log 10 (A/A 0 ) ..………………..(1) where A is the maximum trace amplitude in micrometers recorded on a seismometer and A 0 is a correction factor as a function of distance. Earthquake intensity is another well-known measure of earthquake severity at a point, most notable of which is the Modified Marcelli (MM) scale. Nature of Earthquake Vibration Earthquake involves vibration of the ground typically for durations of 10~40 seconds, which increases gradually to the peak amplitude and then decays. It is primarily a horizontal vibration, although some vertical movement is also present. Since the vibrations are time-dependent, earthquake is essentially a dynamic problem and the only way to deal with it properly is through dynamic analysis of the structure.
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