Earthquake A sudden and brief period of intense ground shaking. Earthquakes are usually associated with volcanic activity or movement along a fault. The point of origin of an earthquake is called the focus. The point on the ground surface immediately above the focus is called the epicentre. When an earthquake occurs, shock waves (seismic waves) travel outwards like ripples in a pond.
(i) primary or P waves, which are the fastest;
(ii) secondary or S waves;
(iii) long or L waves, which despite
being the slowest are responsible for causing the greatest destruction because they travel through the crust only.
Earthquakes are recorded by seismographs and the time-interval between the arrival of P and S waves at a seismic station can allow calculation of distance to the earthquake focus and time of occurrence. The most powerful earthquakes are associated with Features of an earthquake major fault-lines – for example, the San Andreas ‘mega shear’, which gave rise to the great San Francisco earthquake of 1906 and the severe Turkish earthquake on the Anatolian Fault in 1999. The magnitude of earthquakes (a term for the total energy released by an earthquake) is measured by the open-ended Richter Scale, where each point on the scale represents a x30 increase in the amount of energy release. The effects of an earthquake can be measured using the Modified Mercalli Scale, which has 12 gradations indicated by Roman numerals (. Earthquakes represent a serious hazard to human activity accounting for 47% of all fatalities by natural disasters in the last millennium and 35% of all economic costs. The most severe earthquake of the last millennium occurred at Shaanxi, China, causing 830,000 people to lose their lives. Increasingly it is believed that inappropriate building construction and the lack of emergency preparedness are partly to blame for the sheer scale of many disasters.
The point on the surface of the Earth lying immediately above the focus of an earthquake. The latter usually occurs at a depth of 0-50 km; however, ‘deep focus’ quakes, with a depth of origin greater than 250 km, have been identified. The world distribution of epicenters shows a marked concentration around the margins of the Pacific Ocean, along the center-line of the Atlantic, and through the Mediterranean basin into Turkey, Iran and beyond. In other words, along plate margins. Generally, the greatest destruction associated with an earthquake occurs close to the epicenter. There are, however, exceptions. In the 1985 earthquake that damaged large parts of Mexico City and killed 7000 people,
the epicentre was hundreds of kilometers away. The disaster occurred because the city had been built on unconsolidated lake bed sediments that became unstable when affected by the earthquake’s shock waves