Md Habib Mansur

On December 8, a front page news item stated that 97 people were killed as an earthquake hit Indonesia. Bangladesh has also experienced a few quakes in the recent past. So, naturally we become worried with this sort of news. We, the residents of Dhaka where urbanisation has been going on without any proper planning, and in most cases, without properly following any building codes, always remain in fear of a big scale tremor as we are in an earthquake zone.  
We all know that earthquakes are the sudden shaking of the ground. Scientists are not yet able to forecast when or where a big quake is going to hit. From ancient times, there have been many myths regarding the cause of  earthquakes. In the olden days, the Japanese believed that earthquakes are caused by a giant catfish called ‘namazu’! This wiggly creature supposedly lives inside the Earth. Greek philosopher Aristotle opined that quakes were caused by wind blowing through underground caves. A British scientist has claimed that earthquakes were caused by underground water which dripped on Earth’s hot core, creating enough steam to shake the earth like a lid on a boiling pot.
Besides, there are some phenomena people claim to have observed before a big earthquake, including bears coming out of hibernation and fish jumping into air and landing on shore.
However, with the advancement of science and technology all these myths have gone and scientific reasoning is coming forward. Seismology involves scientific study of all aspects of earthquakes. Seismologists measure strength and location of quakes by using a seismograph. The Richter scale, invented in 1935 by seismologist Charles F Richter, measures quake magnitude or strength on a numerical scale.
The Earth consists of a solid metal inner core, with a temperature of about 4000 C; a molten metal outer core; a mantle composed largely of  rock; and two different types of crust _ continental and oceanic.
From core to crust, the Earth is nothing but a big ball of heat and pressure. The crust is broken into huge slabs, called tectonic plates. These plates fit together well, but they are moving. This movement is so slow that our hair grows six times faster than the movement of those plates. The movement creates massive amount of tension in the rocks deep inside and they stretch and bend to absorb the strain. Sometimes, the Earth releases the tension all at once as vibration, and when it reaches the surface of the Earth, it is called earthquake.
Where an earthquake originates is called the focus, or hypocentre. The point on the Earth’s surface directly above the focus is called epicentre. Earthquake waves begin to radiate out from the focus and subsequently form along the fault rupture. A fault is a planar or gently curved fracture in the rocks of the Earth’s crust. Faults may be vertical, horizontal or inclined at any angle.
Volcanic activity sometimes causes earthquake, too. Artificial induction by human activities, such as detonation of large underground nuclear explosion, excavation of mines or filling of large reservoirs may also cause an earthquake.
Earthquakes often cause dramatic geomorphological changes, including ground movement either vertical or horizontal. For example, it is believed that the Indian Subcontinent was attached to Africa once. Then it broke off and drifted on its own, heading northeast for about 100 million years until it crashed into the Eurasian landmass. India also had to push the oceanic crust out of the way by forcing it under the Eurasian plate. As the Indian plate slid underneath Eurasia, it squeezed layers of the old ocean floor up to the jagged folds of the Himalayas, the world’s youngest and highest mountains.
Also, a catastrophic ocean wave may be caused by an earthquake that can shift a large amount of seawater to create a huge tidal wave, called tsunami, like the one that hit the Indian Ocean in December 2006. Tsunamis may also be caused by eruption of underwater volcanoes.

Reference: Encyclopedia Britannica, Microsoft Encarta and Discovery.