Understanding earthquakes, an engineering view: the region should be divided into zones.

(From Business Recorder)

Byline: AFTAB MUFTI, SHAMIM A. SHEIKH, BAIDAR BAKHT

An earthquake is a sudden motion or trembling of a segment of the earth caused by the abrupt release of energy accumulated slowly at a fault line or crack in the crust of the earth.

Some of the energy is released in the form of seismic waves, which are vibrations caused by the movement of rocks within the earth's crust. These vibrations produce the movement we experience during an earthquake.

The point on the earth's surface directly above the location where energy is released suddenly is known as the epicentre of the earthquake. Seismic waves radiate outward from the epicentre similar to the widening ripples on the surface of water when a pebble is dropped into a calm pond.

This article provides some information about earthquakes in general, including the causes of earthquakes and an explanation of how the intensities of earthquakes are measured.

Areas prone to seismic activity on the South Asian Subcontinent are identified, and the role of seismic zoning maps in the development of national construction standards is also discussed with particular reference to the Canadian practice.

WHAT CAUSES AN EARTHQUAKE?

The causes of earthquakes can be divided into two categories: natural and man-made.

NATURAL EARTHQUAKES:

The nearly 50 miles thick crust of the earth is made up of about twenty pieces, known as tectonic plates. Some of these plates are as large as continents. A fault line is the line where tectonic plates meet. According to the glossary of geology for the American Geological Institute, most earthquakes occur along the fault lines.

As two tectonic plates try to move with respect to each other at fault lines, the movement is initially resisted by the jagged mating surface leading to the accumulation of energy. When the mating surface between the tectonic plates can no longer sustain it, the accumulated energy is suddenly released in the form of an earthquake.

Visualise two tectonic plates, one pushing against the other. The plates initially do not move and the energy builds up. Suddenly, the plates slip and slide past one another causing massive vibrations, and producing an intraplate earthquake. The phenomenon of building up and sudden release of energy at a fault line.

THE EARTHQUAKE ACTIVITY IN PAKISTAN IS MAINLY GOVERNED BY FAULT LINES BETWEEN THREE TECTONIC PLATES: Indian, Arabian and Eurasian plates. The fault line between the Eurasian and Indian plates lies in the northern and western sections of the country.

The Indian plate is moving northwards at a speed of about 5cm a year, thus pushing the southern edge of the Tibetan plateau towards the north.

The same force led to the formation of the Himalayas many eons ago. Slips at the Eurasian-Indian fault line cause earthquakes along the Himalayas, all the way to Myanmar. The slip at the African-Indian fault line was responsible for …