One moment, the world seems just the way it was yesterday, the day before, last year, or even the day before the day before. All is well with the world. It’s a beautiful sunny day and you are sitting drinking your morning tea or coffee relaxed and enjoying the day. Suddenly there is a rattling of plates and glasses. Within seconds chairs and tables are rocking violently, the fans sway crazily and crockery is falling off the shelves. Help!! Sometimes plaster falls from the ceiling, walls develop cracks or worse still, they collapse. The very earth is shaking!
This shaking motion is called an earthquake, the result of a sudden release of energy from below the surface of the earth. Few disasters can cause such havoc as an earthquake.
Earthquakes have rocked the world since time immemorial. But what causes it? The earth can be divided into four main layers: the solid inner core, the liquid outer core, the mantle, and the crust. An earthquake is caused by a sudden rupture in a fault. A fault is the fracture within the rocky mass inside the earth’s crust. The depth and length of faults vary greatly.
Stress in the earth’s outer layer cause a pushing effect against the sides of the fault. Due to this motion, rocks slip or collide against each other releasing energy. This released energy travels in waves through the earth’s crust and causes the shaking that we feel during an earthquake.
Faults are divided into three main groups. Reverse fault – when two plates collide and one side of the fracture moves on top of another; normal fault -when two plates are moving apart; and strike-slip or lateral – when two plates slide past each other.
Under the surface of the earth, the two sides of a fault are constantly moving, relative to one another. This movement is known as a fault slip. The movement of these two sides is not smooth and is accompanied by a gradual build-up of elastic strain energy within the rocks along the fault. The rocks store this strain like a giant spring being slowly wound up.
Eventually, the strain along the fault becomes too much and the rocks can no longer bear the tension. The fault then ruptures with a sudden movement releasing all the energy built up over the years. This energy is released in the form of vibrations called ‘seismic waves’.
These waves travel along the surface and through the earth at varying speeds depending on the material through which they move. It is actually these seismic waves that cause most of the destructive effects, which we associate with earthquakes.
The location on a fault where the slip first occurs is called the hypocentre, whereas the position directly above it on the ground surface is called the epicentre.
When these seismic waves reach the surface of the earth, they give rise to strong ground motion causing building and other man-made structures to shake or collapse or develop cracks and fissures.
Earthquakes can also cause landslides, sudden eruptions as in the case of a hot lava flow from a volcano or giant waves called tsunamis. Sometimes new land mass are also formed. Such earthquakes are attributed with the creation of the greatest undersea mountain range (stretching from the Arctic Ocean to the Atlantic Ocean and under the Pacific Ocean) and the longest land mountain range (the Himalaya-Karakoram range).
It is estimated that there are 5,00,000 detectable earthquakes in the world each year. Of these only 1,00,000 are felt, and only 100 of them cause actual damage.
Though there are certain parts of the earth where earthquakes are frequent, the majority of earthquakes occur along the earth’s plate boundaries, which carry the continents and the ocean floor. They occur most between the Pacific plate and the North American plate commonly referred to as the Pacific Ring of Fire.
Although it is known why earthquakes occur, there is no reliable method of accurately predicting the time, place and magnitude of an earthquake. So if you feel the earth shake under your feet, you are either in love or it is an earthquake you are experiencing.