As a result of upheavals below its surface, the earth shakes now and then. This shaking of the earth is known as an earthquake.
Few natural events are as violently destructive as an earthquake. It usually strikes without warning, giving off violent vibrations in the process. These vibrations not only shake the ground but also sometimes crack it open. And then, there is chaos, for earthquakes have been known to wipe out cities and civilisations.
About a million earthquakes occur in one year, but most of them are small tremors. And almost 90 percent of them occur underwater along the “ring of fire” that circles the Pacific Ocean. Other areas that are prone to earthquakes are the Alpine belt stretching between Spain and Turkey, through the Himalayas to Southeast Asia.
Why do earthquakes occur? The earth is made up of many layers. The outermost layer or lithosphere, below the surface, is called the shell or crust. It consists of seven huge slabs or plates, which are as thick as approximately 100 km (60 miles) that move at the speed of 10 centimetres a year. Picture these tectonic plates carrying continents and ocean floors on their shoulders, for that is what they do.
Most earthquakes take place along the boundary between two plates. The slow but constant movement subjects the plate borders to tremendous stress, which builds in the bedrock for decades and centuries.
When the strain produced by these movements increase beyond a certain level, the pent-up energy cracks up the rocks and creates a fracture known as a fault. This sudden release of energy also unleashes ground-shaking vibrations. These vibrations or seismic waves are what make up an earthquake.
If you drop a stone in a pond, ripples are created. The seismic waves travel outward in all directions from the quake’s focus, in a similar manner. Waves that reach the surface spread out in concentric circles from the epicentre. The epicentre is the spot on the ground that lies directly above the focus.
Recording instruments called seismographs have helped geologists identify types of seismic waves, and the most damaging waves among them.
In the 1930s, the Richter scale was developed to compare earthquake intensities worldwide. The scale measures the amplitude or the entire range of ground motion, and not the amount of damage caused. The most powerful earthquakes to be recorded by the scale, which starts at zero, are in the range of eight or nine on the Richter scale.
A major quake is often followed by many smaller tremors known as aftershocks. They release any energy still left in the rocks and occur in parts of the fault where the original quake took place.
One of the most destructive earthquakes in India occurred around the Brahmaputra Basin and neighbouring regions in Assam, in northeast India, in 1950. Besides major landslides and rifts, it destroyed hundreds of villages and killed many people.
In recent times, one of the worst earthquakes in India took place in 1991 in the Chamoli-Uttarkashi region of northern India. As a result, over 2,000 people were killed, more than 1,800 were injured and 18,000 buildings were destroyed.
Earthquakes can be severe enough to alter the topography of a region. An earthquake that occurred in Japan in 1923 caused major land upheavals throughout the Kanto region, which was the epicenter. The floor of Tokyo Bay rose three metres, while the nearby Izu Peninsula was shoved nearly four metres to the west. It was the worst in Japan’s history.
Japan and neighbouring regions are among the most earthquake-prone regions in the world. The San Andreas fault in California is another. The fault makes the region very prone to earthquakes, the most recent one being in the Landers region, in 1992.
While scientists have still not been able to predict an earthquake, they have identified certain indicators that foretell an earthquake. This they have done by studying areas of the world with a high risk of earthquakes. So small earthquakes and tectonic plate movements, and swelling or tilting of the ground are some omens.