Earthquake – Important Notes
When the Lithospheric plates move, the surface of the earth vibrates. The vibrations can travel all round the earth. These vibrations are called earthquakes The place in the crust where the movement starts is called the focus. The place on the surface above the focus is called the epicentre.Vibrations travel outwards from the epicentre as waves.Greatest damage is usually closest to the epicentre and the strength of the earthquake decreases away from the centre. Earthquakes occur when the ground is subjected to so much force that it fractures or breaks. Earthquakes occur all the time all over the world, both along plate edges and along faults. Most earthquakes occur along the edge of the oceanic and continental plates. The earth’s crust (the outer layer of the planet) is made up of several pieces, called plates. Earthquakes usually occur where two plates are running into each other or sliding past each other.
There are three types of earthquake waves:
1. P waves or longitudinal waves
2. S waves or transverse waves
3. L waves or surface waves
Foreshock is an earthquake that occurs before a larger earthquake, called the mainshock.
An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. Aftershocks are formed as the crust adjusts to the effects of the main shock.
Earthquakes in India
Strong and damaging earthquakes have been felt in all parts of India and the rest of south Asia. The most seismically active areas are those in the Himalayan regions of the sub-continent and in the Andaman and Nicobar Islands due to the peculiar regional tectonic setup. The peninsula area, was generally thought to be “seismically safe”. But recent large quakes such as Koyna & Killari, have proven otherwise. Several damaging shocks have hit this region in historical and ancient times.
- The largest known Indian earthquake occurred is the 2001 Gujarat earthquake, also known as the Bhuj earthquake, occurred on 26 January which was also India’s 52nd Republic Day.
- At least 20,005 people killed, 166,836 injured, approximately 339,000 buildings destroyed and 783,000 damaged in the Bhuj-Ahmadabad-Rajkot area and other parts of Gujarat. Many bridges and roads damaged in Gujarat. At least 18 people killed and some injured in southern Pakistan.
- The earthquake was followed by a large number of aftershocks. The Indian Metrology Department (IMD) recorded more than 500 aftershocks of magnitude 3.0 and above, which continued through the month of March
How is it measured
When and earthquake occurs it generates seismic waves which radiate away from the rupture point like waves in a pond, but also travelling downwards through the earth. An earthquake is measured with a machine called a seismograph. The magnitude of the earthquake is measured on the Richter scale.
Poor construction technique, where slab walls and floors are not tied together correctly, for example, makes buildings far more vulnerable to earthquake damage; buildings where the bricks have been held in place with the correct mortar tend to survive much better.
The resonant frequency depends on the height of the building: low frequency shaking might cause tall buildings to shake violently while having little effect on low-rise buildings nearby, although higher frequencies of vibration might have the opposite effect.