An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth’s crust that creates seismic waves. The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured with a seismometer; a device which also records is known as a seismograph. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale. The depth of the earthquake also matters: the more shallow the earthquake, the more damage to structures (all else being equal).
At the Earth’s surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake’s point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.
The branch of science that deals with the study of earthquake and the structure of the earth is termed as earthquake engineering.
The main objectives of earthquake engineering are:
- Understand the interaction between buildings or civil infrastructure and the ground.
- Foresee the potential consequences of strong earthquakes on urban areas and civil infrastructure.
- Design, construct and maintain structures to perform at earthquake exposure up to the expectations and in compliance with building codes.
- A properly engineered structure does not necessarily have to be extremely strong or expensive.
Types of earthquake
1. On the basis of location
A) Interplate à The earthquake which occcur along the boundaries of the tectonic plates are called interplate earthquake. For eg.Assam earthquake of 1897 have 8.7 magnitude on richter scale was interplate e/q.
B) Intraplate à The earthquake which occur within the plate itself away from the tectonic plate boundaries are termed as intraplate earthquake.
2.On the basis of their cause
a) Non-tectonic earthquake à The earthquake which are of non-tectonic origin are termed as non-tectonic earthquake.
These are further classified as;
i) Superficial earthquake:- The earthquake which are caused by dynamic agencies operating upon the surface of the earth.
ii) Volcanic earthquake:- The earthquake which occur due to different volcanic activities.
b) Tectonic earthquake à The earthquake which are of tectonic origin and occur due to structural disturbances within the earth’s crust are called as tectonic earthquakes.
3.On the basis of focal depth
a) Shallow earthquake à The earthquakes in which focal depth is limited up to 71km are called shallow earthquake.
b) Intermediate earthquake à Those earthquake in which focal depth varies from 71km to 300km are termed as intermediate earthquakes.
c) Deep earthquake à Those earthquake in which focal depth is greater than 399km are termed as deep earthquake.
4. On the basis of intensity
b) Very feeble
d) Fairly strong
f) Very strong
j) Very disasterous
5. On the basis of magnitude
a) Micro earthquake (M=3.0)
b) Intermediate earthquake (M=3-4)
c) Moderate earthquake ( M=5-5.9)
d) Strong earthquake ( M= 6-6.9)
e) Major earthquake ( M= 7-7.9)
f) Great earthquake ( M>8.0)
Causes of earthquake:-
1.Superficial causes à Earthquakes of mild intensity which occur over the ground surface caused by the dynamic agencies operating upon the surface of the earth are termed as Superficial ones.
The various surface causes providing seismic tremors are :-
i) A huge lanslide or a rock fall along hill slope.
ii) Giant sea waves and crashing breakers along sea shores.
iii) Running water, descending falls and cascades upon valley floor.
iv) Heavy vibrating machinery in Industrial area.
v) Movement of locomotives and other heavy vehicles on earth surface.
vi) Man made explosives and other nuclear tests.
vii) Mining blasts in mining areas.
viii) Large scale heavy excavations causing land subsidence, which sets minor tremors in the vicinity.
Some of the other causes seismic shocks are due to cultural noise or disturbed vibrations generated by industry and the traffic.
2. Volcanic causes à Volcanic activities taking place indifferent parts of the World. Oftenly produce volcanic explosions during which the surface the earth trambles.
The thus generated is sometime so strong that it causes earthquakes in the nearby areas.
Volcanic earthquakes caused by high pressure exerted by movement of molton lava is known as magma with in the earth’s crust are generally shallow earthquakes of mild intensity. Peninsular and extra – peninsular india have observed volcanic activity on a large scale in the past bay of bengal and andaman regions have extinct volcanoes.
3. Tectonic causes à Tectonic causes are those which originate within the earth’s crust and are neccessarily associated with the relative movement of rock masses forming the crust of the earth. The seismic shocks which occur due to sudden release of enormous strain owing to crustal movements are termed as tectonic e/q.
It established that all major e/q causing large scale devastation on the surface of the earth are of tectonic origin.
Tectonic e/q occur due to :-
i) Displacement of rock masses along pre- existing cracks or faults.
ii) Development of new fault planes. Bhuj earthquake in Gujrat ( INDIA) in the year 2001 was also of tectonic origin.