(World Geography) - Earthquakes, Theories, Plate Tectonics, Cratonic & Seismic Zones
TYPES OF EARTHQUAKES
A) Classification on the basis of causative factors:
1) Natural earthquakes are those which are caused by natural processes i.e. due to endogenetic processes. These are further subdivided into following subcategories:
a) Tectonic earthquakes are caused due to dislocation of rock blocks during faulting activities. The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane. E.g. 2001 earthquake of Gujarat.
b) Volcanic earthquakes are caused due to volcanic eruptions of explosive and fissure types. The Intensity and magnitude of earthquake depends on the intensity and magnitude of volcanic eruptions. E.g. severe earthquakes caused by Krakatao volcano in 1883.
c) Isostatic earthquakes are triggered due to sudden disturbance in isostatic balance at regional scale. Generally earthquake in the active zones of mountain building are included in this category.
d) Plutonic earthquakes are deep focus earthquakes whose centres lie at depths from 240 to 670Km.
2) Artificial or man induced earthquakes are caused by anthropogenic activities such as underground explosions, mining, large reservoir induced pressure etc.
B) Classification on the basis of focus of earthquake:
a) Shallow focus earthquake has its seismic foci location at depth from ground surface to 50km.
b) Intermediate focus earthquake has it’s foci at depths between 50 to 250 km.
c) Deep focus earthquake have centres at depth from 250 to 700 km.
Theories of Earthquake
Earthquake are not randomly distributed and there are zones that are more prone to them than others.
Elastic Rebound Theory: It suggests that crust and lithosphere are capable of withstanding certain stresses to a point beyond which the crust goes through a series of deformation until it yields and breaks with sudden snapping of the earth’s crust the slabs go though a series of vibrations before it comes to rest finally and these vibrations manifest as earthquakes. This theory though not incorrect is hardly a comprehensive insight into the nature of force that causes the earthquakes. It does not tell why do we have such forces and why do earthquake distribution has a definite pattern on earth.
Plate tectonics’ Explanation of Earthquakes
As per Plate tectonics earthquakes are a consequence of interplate interaction. Along convergent boundary compressive forces crush and cause earthquakes while along divergent boundary tensile forces stretch and snap the earth creating earthquakes. Earthquakes can also be due to plate motions and the friction of lithospheric plates while they drift along asthenosphere. Divergent boundaries and convergent boundaries between two continental plates will continental plates will only have shallow focus earthquake. Intermediate and deep focus earthquakes are almost entirely limited to ocean floor subduction.
TERMS ASSOCIATED WITH EARTHQUAKES – Focus, Epicentre, Foreshock, and Aftershock
The subsurface area along a fault plane, where the seismic waves are initiated, is the focus, or hypocenter, of an earthquake. The area at the surface directly above the focus is the epicentre. Shock waves produced by an earthquake radiate outward though the crust from the focus and epicentre. A Foreshock may occur after the main shock, sharing the same general area of the epicentre. Foreshock which precedes the main shock is also possible. The pattern of foreshocks is now regarded as an important consideration is the forecasting effort.
The earthquake events are scaled either according to the magnitude or intensity of the shock. The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10. The Richter scale is logarithmic. Each whole number on it represents a 10-fold increase in the measured ware amplitude. Translated into energy, each whole number signifies a 31.5-fold increase in energy released. Thus, a magnitude of 3.0 on the Richter scale represents 31.5 times more energy than a 2.0 and 992 times more energy than a 1.0. The magnitude of a quake is measured with the help of seismograph.
The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The intensity at a point depends not only upon the strength of the earthquake (i.e. magnitude) but also upon the distance from the earthquake to the point and the local geology at that point. While the intensity decreases with the distance form the local epicentre, magnitude for an earthquake remains the same, irrespective of where it is measured. The range of intensity scale is on observed effects. The lower number of intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scales are based on observed structural damage. Normally damages are associated with intensity V or more.
The distribution of earthquake coincides with certain areas, as follows:
1) Circum-Pacific belt: It includes the epicenters of coastal margins of north and South America and East Asia. This belt accounts for 65% of the total earthquake of the world. It presents four ideal conditions for the occurrence of the earthquakes viz.
a) Subduction zone of convergent plate boundaries,
b) Junction of continents and ocean margins,
c) Zone of young folded mountains and,
d) Zone of active volcanoes.
The pacific plate boundary is being subducted below American and Eurasian plate boundaries. The coastal margins of Asia and island arcs and festoons (Kamchatka, Sakhalin, Japan, Philippines) experience both tectonic and volcanicity induced earthquake in substantial number.
2) Mid-continental belt: This is Alpine-Himalayan belt which represents zone of plate convergence. The Indian and African plates are subducted below Eurasian plate causing isostatic and fault induced earthquakes. About 21% of total seismic events of the earth occur in this belt.
3) Mid-Atlantic belt: It includes epicenters located along mid-Atlantic ridge and several islands near the ridge. This belt records moderate and shallow focus earthquakes. The spreading of sea floor, fissure type of volcanic eruptions and transform faults are causes of earthquakes in the region.
Earthquakes along stable cratonic zones
Earthquakes are very rare in stable cratonic zones that have hard resistant rocks however there are instances of such earthquakes where these earthquakes are a consequence of rare events where ancient extinct faults have been reactivated. Eg, the Bhuj earthquake was due to reactivation of Allahbund fault.
SEISMIC ZONES OF INDIA
Bureau of Indian standards has grouped the country into four seismic zones viz.
Zone-II, III, IV and V. of these, Zone V is the most seismically active region, while zone II is the last. The Modified Mercalli (MM) intensity, which measures the impact of the earthquakes on the surface of the earth, broadly associated with various zones is as follows:
|Seismic Zone Intensity||on MM scale|
|II (Low intensity zone)||VI (or less)|
|III (Moderate intensity zone)||VII|
|IV (Severe intensity zone)||VIII|
|V (Very severe intensity zone)||IX (and above).|
Broadly, Zone-V comprises of entire north-eastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar Island. Zone-IV covers remaining parts of Jammu & Kashmir and Himachal Pradesh, Delhi, Sikkim, northern parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan. Zone-III comprises of Kerala, Goa, Lakshadweep, remaining parts of Uttar Pradesh, Gujarat West Bengal, parts of Punjab, Rajasthan, Maharashtra, Odisha, Andhra Pradesh, Tamil Nadu and Karnataka. Zone-II covers remaining parts of the country.