LOCATING TERRA FIRMA

• The word tectonics originates from Greek, meaning "to build".
• As for plates- it is a geological term denoting large slabs of rocks.
• The theory states that the Earth's outermost layer is made up of plates vastly different in size and shape.
• The world we know is thus based on several plates. The interaction of all these plates cause land movements.
• The continents are built of differential blocks of the crust, with interiors, which are relatively stable and older in age, known as cratons.
• The bordering areas are the zones of complication, and are formed of younger rocks.
• They may however, also contain some remnant rocks from ancient mountain chains

• India was a part of Gondwanaland during the Paleozoic and Mesozoic eras.
• The Pangea broke up and evolved into five continental masses.
• The Gondwanaland on which our present day India sits, includes the Indo-Gangetic plain as well as the Himalayan mountain ranges although modified with time, while the pen insular part consists of the original fragment of the Gondwana plate.
• The Indian plate is bordered by the African plate in the west, by the Pacific plate in the east, the Antarctic plate on its south and by the Eurasian Plate in the north.
• On its northern boundary, the northward moving Indian plate is said to be colliding with the Eurasian Plate; the subduction zone lying at the foot hills of the Himalayan range.
• The processes of these plate movements also bring about the mineralogical variation within the plate mass due to the varying pressure and temperature conditions.
• The existing crustal deformations and newly developed fault and fold make the areas more sensitive producing seismic waves of varying intensity and amplitude.
• Thus these regions are extremely quake prone, as they strive to regain the equilibrium by releasing huge amounts of energy.
  
  
  

• There are five earthquake zones in India based upon their susceptibility to earthquakes.
• The divisions are based on their distance from the Main Fault Boundary, which thrusts between the Siwaliks and the inner Himalayas and may be an intercontinental convergence zone, which in all possibility extends to the south of the Salt range.
• Due to continuous movement, the plates are developing strains in its central part.
• These tensions when released take the form of highly destructive earthquakes.
• Geo physical studies show that transverse faults of the plates have now accumulated enough strain to cause earth quakes. In fact, the Indian plate is showing signs of breaking up.
• Another shocking revelation shows that the sub-plates in the main Indian plate are now becoming active.
• The broad categories of seismic regions and their causes are listed below

EXTRA PENINSULAR REGION

Lithologically this region is composed of ultra basic, basic and intermediate volcanic and hypabyssal type of rocks. Folding and thrusting type of structures and intense orogenic activities are reported in this region. According to the Plate Tectonic theory the Indus-Suture Zone constitutes the subduction zone along which the "Indian plate" collided with the "Tibetan Plate" giving rise to orogenesis of the Himalayas. Thus the Tethys sea gap was closed by northward moving Indian Plate against the Central Asian landmass. The subduction of this plate underneath the eastern Eurasian Plate pushed up the Tethyan sea to form the Himalayan chain.

1. North Eastern India

• Mechanism of Seismicity—Thrusting fault.
• The axes of compression are more horizontal than vertical and are roughly perpendicular to the mountain arcs.
• The direction of under thrusting of the Indian plates is towards the north, beneath the eastern Himalayas and towards the east, beneath the Burmese arc.
• The magnitude of shock waves prevailing in this region ranges from 6.0 to 6.5 and in some cases it may reach 7.0 to 8.0 on the Richter scale.
• The affected region includes the states of Assam, Arunachal Pradesh, Meghalaya, Manipur and Tripura.

2. Bihar-Nepal regions

• Mechanism of Seismicity- Sudden release of strain between the uprising Himalayas and the downwarped alluvial belt of the Gangetic plain.
• Earthquake shocks are of low to medium magnitude.
• The regions affected are Monghyr (in Bihar) and Bhatgaon (in the Kathmandu valley)

3.North-western Himalayas

• Mechanism of seismicity- Their exact genesis is unknown, although some have attributed it to movement along one of the reverse fault in the Himalayan zones.
• This region is prone to medium magnitude shocks. Two important zones in this region are,
  • Indus-Kohistan Seismic zone (IKSZ)
  • Hazara Lower Seismic zone (HLSZ)
• Examples of affected regions in this zone are Kangra valley, Dharmsala, Mandi, Kalas Deharadun, Mussouri, Chamba, Hindu Kush (in Great Pamir), Buner and Hazara region, Jammu and Kashmir, Punjab and Pakistan.

4.Baluchistan

• Mechanism of Siesmicity- It is caused due to thrust plane or strike-slip faulting.
• In this region the Jurassic formations are thrust over Cretaceous rocks (Mesozoic era), which dip to the North East.
• The east-west trending mountain ranges from Quetta to the Sulaiman are a part of the fold and fault belt related to the Indo-Eurasian plate boundary.
• Shocks that occur here are of medium magnitude.
• The regions affected are Lovalai district (Baluchistan), Kohan, Muri Hills, Kohlu Village, Jhalawan, Chaman, Khjak range, Sind, Quetta.

PENINSULAR REGION

This region comprises of granitic gneisses and migmatite type of rock. Orogenic activity gives rise to physiographic and structural features. Prominent examples of such activity are the Dharwar folding, Eastern ghat folding, Aravalli folding and Satpura folding.

1. The Deccan

• Mechanism of Seismicity- Results from the marginal activity along the coast, a feature of stable landmasses.
• Low magnitude shock occurs in this region. Affected regions are central part of India, Godavari valley near Bharadrachalam and Hyderabad.

2. The Koyna region:

• Mechanism of seismicity- Dam induced Seismicity.
• The cause of this earthquake zone is attributed to excessive loading of water in the Shivaji Sagar reservoir formed by damming of the Koyna river.
• Low magnitude shocks occur due to Dam Induced Seismicity.

3. Gujarat

• Mechanism of Seismicity- Plate movement along a pre-existing fault in the underlying Eocene sediments.
• High Magnitudes shocks occur here. Examples of affected regions are Rann of Kutch, Bhuj, Anjar and Broach.

OCEANIC OR COASTAL REGION

This region of Indian sub continent is generally composed of Sedimentary rocks, with several folded structures. However, the oceanic ridges are of basaltic and doleritic composition with seamount like topography and several major and minor structural features.

1. Bay of Bengal

• Mechanism of seismicity,- Tectonic ac tivity here depends upon the sea-floor epicenter that lies on continental mar gin. Examples include Port Blair (Andaman & Nicobar Islands) where any shock experienced can also be felt in Calcutta, Visakhapatnam and Madras Port areas.

2. Makran coast

• • Mechanism of seismicity- Faulting due to tectonic activity is related to the great Makran fault. Magnitude of earthquakes that occur in this region is extremely high.
  • Examples of regions affected include the Markan ports region and the West Coast of India.