EARTHQUAKES

Earthquake is a term used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by sudden stress changes in the earth like the slip on a fault or volcanic activity. Earthquakes are essentially vigorous shaking of the earth's surface.

They may be related to the disruptions at the surface or disturbance from within. They can be natural or man-made. Construction, developmental activities, mining, quarrying Reservoir induced seismicity. Eg - the Koyna earthquake. Landslides due to human factors cause earthquakes.

Types

There are 3 types of earthquakes on the basis of depth of focus.

1. Shallow focus earthquake - Within (15-30 km).
2. Intermediate focus earthquakes - Between (70-300)km
3. Deep-focus earthquake - Beyond 300 km.

Mechanism

The mechanism of Earthquakes can be explained through 2 theories

• Elastic Rebound Theory
• Theory of plate tectonics

Elastic Rebound Theory.

The earth's crust is like an elastic medium. It can withstand stresses to a limit. But if the strain crosses the elastic limit the crust can yield and break and vibrates before coming to rest. The earthquakes are the result of vibrations.

This explanation is not incorrect . But it is too simplistic and too trivial. It doesn't explain the nature of forces and their genesis that cause the stresses.

INTERPRETATION OF EARTH QUAKE - Through tectonic theory.

Tectonics can explain the intensity, distribution and different type of E.Q.

EQ is the consequence of mantle dynamism, Plate movements and the processes related to this.

EQ are features of plate boundaries. The distribution of EQ and WWSSN mapping was the basis of identifying plate boundaries.

Along Divergent boundaries.

• Shallow focus EQ.
• Not as devastating.
• Even if magma sourced from deep mantle, it is very fluid and moves relatively smoother.
• The cause of EQ is crust breaking and plates moving
• At MORs, transform fractures can cause EQ. They are relatively violent.

Along Convergent boundaries.

1. CB which have subducting ocean slab (O-O and O-C)
2. CB which don't have subducting ocean slab (C-C)

CB which have subducting ocean slab (O-O and O-C)

It can have all 3 types of EQ. Shallow focus, Intermediate focus and Deep focus.

Shallow focus

• Because of plate movements
• Because shear fracture along the trench and deformation at the collision the slab.
• Volcanic eruptions are Andesitic
• Granitic magma at Ocean - continent convergence
• These shallow Focus EQ are destructive

Intermediate focus EQ - Along the Subduction Zone, the friction between the mantle and slab, local turbulence, and eddies create Intermediate focus EQ.

Deep focus - In the deeper mantle, the EQ may be due to ocean slab fracture as the slab descends into deeper mantle and also due to violent conversion of olivine into perovskite and ______.

Benioff Wadati zone

• Earth quake concentration along The subducting Zone is BWZ
• BWZ plane is 38 - 40° inclined to horizontal
• It Corresponds to Angle of Repose of subducting slab

Pacific Ring Fire

• PROF is Ocean - Ocean and Ocean-continent type of Boundary
• The boundaries are associated with Trenches
• Hence most earthquake prone with destructive consequence.

CB which don't have subducting ocean slab (C-C)

Along Continent - Continent convergence Boundary

• Related to Himalayan type of orogeny
• Shallow focus and destructive
• Dont have Intermediate and deep focus EQ because there is no subducting Ocean slab.

Intraplate volcanism ( hotspot location)→ shallow focus EQ. " Not destructive type Sourced from the deep mantle and is of very fluid type and can move through the mantle without much resistance.

In General EQ are features of unstable mobile belts of the continents which are Generally plate margins and they have active orogeny.

The cratonic and shield zones of continents are the. stabilized pre cambrian old ancient blocks and generally do not have EQ.

Eg - peninsular plateau of India

But these cratonic blocks may have ancient faults that may can become activated for reasons not Known. They can have Random infrequent EQ.

Eg - Jabalpur EQ, Bhuj EQ, Latur EQ ( due to Allahbund fault reactivation)

Distribution

The following are the major EQ belts of the earth.

1. Circum Pacific Belt (Most violent )
2. Trans Eurasian Mt Belt
3. African Rift valley
4. MORs Atlantic MOR being the most active one.