Here we are providing the short notes on Geography which will be help to crack General Awareness part in may examinations like SSC, Railway and other competitive exams.
- a) An earthquake is the sudden release of strain energy in the Earth’s crust resulting in waves of shaking that radiate outwards from the earthquake source.
- b) The point at the surface directly above the focus is called the earthquake epicentre.
- c) When the earth moves in an earthquake, it can cause waves in the ocean, & if a wave grows large enough, it’s called a “tsunami”. Underwater earthquakes sometimes produce largetical waves called Tsunami.
- e) Earthquakes are measured with a seismometer.
The magnitude of an earthquake, & the intensity of shaking, is measured on a numerical scale. On the scale, 3 or less is scarcely noticeable, & magnitude 7 (or more) causes damage over a wide area. The point of origin of earthquake is called Seismic focus. The point on the earth’s surface vertically above the earth’s surface is called Epicentre.
- f) The passage of earthquake waves is recorded by Seismograph. The magnitude of waves is measured on Richter’s scale. For measurement of the intensity of the earthquake (damage caused), the Modified Mercalli Intensity Scale is used.
Distribution of Earthquakes
- a) Around the Pacific Ocean along a belt of volcanoes known as the Ring of Fire. 68 per cent of the volcanoes are experienced in this region.
- b) From the middle of Asia (Himalayas, Caspian Sea) through the Mediterranean Sea to West Indies. 21 per cent earthquakes are experienced in the region.
- c) Mid-Atlantic ridge belt which accounts for 11 percent of the earthquakes.
TYPES OF SEISMIC WAVES
There are two types of seismic waves, body wave & surface waves.
- Body waves travel through the interior of the Earth. They follow ray paths refracted by the varying density & stiffness of the Earth’s interior which in turn, vary according to temperature, composition, & phase.
Body waves are divided as
P-WAVES (Primary Waves) are compression waves that are longitudinal in nature. These waves can travel through any type of material, & can travel at nearly twice the speed of S waves.
S-WAVES (Secondary Waves) are shear waves that are transverse in nature. These waves typically follow P waves during an earthquake & displace the ground perpendicular to the direction of propagation. S waves can travel only through solids, as fluids (liquids & gases) do not support shear stresses. S waves are slower than P waves, & speeds are typically around 60% of that of P waves in any given material.
- Surface waves are analogous to water waves & travel along the Earth’s surface. They travel slower than body waves.
There are two types of surface waves:
Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water.
Love waves are surface waves that cause circular shearing of the ground. They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity, & have the largest amplitude.
The asthenosphere separates the strong, solid rock of the uppermost mantle & crust above from the remainder of the strong, solid mantle below. The combination of uppermost mantle & crust above the asthenosphere is called the lithosphere. The lithosphere is free to move (glide) over the weak asthenosphere. The tectonic plates are, in fact, lithospheric plates.