The word ‘Seismos’ which means Earthquakes have originated from the Greek language. An Earthquake is the sudden vibration of the surface of the Earth caused by the elasticity or the isostatic adjustment of the rocks beneath the surface of the Earth (mostly due to lithospheric plate movements).
Hypocentre and Epicentre
- The point of origin or the source of an earthquake is known as hypocenter or Focus. A hypocenter is below the surface, where the first rock displaces and creates the fault. Epicentre is the point on the Earth’s surface that is directly above the hypocenter or focus.
- In Earthquake, there is a sudden release of energy in the Earth’s crust, which leads to a series of motions because of the shock waves generated due to this energy release called seismic waves. These seismic waves originate within the focus and spreads towards all directions.
Types of Earthquake Waves
Earthquake waves are basically of two types: Body waves and Surface waves.
These are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves. There are two types of body waves. They are called P and S-waves.
Primary (P) waves
- P-Waves – They move faster and are the first to arrive at the surface.
- These are also called ‘primary waves’. The P-waves are similar to sound waves and compress the material in the same direction as they travel.
- They travel through all media i.e. gaseous, liquid and solid materials. From studying the speed of these waves it is inferred that earth has an inner solid core made up of extremely compact material.
Secondary(S) – waves
- S-Waves – They arrive at the surface with some time lag. These are called secondary waves. An important fact about S-waves is that they can travel only through solid materials.
- So they are also called Sheer waves, as they are able to make changes in the material due to sheer stress only, which is only possible in solids. Earthquake waves are generated due to the release of energy at the focus and travels in all directions. The study of S waves helped to understand the structure of the interior of the earth.
- The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.
- The body waves interact with the surface rocks and generate new set of waves called surface waves
- These waves are travelling through the surface of the earth.
- L waves are similar to water waves. It have long wavelength, low velocity and are confined to the Earth’s crust. They cause most of the structural damages.
- The surface waves are the last to report on seismograph. These waves are most destructive.
- They cause displacement of rocks, and hence, the collapse of structures occurs.
- These waves are classified into 2 types;
- Rayleigh Waves (R Waves)
- Long Waves (L Waves)
Shadow Zone of an Earthquake
- Earthquake waves get recorded in seismographs located at far off locations.
- However, there exist some specific areas where the waves are not reported on seismograph.
- Such zones are called the ‘shadow zones’.
Types of Earthquakes
- Earthquakes can be generated by a number of sources, most of which are result of natural tectonic processes, usually caused by the convergence, divergence, or transformation between two lithospheric plates.
- Earthquakes can also be generated by volcanoes as magma is ejected into the Earth’s crust.
- For example, earthquakes in the island of Hawaii are generally volcanic earthquakes.
- Some of the Earthquakes are artificially generated by nuclear test explosions.
Tectonic Earthquakes are most common and generated due to folding or faulting caused due to plate movements. This type of Earthquakes is the most destructive ones.
Earthquake associated with volcanic activity are called volcanic earthquake. These are confined to areas of volcanoes and active volcanic activities. The ring of fire region in the Pacific Ocean is best example of these types of earthquakes.
They are confined to the areas of intense mining activity. Sometimes the roofs of underground mines collapse causing minor tremors.
This is minor shocks occured due to the explosion of the nuclear explosives.
Reservoir Induced Earthquakes
Large reservoirs may trigger the seismic activity because of the pressure induced on huge rock stratas by large mass of the water.They are called reservoir induced earth quakes.
- It is a series of low magnitude earthquakes that occur in a localized region and over a period of time ranging from days, weeks to even months.
- The Earthquakes come in three forms of clusters called foreshocks, mainshocks, and aftershocks
- Foreshocks are quakes that occur before a larger one in the same location; around a quarter of all mainshocks happen within an hour of their foreshock. Mainshocks and aftershocks are better known.
- Mainshocks are of the highest magnitude vibrations. The major causalities are caused by the mainshocks.
- Aftershocks are smaller quakes that occur in the same general geographic area for days-and even years-after the larger, mainshock event.
- Earthquakes originate at depths ranging from about 5 to 700 kilometres. Nearly 90% of all earthquakes occur at depths of less than 100 km.
- Shallower the depth, more destructive an earthquake is.
Mechanism of Tectonic Earthquakes
- Theory of plate tectonics explains that earth’s crust is formed by a number of large plates that move very slowly in various directions on the earth’s surface.
- These plates are 60-200 km thick and float on top of a more fluid zone called Asthenosphere, much in the way that icebergs float on top of the ocean.
- Most earthquakes occur near a boundary between two plates. As one plate pushes past or moves over another, great stresses build up in the rock along the edges of the plates because friction prevents them from sliding past each other.
- Subsequently, the stresses become great enough so that the rocks can rupture. The edges of the plates slip a short distance in different directions, causing an earthquake.
- Greater the stress, greater is the intensity of earthquake. The plate movements are of three kinds:
- In divergent movements the plates move apart from each other and a crack develops between them.
- This is most common type of movement in mid-oceanic zones hitch results in the formation of mid oceanic ridges (mid Atlantic, east pacific, and chagos-laccadives ridges) here rocks breaks due to tensional force created by the diverging plates.
- The divergent margin is known as constructive margins because of the formation of new crust.
- In convergent movements the plates move towards each other and the border overlap.
- This is most common type of movement in subduction zones where the dense oceanic plates collide and slide beneath the continental plates.
- The region where subduction takes place is known as Benioff zone. The converging plates can interact with each other in three different ways:
Oceanic-continental convergent plate: when one of the converging plates is an oceanic plate and the other one is a continental plate, the oceanc plate gets subducted below the continental plate because of being denser.
Oceanic-oceanic convergent plate: when both the converging plates are oceanic in nature, the relatively denser plate will be subducted which results in the formation of trenches and volcanic islands.
Continent-continent convergent plate: when both the plates are continental, the subduction zone is not so active and it results in the formation of long chains of Fold Mountains. These regions are geographically unstable.
Transform boundaries are places where plates slide sideways or parallel, past each other. At transform boundaries lithosphere is neither created nor destroyed. Many transform boundaries are found on sea floors, where they connect segments of mid oceanic ridges.
Earthquake belts are the narrow geographic zones on the earth’s surface along which most earthquake activity occurs. There are three major belts of earthquakes in the world. They are as follows
This belt is seen in the Pacific Ocean. 70% of the earthquakes occur in this belt. This zone includes the regions of ‘ring of fire’ which is well known of great seismic activity and countries such as Japan, the Philippines, and Chile. This path coincides with the “Pacific Ring of Fire”.
Pacific Ring of Fire
Pacific Ring of Fire is a horse-shoe shaped 40,000 kilometre area with 75% of Earth’s active and dormant volcanoes. It is the area with large number of Volcanic Eruptions and Earth quakes. The most active Volcanoes are located in Chile, Mexico, United States, Canada, Russian Far East, Japan, Philippines, Indonesia, New Zealand, & Antarctica.
20% of the earthquakes occur in this region which is seen along the meditteranean-himalayan belt.
10% of the earthquakes occur in this region which is seen along the central part of the Atlantic Ocean from north to south (S shape)
Impact of Earthquake
- Soil liquefaction occurs when a saturated or partially saturated soil substantially loses strength and stiffness in response to an applied stress in which the soil that is ordinarily a solid behaves like a liquid.
- Earthquakes can cause soil liquefaction where loosely packed, water-logged sediments come loose from the intense shaking of the earthquake.
- The liquefaction is more prominent in areas such as river valleys, river plains and deltas.
- The randomly bunched together soil particles have spaces formed between them.
- These spaces, called pores, can be filled with water or air. The pressure of the material in the spaces holds the particles apart and stabilizing the soil in its present configuration.
- The effect of a seismic wave on granular soil and pore increases the water pressure and forces the particles apart as well as disrupts the contact point of the particles themselves.
- At this point in time, the soil will flow like a liquid. The end product is the collapse of the particles so that there is less space between them.
- The water that was in that space is then forced upward.
The impacts of the Liquefaction are as follows:
- The underlying layer of water rich sand compacts and sends a column of water and fine sand up and out onto the surface.
- This phenomenon is called Differential Compaction.
- At the same time, depth of lakes, ponds, borrow areas, and other depressions becomes lower, because the sand is pushed through the ground.
- The buildings sink into the ground after the earthquake.
Ground rupture is another important effect of earthquakes which occurs when the earthquake movement along a fault actually breaks the Earth’s surface
Landslides are caused by earthquakes both by direct rupture and by sustained shaking of unstable slopes. They can easily destroy buildings in their path, or block roads and railroad lines, or take hilltop homes with them as they tumble. They even can dam rivers on occasion
Tsunamis, which are popularly known as ‘Giant waves’, are a grave hazard to many parts of the world, particularly around the Pacific Ocean basin where earthquakes occur occassionally. Tsunamis are a series of water waves caused when the seafloor moves vertically in an earthquake. Tsunamis are a series of waves, and may start with a gentle withdrawal of water, followed by a very abrupt arriving wave, followed by another withdrawal.