Earthquake is a sheer vibration felt by us on the surface. But it causes catastrophic damages. Let us see learn all about the deadly disaster in detail!

## PLATE TECTONICS

Consider the earth as a ball. This ball is not made from a single piece or a mould, but is joined together by many pieces of lithosphere (land). It’s is very similar to solving the puzzle with seven pieces to form a sphere. The only difference is that this solved puzzle (the earth) has rocks and iron in its inside. The puzzle pieces are the tectonic plates.

## WHAT ARE THE SEVEN TECTONIC PLATES?

• African Plate
• Antarctic Plate
• Eurasian Plate
• Indo-Australian Plate
• North American Plate
• Pacific Plate
• South American Plate

When these plates are put together the earth’s outer surface, the lithosphere is formed.

The above figure shows the earth’s tectonic plates after there are virtually removed.

## HOW WERE THE CURRENT TECTONIC PLATES FORMED FROM A SINGLE PLATE?

As said earlier, the earth’s surface had only one plate altogether. The plate or plates are always in constant motion. The plates move away from each other or towards each other, that is, they either get pulled away or pushed towards or slided over. This constant pulling or pushing of the plates has led to the current existence of seven tectonic plates.

WHAT MAKES THE PLATES MOVE?

The earth is divided into three layers namely

• crust
• mantle
• core (inner core and the outer core)

The crust is the outer most layer where the trees grow followed by the mantle and the core. The mantle has metals and rocks in their molten state. Hence the crust is floating on the liquid mantle or the seven tectonic plates are floating on the liquid mantle.

## MANTLE CONVECTION CURRENTS

As we move deeper towards the earth’s core, the temperature increases. Therefore, the liquid matter at the bottom of the mantle is hotter than the matter at the top of the mantle.

Hence, there is a flow of CONVECTION CURRENT in the mantle that makes the plates in the crust move.

## WHAT ARE CONVECTION CURRENTS?

When a liquid is heated, the bottom part of the liquid is heated first. As it gets heated, it becomes lighter as it’s density reduces. The liquid matter lighter in weight rises up and the comparatively denser liquid at the top sinks down. Now, this gets heated up. When the current liquid at the bottom is heated more than the one that rose above, this liquid rises up sinking the matter at the top. This cycle keeps continuing. Hence, there is a circulating current. If an object is made to float on this circulating current, it would start moving.

Therefore, the mantle convection currents keep the plates in motion. The plates move at a rate slower than the rate at which the human nails grow. Speed at which the plates move is 1-4 cm/year. How do the motion of these tectonic place make earthquake happen on the earth’s surface?

## HOW DOES AN EARTHQUAKE OCCUR?

Everyday hundreds of earthquakes occur on the earth’s surface, most of which are hardly felt. Earthquakes predominantly occurs on the weakest points of the crust, which are nothing but the line of crust where the two tectonic plates join called the FAULTS. The plates are continually in motion. When the plate plates collide into each other, their free movement is hindered. Elastic strain energy starts building up along the fault line. Eventually, the strain along the fault becomes too much. As the days pass by, when the strain has exceeded the threshold the fault ruptures releasing enormous amount of energy and tremors. This causes the shaking effect that causes catastrophic disasters on the earth’s surface. Do these catastrophic disasters constantly occur at the same place? NO!

## Which places on the earth are more prone to earthquakes and why?

The fault line on the earth’s surface are the regions more prone to earthquakes as they are the weakest points and also the points where the movement of the plates can be hindered. Movement hindering causes the build up of elastic strain energy and later earthquakes. The following image shows the points that are more prone to earthquakes.

### WHAT ARE FOCUS AND EPICENTER

The point of the origin of the earthquake is called the FOCUS. The point on the earth’s surface directly above the focus is called EPICENTER.

Hence, focus lies inside the earth and epicenter lies on the earth.  Almost in the occurrence of 90% of the earth quakes the epicenter is located in the seabed. Based on the location of the foci, the earthquakes are classified as follows

1. Deep focus earthquakes (Focus located at a depths greater than 300 km)
2. Intermediate focus earthquakes (Focus located at depths between 55 km and 300 km)
3. Shallow focus earthquake(Focus at a depth less than 55 km)

When the epicenter is located in a city, the earthquake might turn the city upside down. Once such after effect of earthquake is earthquake liquefaction.

## EARTHQUAKE LIQUEFACTION

The loosening of soil after the occurrence of earthquake is called earthquake liquefaction which is a form of soil liquefaction. A state of ‘soil liquefaction’ occurs when the effective stress of soil is reduced to essentially zero, which corresponds to a complete loss of shear strength. Soil liquefaction can occur due to many reasons like,  increase in load on an embankment, cyclic loading, earthquakes, etc. The major contributor of soil liquefaction is earthquakes.  The following picture is a result of  earthquake liquefaction.

The above image shows a building at Niigata, a city of Japan. It is a picture taken after the earthquake. After the earthquake the soil lost it’s shear strength and could no longer hold the building. The building is a multi-storey where more than thousands of people reside and the earthquake has brought its roof to the ground, turning it upside down, causing dreadful and unimaginable damages in a very few seconds. Such a dreadful natural disaster can never be prevented. But we can save our self by predicting. What are the technology or device that help in detecting an earthquake.

## SEISMOMETER

Seismometer is a device used to detect the earthquake. The following image shows a rough sketch of seismometer

The seismometer has a weight and spring arrangement. The weight is hung from by a spring attached to a frame. The weight is held still by electromagnetic forces and is restricted from motion even when the ground moves. The frame moves parallel to the earth’s surface. Hence when a vibration occurs the frame moves but the weight stands still. The weight has a pen attached to its end and therefore when the vibrations are felt the pen moves along with the weight drawing peaks of lines warning earthquakes. Are these vibrations felt by humans? NO! The vibrations felt by seismometer is not felt by humans. They are very small in amplitude. Hence the usually a seismograph is generally used to to predict an earthquake

## WHAT IS A SEISMOGRAPH?

With seismometer it was very difficult to accurately detect the arrival of the quakes ! Hence, the seismometer was remodelled, and the remodelled seismometers are called TELESEISMOMETERS. In teleseismometers, every single movement made by the weight is converted into an electric signal, amplified and then converted into digital signal are are stored in the computers in the form of graphs called SEISMOGRAPHS.

## WORKING IN DETAIL

When the tectonic plates are in free in motion. before they collide into each other, the graph(seismograph) records normal waves. The peaks cannot be seen. Once, the plates collide into each other, the stress starts developing and after few years and before few days of the happening of the earthquake, the the normal seismograph varies in its plotting showing a small peaks indicating the possibility of earthquake. Hence, it just shows the possibility with a small variation and as already discussed more than hundred earthquakes occur in a day. How do seismologists accurately warn us the DEVILS then? Seismologists have recorded the earthquake measure (RICHTER) and have fixed thresholds.

## WHAT IS RICHTER SCALE?

The magnitude of the earthquakes is measured on the Richter scale, invented by Charles F. Richter. The Richter magnitudes are based on a logarithmic scale (base 10), which means, for each whole number that goes up on the Richter scale, the amplitude of the ground motion recorded by a seismograph goes up ten times.  A magnitude 5 earthquake would result in ten times more than the level of ground shaking as a magnitude 4 earthquake (and 32 times energy would be released).

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