Introduction to Earthquakes


Our world is a gift from nature, nature can sometimes become unpredictable and unmanageable, it causes devastation. Examples of such natural occurrences are hurricanes, tornados, wildfires, volcanic eruptions, flooding, earthquakes and tsunamis. These are usually not caused directly by humans, but their effects live with us for a long time. In this lesson, we shall look at one of such natural occurrences...earthquakes!

What is an Earthquake?

Simply, earthquakes are the rumblings, shaking or rolling of the earth's surface. It is usually what happens when two blocks of the earth suddenly slip past one another or break apart from each other as a result of tension caused by prolonged energy build up. 

Earthquakes come in many forms. It can be felt as a shock under your feet or may be very powerful and destructive enough to flatten an entire city. They can happen anywhere, land or sea.

Foreshocks, Main shocks and Aftershocks:

there are smaller shocks that occur before (foreshock) and after (aftershock) the main earthquake (mainshock). Sometimes foreshocks are so big and scientists are unsure if it is the main shock. Foreshocks and aftershocks can occur for days, weeks and months of the main earthquake.

Earthquakes are also called temblors. 

It is important to understand 
the earth’s makeup to help understand earthquakes better.

In this diagram, you will notice that the inner and outer core of the earth (middle part) are liquid in nature, containing iron and nickel of extreme temperatures (5,500°C). 

The Mantle is semi-molten rock, also called magma. The outer is the crust, which is the hard part of the earth that forms the surface. This outer crust includes the land on which we live, the oceans and ocean deeps and anything within 40km (approx) down the earth's surface.

Earthquakes are developed in the outer crust of the earth.

Important terms to know about earthquakes

Let us take a moment to learn about these terms to help us understand earthquakes better.

Tectonic Plates:

These are huge layers that make up the earth’s upper layers. They continually stretch, move, slide, and collide with each other. Even though they are constantly moving, we do not feel it. Each plate is about 50 to 250 miles (80 to 400 km) thick.

Faults (or Fault plane or fault lines):

These are weak lines that can develop in the plates, usually on the surface of the earth. There are different types of faults and the major types include dip-slip normal, dip-slip reverse, strike-slip and oblique-slip. 

The hypocenter is the location below the earth’s surface where the earthquake starts. The epicentre is the location directly above it on the surface of the earth.

Seismograph and The Richter Scale (RS): 

The seismograph is a device that scientists use to measure the magnitude of an earthquake. The Richter scale, on the other hand, is a scale or measure that is used to compare earthquakes. It is calculated in levels of ten. Example, an earthquake measuring 4 on the RS is ten times more than a measurement of 3, and an earthquake measuring 8 on the RS is 10 times more than one that measured 7 on the RS. As a guide, an earthquake measuring 3-5 is considered minor, 5-7 is moderate, 7-8 is major and 8 or more is considered great and usually very devastating.

How do earthquakes form?

Let us imagine what goes on in the outer crust with the help of this diagram.

Earthquakes develop in the crust of the earth. The crust involves the earth's surface, submarine levels, down to the ocean floors. The inner part of the earth contains massive energy. Some of this energy escapes through cracks and other volcanic activity, but the bulk of it is stored within the earth’s inner part, contained in the crust.

The earth’s outer crust is held in place like a completed jigsaw puzzle, with rough edges and lines. The energy stored here causes the pieces to slide, glide, knock and move around each piece. These pieces best describe what we call ‘Tectonic plates’.

After a period of time, the built up energy and movement causes huge tension in the plates, and there is massive pressure on the fault lines. This intense pressure resulting from energy build up causes the fault lines give way, and plates move over, against or apart from each other, this shakes the ground and anything on it, tearing down houses and structures. This is what we see as earthquake.