Earthquake seems to be a part of our life now as we come across earthquake from one part or another part of the world. It is one of the main natural disasters that happen. It may vary in the effect on each place. Let us see what earthquake is, its causes and effects, how earthquake is measured and also types of earthquakes.
What is Earthquake?
Earthquake is a natural disaster that can be broadly defined as a series of vibrations that are induced from the earth’s crust. A sudden movement of the earth’s crust, sudden release of extreme energy, shaking or trembling of the crust of the earth due to shifting of rocks under the earth’s surface or underground volcanic forces can be called as earthquakes. This is often accompanied by terrible noise. These waves can sometimes traverse half a hemisphere destroying the cities completely. The location where earthquake starts is called hypocenter or focus. The location just above the hypocenter is called epicenter. The main earthquake is called mainshock. There are many aftershocks that occur after the mainshock. The amount of energy released during earthquake is called magnitude.
CAUSES OF EARTHQUAKES
Most of the scientists believe that earthquakes are formed due to the movement of the earth’s plates. It is a very natural phenomenon and a theory commonly known as plate tectonics. As, we all know earth is divided into 3 layers namely – Crust, Mantle and Core. The outermost layer of earth is known as the crust which is made up of granite and basalt. About 70% of the crust is ocean. The crust has several continental plates which drift every year. The second layer is called mantle. 80% of the earth is mantle. The mantle consists of lithosphere which is the upper mantle and asthenosphere which is the lower mantle. The core is the innermost layer of the earth. One third of the mass of the earth is core. The core has a liquid outer core made up of iron and it is very dense. It also has a solid inner core made of nickel and iron. The magnetic field of the earth is created when the liquid outer layer spins. So, as discussed above, the earth’s crust is made up of several large and hard plates. These plates move slowly and continuously above the mantle. So collision and stress is caused among the plates. The stress among the plates can also occur when one plate goes over the other or when the sides if the plates collide. As the stress increases, the crust breaks and the energy is released. This released energy moves to and fro in the form of waves called seismic waves. Thus these naturally created waves shake the earth forming earthquakes.
MEASUREMENT OF EARTHQUAKES
The first widely used gadget for measuring earthquakes were the Richter scale which was developed by a Charles Richter in the year 1934. According to this scale, the largest wave was recorded in a seismometer and then the distance between the earthquake and the seismometer was measured. But unfortunately, the measurements in this Richter scale were not accurate. Main 2 devices are used by seismologist to measure earthquakes. They are: seismograph and seismoscope. Seismograph is an instrument to measure seismic waves of the earthquakes. Seismoscope is an instrument to measure the time or occurrence of earthquakes. Nowadays earthquakes are calculated by its magnitude and intensity. Logarithmic scale is the base for calculating magnitudes. This means that the higher the whole number on the magnitude, the motion recorded by the seismograph goes up ten times. This scale has no upper limit. Thus, magnitude is the amount of seismic energy at the hypocenter of the earthquakes. Intensity on the other hand is observed by the ground shaking of the people and buildings. This varies from place to place. According to the U.S geological center, the earthquakes are assigned to a class as per their magnitude. The classes are as under-
- Great : magnitude is 8.0 or above- tremendous damage
- Major : magnitude 7.0 – 7.9- heavy damage
- Strong : magnitude 6.0 – 6.9- severe damage
- Moderate : magnitude 5.0 – 5.9- considerable damage
- Light : magnitude 4.0 – 4.9- moderate damage
- Minor : magnitude 3.0 – 3.9- light damage
- Micro : magnitude less than 3.0- slight treble.
TYPES OF EARTHQUAKES
There are many classifications for earthquakes. Some of the classifications are as under:
- On the basis of origin
- On the basis of dept of focus
- On the basis of magnitude and intensity
On the basis of origin, earthquakes are divided into tectnotic and non tectnotic earthquakes. Tectnotic earthquakes occur when the earth’s plates shift due to some geographical force. Non tectnotic earthquakes occur due to the surfaces and collapse of the cavity roofs.
Dept of focus:
Here the depth of the focus is measured and further analyzed by seismograph. Here the further sub divisions are:
- Surface earthquakes : these earthquakes have depth of focus of less than 10,000 meters and are called surface earthquakes.
- Normal earthquakes : these earthquakes have depth of focus between 10-50 kilometers and are called normal earthquakes.
- Intermediate earthquakes : these earthquakes have depth of focus between 50-300 kilometers and are called intermediate earthquakes.
- Deep earthquakes : these earthquakes have depth of focus between 500-700 kilometers and are called deep earthquakes.
Magnitude and intensity :
Sometimes earthquakes are measured on the basis of magnitude i.e. the amount of seismic energy at the epicenter and intensity i.e. the total damage and destruction caused due to earthquakes.
Earthquakes can be again classified into:
- Shallow fault earthquakes : When the rock breaks under the feet it is called a fault. So earthquakes occur within and through these faults. These faults are close to the surface of the earth, a small quake can cause a lot of damage.
- Subduction zone earthquakes : These earthquakes happen in the Subduction area of the earth along the Subduction zone. Huge shift in land level happens during this earthquake. The waves that occur are tremendously high and spread over a larger area.
- Deep earthquakes : These earthquakes occur in the deep ocean slabs beneath the continental crust of the earth.
EFFECTS OF EARTHQUAKES
- Causes widespread damage and loss of human life.
- Causes ground displacement along the fault.
- The shaking of the ground causes landslides and avalanches.
- Causes rupturing of dams and can cause floods and tsunami.
- The broken electric lines and gas lines due to the earthquakes can cause fire.
- Kills and injures hundreds.
- The entire city can be devastated.
- Liquefaction of the soil takes place
To conclude, earthquakes are the most powerful and destructive natural force which is hazardous to mankind. Earthquakes are unpredictable disasters that arise with no warning and have plagued humans throughout history. The energy released earthquakes fracture the surface of the earth. Forecasting earthquakes are a bit difficult but earthquake warning systems have been developed. Earthquake engineering and earthquake insurance have to be developed in a larger angle so as to prepare for the aftereffects. But not all earthquakes are bad. Sometimes when earthquakes are created, it has even created notable landmarks in the world. So the place of occurrence is very important before stamping it as good or bad.