Why so many earthquakes ?

Devjit Yumnam *

An earthquake with magnitude 6·8 occurred near Imphal, Manipur on January 4 2016 :: Pix - Deepak Oinam

In times of confusion and fear a voice of logic and reason easily gets lost. Knowing what is happeneing and why is a luxury only logic and facts can provide. This is a thesis on why we are experiencing so many high magnitude earthquakes in such a small time span.

We have all studied the structure of the earth as a child in our school days. Do you remember that the earth is devided into different layer according to the composition. The outer most layer (ignoring the atmosphere) is called the crust, consisting of the soil and rocks. Followed by the mantle and the core.

To help you better explain the idea, let us assume that we have a glass container filled with water, we then put some granular wood powder on it and put it on a stove . What happens? The wood powder floats on top and the water stays below. In this situation the wood powder on top represents the crust and the water below represents the molten core (and mantle) of the earth. Below the crust the earth exists in a fluid state. There is a more detailed layering of the earth which anyone can google and easily find out. For the sake of my explanation all you need to understand is that the crust floats on a molten fluid core. Like wood floating on water.

Now let us turn on the stove, what do we observe, the bottom of the vessal gets hot and it heats up the water in contact with it. As the water heats up it expands, becoming less dense than the water around it. The decrease in its density makes it lighter and it rises up. The void left over by this is filled up by the sorrounding cooler water which again in turn gets heated and rise. This results in the formation of a steady upward stream of hot water. As these water reach the top they give off their heat, cool off increasing its density and fall down hence completing a vertical loop of water stream. This is what we call a convection current.

During this time we observe that the wood granules which were starionary and stable in their respective positions when the stove was off now starts moving around. Spots where there are no granules and others where they are hurdled together can be observed. The clear spots are due to the convection current breaking the surface and pushing the particles away and the spots of hurdled granules are due to the push from the current. Its what you would see happening to tea granules when making tea.

The earth's crust is broken up into plates called tectonic paltes. The spots where there are no granules correspond to the region of the plate where the crust moves apart, volcanes can be seen in these area. It is the molten core convection current breaking the surface. The regions where the tectonic plates are pushed together corresponds to the ruddled up particles of the vessal. The himalayan mountains is a good example of such a region.

Lets do another experiment. For this you will need a nail, a copper wire, a battery and a compass. Take the copper wire and wind it on the nail as many times as you can, more the better. Now connect the two end of the wire to the battery and bring it near the magnetic compass. You will notice that the needle of the compass turns towards the nail.

Change the terminals of the battery and observe again. You will notice that the other end of the magnetic needle gets attracted unlike the previous case. What is happening here? Well, the charge stored in the battery starts flowing in the copper wire once you connect the two ends to it. Whenever a charge flows or moves it creates a magnetic field around it. The magnetic compass senses this magnetic field and gets deflected when brought near the nail. When the ends of the wire are connected in reverse the charge flows in the opposite direction so the magnetic field produced by it also changes its direction, which is indicated by the reversal of the magnetic needle.

Inside the earth due to the very high temperature elements like iron, nickle, etc exsist in their charged state called ions. Unlike the convection current of water in the vessal, the convection current inside the earth is the movement of charged ions of metals. The circulation of these ions inside the earth like in the case of the copper wire creates a magnetic field. As the scale of this is very big, it thus creates a very strong magnetic field which can be felt by a magnetic compass anywhere on earth. This is called the earth's magnetic field, the north and the south poles arise from it.

A phenomenan called the magnetic pole reversal has been documented from studying the orientation of magnetic field in magnetic rocks and computer models simulating the behaviour of the earth's core done on super computer have given similar results. The magnetic field of the earth completely reverses. The northpole becomes south pole and the south pole becomes the north pole. It happens roughly every 10,000 years, we have passed the due date for our reversal by qiute a bit.

The magnetic field will not vanish completely during a resersal, but many poles might form chaotically in different places, until it stabalizes. The underlying cause of the magentic field reversal and anomalies is the change in the behaviour of the convection current flowing beneath the earth. If the behaviour of the convection current changes than the nature of the forces acting on the tectonic plate will change.

Like in the case of our experiment, if we turn the stove off or reduce the heat, the granules will try to resettle themseves. The pushed up granules will start spreading out. Similarly if the forces acting on the tectonic plates change then the plates will try to resettle and change in accordance to the changing forces, releasing centuries of built up energy in the process.

It is logical to assume smaller plates will be the first ones to move as their threshold energy for keeping in place will be less. After the small plates have moved the bigger plates will start moving, releasing bigger energy. The magnitude of the earthquake caused will be directly propotional to the size of the plate. The earthquakes in India was due to the movement of the indian plate, that of Japan was due to the movement of Filipinno plate and that of Ecuador was due to the movement of the Nazca plate.

Three similarly sized plates producing earthquakes of magnitude 7- 8. None of the big paltes have moved, if they do they will release much greater energy.

Until we have complete pole reversal and stabilization of the convection currents we should be ready for some big shakes.