HOW EARTHQUAKE OCCURS?

Latest earthquake occurred dated on 28.01.2020 in Caribbean cities like Jamaica,Havana,Kingston, Miami indulged me to write on the subject of earthquake occurrence and it’s cause.

This blog will help many laymen to understand the cause behind the occurrence of earthquake. There are different myth behind the earthquake occurrence among different people. Therefore, as a structural engineer, it’s my duty to explain earthquake in a simpler way. There are two disturbance which are geologically proved are solely responsible for seismic activity. One is disturbance at tectonic plate boundaries and other is on the fault planes. Either of the two plays a vital role for any seismic activity which are explained below.

1. Disturbance on Tectonic Plates Boundaries:

Actually all natural or artificial structures are supported over the earth crust. It is the uppermost part of earth.Below this crust,there is layer of earth which is known as mantle. Mantle is further divided in two layer. Upper mantle & lower mantle.There is a layer above upper mantle known as Mohorovicic discontinuity or Moho. This layer “Moho” behaves like a viscous fluid having convective force due to high temperature. Earth crust being a rigid layer float over Mohorovicic discontinuity as Moho is a viscous fluid mass. How earth crust floats over Moho can be clearly understood when it can be correlated with a plate placed over utensil having water continuously boiling over it .Due to convective force of boiling water,plate starts floating it, in the same manner, earth crust floats over Moho being it a viscous fluid generating convective forces. Geological survey of earth has revealed that earth crust has been dived into twelve plates. All these plate are called as Tectonic Plates. All these plates are resting over Moho Discontinuity & moves with respect to each other due to convective force . As the convective forces are highly unsymmetrical and complex, sometime it exerts force to earth tectonic plate to make it move towards each other or sometime it exerts force on tectonic plate to make plates departs from each other. This process of colliding of one plate with other is convergence while departing of plates with respect to other is called divergence of plates. When two plates collides with each other, then huge amount of energy is released in the form of seismic waves. This release of high energy in the form of wave is called as Seismic Wave. When this wave strike to the structures, it exerts forces on the structure due inertia which is known as Seismic Forces. When two tectonic plates move away from each other, then volcanic eruption takes place in which molten Magma comes out on the earth crust. As this volcanic eruption is vigorous, it also produce vibration to the ground but is not much significant as compared to the earlier case when two plates collide with each other.

2. Disturbance at Faults Plane

Below the earth surface, there are many rocks which are in distorted or in fractured form. These fractures are formed as result of high pressure. These fractures are in unstable condition. When two parts of rocks have relative movement or deformation or dislocation then there is release of large amount of energy in the form of seismic waves. As there are many form of plate tectonic movement like convergence or divergence in vicinity of plate boundaries, in the same way different names of distortion in rock through fault boundaries are named based on the relative movement like Strike -Slip faults, Dip-Slip faults, normal faults or reverse faults. I invite you to learn more through these terms. Please read other literature for thorough knowledge of seismology.

Thanks & regards!

Mohammad Sohel Akhtar

(Structural Engineer)