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Introduction

 

Earthquakes are ground tremors that are principally caused by the disturbance of the mechanical equilibrium of the Earth’s rocks. It is today known that the Earth’s crust is made up of a number of lithospheric plates, which are continuously moving, resulting in the development of forces at their outer limits as well as in their interiors. When the exerted forces exceed the upper limit of elastic deformation of the rocks, then these are ruptured and there is a sudden and violent release of energy that is transmitted in all directions in the form of a wave motion, which in effect constitutes the earthquake. When this wave motion reaches the Earth’s surface it can cause damage to buildings and other structures, landslides, land subsidence and fracturing of the ground as well as elevation and hydrographic changes. The point or area of rupture of the rocks is called the focus or hypocentre of the earthquake while the point on the Earth’s surface above the focus is called the epicentre.

 

Special instruments, called seismographs, record the ground tremors of earthquakes and provide seismologists with the necessary information to study the causes, the genesis mechanisms, the transmission of seismic waves, as well as their effect on structures and  on Man and the environment.

 

The graduation of earthquakes is carried out by means of two parameters, the magnitude and the intensity. Magnitude is a measure of the quantity of energy that is released at the focus of an earthquake and is measured on a logarithmic scale that runs from 1 to 10 and is called Richter scale. Intensity is a measure of the fierceness with which an earthquake manifests itself at a particular place and is measured from I to XII on the modified Mercalli scale. Intensity I on this scale is defined as the intensity of an event felt by very few people, whereas intensity XII is that of a catastrophic event that causes total destruction.

 

 

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