An earthquake is a movement of the earth, caused either by volcanic activity below the surface or by a large area of earth, weaker than that which surrounds it, slipping a little downwards.
Earthquakes need not be severe. In some parts of the Pacific they are a daily, and not especially frightening, occurrence.
The motion occurs in very different ways, having sometimes a perpendicular, sometimes a horizontal undulating, and sometimes a whirling motion. It also varies much in degrees of violence, from a shock which is hardly perceptible to one which bursts open chasms and changes the appearance of the ground itself. During these shocks sometimes smoke and flames, but more frequently stones and torrents of water are discharged. Research Earthquake
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