Not only earthquakes

Earthquakes have a very strong destructive power, on one hand due to the direct effects of the vibrations, on the other because they can provoke numerous undesired effects, at times of a magnitude that is even greater than the damages provoked by the tremors. Very strong earthquakes can induce dramatic modifications in the landscape, setting off landslides, opening fissures in the ground, deviating watercourses and consequently causing floods and triggering off or summing to volcanic eruptions. Geysers of sand and mud, liquefied by the vibrations, may form, and their jet may reach over 6 metres in height. In the urban centres, destruction caused by seisms may lead to the breaking of gas pipes or oil pipelines, may provoke short circuits, causing fires or explosions. Many of the victims recorded, are due to the fires. When the hypocentre is in the sea, some types of movements may cause the feared tsunami waves, whose devastating effect is still under our eyes with the dramatic images of what occurred in South East Asia some months ago.
Some statistics
Every year,  seismographs all over the world record over 600,000 tremors of intensities below 2 of the Richter scale, that can be perceived only by instruments. Other 300,000 earthquakes with an intensity between 2 and 2.9 can be felt only by persons who are particularly sensitive. 49,000 more earthquakes with a magnitude between 3 and 3.9,  are felt by those who live near the epicentre, while 6,000 tremors, of intensities between 4 and 4.9 provoke minor damages. For the higher intensities, that always provoke damages, approximately 1,000 are of an intensity between 5 and 5.9, 120 between 6 and 6.9, approximately 14 with a magnitude between 7 and 7.9, while earthquakes with an intensity between 8 and 8.9 occur every 5-10 years. Fortunately, earthquakes of a magnitude greater than 9 are rare, approximately 1-2 times in a century. But these are only statistics, or annual averages – in South East Asia two seisms of a great intensity took place (greater than 9 and approximately 8.7) in around 3 months, which completely distorted the statistics, and this does not exclude that similar events might take place again in the short term.

 

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