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Devastating earthquakes have always occurred during human history and, according to the chronicles reported since the invention of writing, they have often made many thousands victims in towns, until many hundreds thousands, with a total destruction of all buildings, never built with anti-seismic criteria. This is possible today, but not always these criteria are respected, even in the most seismic areas. This happens for the lack of controls and for the tendency of builders to limit costs at the expense of security.

For this reason, the entity of destruction and human losses, also when faithfully reported, is not sufficient alone to evaluate the intensity of an ancient earthquake, given that also an event of 6-7 degree by the modern Richter scale (explained below), not among the most destructive recorded (about 9-9.5 degrees) was able to destroy all the buildings, if made with poor technique and scarcely resistant materials.

Only after the invention of the first modern seismographs in the XIX century, it was possible to compare the intensity of earthquakes by using an absolute scale, the Richter scale (introduced by Charles Richter, of the California Institute of Technology, in 1935) whose units are logarithmic; this means that an earthquake of 6th degree is 10 times more powerful than a 5th degree one.

So, since 1900, the most powerful earthquake recorded occurred in Chile on May 22nd, 1960 and its intensity was 9.5 on the Richter scale. It had its epicentre at Canete near the harbour town of Valdivia, 900 Km south of Santiago, the capital and occurred at 14:55. The sea water in the harbour of Valdivia increased its level of 4 metres before starting to withdraw; the earthquake had produced an enormous tsunami that hit the coasts of Valdivia region, between Talca and Chiloe and the first wave, 8 metres high, struck the coast at 16:20, followed by a second wave 10 m high, ten minutes later.

The victims number is very uncertain, reported from 2200 to 6000 and it was limited only for the scarce population of that region. The tsunami started from Chile in that occasion launched its waves across the whole Pacific Ocean, making hundreds of victims in the Hawaii, Japan, Philippines, New Zealand and reaching the Aleutian Islands, in Alaska.

In Chile, this earthquake caused a diffuse soil subsidence that destroyed many buildings and changed the course and level of many rivers, also blocked by landslides that formed dangerous dams, ready to pour the water downstream.

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