Earthquake study is called Seismology and is a relatively recent science.
Up to the XVIII, objective records are scarce and there was no real comprehension of the phenomenon. From explanations related with divine punishment or answers of the Earth to human misbehavior, they passed to pseudo-scientific explanations like they were originated by air liberation from caverns at the depths of the planet.
The first earthquake recorded took place in China in the year 1,177 B.C. There is a Chinese Earthquake Catalogue that mentions a dozen more phenomenons in the following centuries.
In the European History of the first earthquake, the year 580 A.C. is mentioned, but the first one clearly described dates back to the mid XVI century.
The oldest earthquakes registered with photographs or accurate descriptions in America took place in Mexico, during the late XIV century, Chile in 1570, in Quito, Peru (no Ecuador) in 1587, in Chile, May of 1647, Jamaica, 1692, in Massachusetts, USA, 1744 and 1755 and in Peru in 1746, although a clear description of their effects is not available.
From the XVII, several stories about earthquakes begin to surface, but they appear to be distorted or exaggerated.
Northamerica reports an important series of earthquakes placed between 1811 and 1812 near New Madrid, Missouri, highlighting one that was estimated around 8 degrees on the morning of December 16th of 1811. Other two considerable earthquakes took place on January 23 and February 7 of 1812, especially the last one, whose aftershock lasted for months and was felt in areas as remote as Denver and Boston. Since they were not so populated then, cities did not register too many deaths or damage.
This didn’t happen in 1906 when more than 700 victims were produced and the city was devastated by the earthquake and subsequent fire in the greatest earthquake in the history of the US. 250,000 people were left homeless.
In Alaska, on March 27 of 1964, an earthquake with even more energy was recorded, but since the area had a low demographic density, population damages were not as serious, registering only 107 victims, which is not as much if we consider that the earthquake was felt in an area of 500,000 square miles and uprooted trees in some areas.
MEASURING EARTHQUAKES
It is performed through an instrument called seismograph, which registers the Earth’s vibration produced by the earthquake (seismogram) in a paper. It informs the magnitude and duration.
This instrument registers two types of waves: Superficial, that travels the earth’s surface and produces its greatest vibration (and probably the most damage) and the central, which travel through the Earth from its depths.
The central waves, in turn, have two types of waves: The primary waves (“P”) or comprehensive and the secondary waves (“S”) or shear. The interesting part of these waves is that the “P” waves travel through the magma (melted rock area) and reach the surface first since they achieved greater speed and push the small material particles in front of them and devastating another portion behind.
The “S” waves on the other hand, displace material in straight angles from them (that is why they are also called “transversal”).
A typical sequence of an earthquake is: First the arrival of a deaf sound caused by the comprehensive (“P”) waves, then the shear (“S”) waves and finally the “rumbling” of the earth caused by superficial waves.
