Seismic shaking or earthquake is a tremor of the earth's surface produced by natural forces within the earth's crust and at varying depths.
The quakes are caused by the clash of rocky slabs at depths ranging from 50 to 900 km below the ground.
Other factors considered are gas displacements such as methane and volcanic activities. There are two types of earthquakes: those of natural origin and the induced ones.
Most seismic quakes are of natural origin from Earth, are called tectonic earthquakes. The force of the tectonic plates slides over the asthenosphere and may collide, move apart or slide across each other.
Through these forces the rocks change to their point of tension, later the rocks begin to break and release an accumulated energy during the displacement process.
The energy is then released through seismic waves across the earth's surface and interior.
Induced earthquakes are basically the result of man's action. They originate from explosions, mining of minerals, water or fossils, or even from falling buildings; however, the intensity presented is much lower than that of tectonic earthquakes.
Among the consequences of a seismic quake we mention:
• Soil vibration of varying intensities,
• fault opening,
• Changes in earth rotation.
The consequences of a seismic quake usually cause harmful effects to man such as injuries, deaths, financial and social damage, collapse of buildings, destruction among others.
The regions most susceptible to earthquake are those near tectonic plates such as western South America where the Nazca plate and the South American plate are located; and in regions where new plates are formed, such as in the Pacific Ocean where the Fire Belt is located. The length of an earthquake failure can range from centimeters to kilometers.
To measure the size of seismic quakes a scale is used. The most used is that of seismologist Charles Francis Richter. Its scale ranges from 0 to 9 degrees and calculates the energy released by the tremors. Another widely used scale is Mercalli, which measures earthquakes by the extent of the damage. This scale is divided into 12 categories according to their intensity.
Check out the description of the 12 degrees of the Mercalli scale:
1) Not felt;
2) Directed by people at rest or on higher floors of tall buildings;
3) There is slight vibration, and hanging objects sway;
4) There is moderate vibration, such as that caused by earthmoving machinery, the windows and crockery rattle, and the cars sway;
5) Sense outside the home, able to wake people up. Small objects and pictures fall;
6) Sensed by all, causes displacement of furniture and damage such as breakage of crockery and glazing and cracks in plaster;
7) Noticed by people who are driving, those who feel have difficulty staying upright. Chimneys, architectural ornaments and furniture break. Church bells ring. There are large cracks in plaster and masonry, some houses collapse;
8) Branches and logs break. Moist soils are cracked. Elevated water towers and monuments, for example, are destroyed. Brick structures, fragile timber houses, irrigation works and dikes suffer serious damage;
9) There are cracks in soils causing sand craters. Unarmed masonry buildings collapse and fragile concrete structures and underground pipes suffer damage;
10) Slips and cracks appear very scattered on the ground. There is destruction of bridges, tunnels and some reinforced concrete structures. There is damage to most brickwork, dams and railways;
11) Soils suffer permanent disturbances;
12) Almost total damage.
The following image shows the rank on the Richter key.