These are a clear testimony to the vitality of the earth, certainly all the volcanoes are the most visible effect of the fact that our planet is alive (see the flow of heat) and this is largely due to the movement who suffer from the plates (see plate tectonics) above the mantle.
But what is this energy? Well imagine it’s like to have in my hands a stick of wood: if you start to bend it offers a resistance to bending which is expressed in the form of elastic energy, rocks behave the same way: ie if a piece of rock begins to deformed, it will offer some resistance (which changes depending on the type of rock), but when the forces that hold together the rock are exceeded those that deform then this is broken and there is an abrupt shift of the two parties ISSUE ‘energy that had accumulated during the deformation and returning to a state-deformable. The shift occurs both vertically and horizontally.
Usually these cracks, and the consequent shifts, there are long lines preferential called faults, and the precise point from which spreads the earthquake hypocentre said, while the same point, carried upright on the earth’s surface , is called the epicenter. But what are these faults? A fault is essentially a fracture in the ground, deep even several kilometers, along which the ground motions occur. In fact, a fault is simply a line of least resistance in the rock under pressure and therefore rupture always takes place along this linea.In figure next to the sequence of rupture: 1 and 2 ply, 3 rupture with release of energy stabilization in the 4 new form. Besides a simple scheme of 3D propagation of various types of faults waves.
There are also very different, but all have in common the fact that along the line you have a relative movement of the rocks. Interesting is the case with the famous San Andreas Fault that runs along the west coast of the United States. This is a type of strike slip faults, that the ground motions always occur in the horizontal plane (such as a hand goes toward the North while the other goes to the South) and slowly approaching the city of Los Angeles to San Francisco the rate of about 2 centimeters per year. This may seem a very small number, but in reality if we think in geologic time (millions of years) this movement is very fast.
But back to our earthquakes: we said that these arise because at a certain point, the rock breaks along a fault line, in depth, and releases all the energy that had built up to withstand the motion. This energy dissipates into the ground dall’ipocentro in all directions in the form of waves (also partly in the form of heat) which can be: in order of volume, ie involving a volume and therefore in this case, the land itself, and surface waves that propagate only on the surface of the waves terra.Le volume can be further divided in P-wave that is primary (also called longitudinal), those who arrive first, and then those who are traveling within the earth with the velocity higher (order of 6 kilometers per second) and are also different ways to travel in terreno.
Now is possible to identify very precisely the point where you format a earthquake.
In figure a diagram of the area affected by the California St. Andrew’s Fault (Earth Science, Casati, Ed clup).
Besides being the fastest they alternately compress and release the ground in their direction of propagation just like sound waves and in fact when this kind of waves reach the surface undergo a refraction in the air and can be transmitted to the atmosphere under form of sound waves.
Then we have the S-waves that is secondary (also called transverse) because they are slower (in fact arrive per second) and make the ground move alternately up and down angles to the direction of propagation and by their nature can not travel in liquids.
As we have said over the waves of volume we also surface, and these are also called for long because they travel long distances and are very similar to those that appear when you launch a stone on water, such waves are those that cause damage to homes and foundations. This energy is discharged usually with a strong main shock, usually preceded by warning small quakes (called foreshocks) and followed by a series of shocks many such replicas. But sometimes the earthquakes may occur directly with the main shock and of course are the most dangerous. The release of energy due to the efforts acting on the lithosphere (the hard shell that covers the ground, you see inside the earth) can also occur continuously, as the two blocks on either side of the fault can slide gently side by side ‘ more smoothly and without sudden accumulation of energy, so without causing major earthquakes.
The earthquakes that are associated with movement of the ground along the faults are the most common but there are also associated with earthquakes, volcanoes (see volcanoes), and their activity (movement of magma, gas pressure etc. ..). In figure representation Schematic of the propagation of waves in the first case of P waves of compression, shear S waves below. Under the propagation of surface waves.
Historically man has constantly sought to classify different types of earthquakes. Almost all the classifications were based on properties more evident: their intensity. In Italy the best known is the Mercalli scale, invented in 1897 by Giuseppe Mercalli, based only on the extent and amount of damage: that is, when an earthquake happened was an estimate of the damage and in light of these was assigned to a given earthquake value ranging from 1 (no damage, only the tools warn) to 10 (total destruction). That Mercalli is now superseded by one based on values more objective: The Richter scale (invented by Charles Richter in 1935).
But you’ll wonder how does one measure the seismic waves of surface with a tool that supports the earth, If all the surface itself moves?
To overcome this problem we use the seismograph. A seismograph is an instrument consisting of a roll of paper and a “pen” that writes on paper on the roll. The trick is that the pen is held suspended by a spring that makes the stylus to keep the same position, while during the earthquake, the roll of paper will go up and down along the ground motions. The nib is more or less the same position because the spring, which is attached, the movement of the soil and not send them to this.
With these tools, and the same earthquake, experts can study the interior of the planet and see what’s below the crust on which we live (in fact you can not tell anything directly since no one has ever been in the center of land and even the most modern techniques do not allow oil drilling to go to depths of 10 – 15 km, and has also been able to divide the interior of the earth in various parts like the crust, mantle and core (but this is another topic, see the inside of the Earth). In the picture above two types of seismograph used, first measure the vertical movement, the second horizontal ones, and next an example of a seismogram with the arrival, at different times, different types of seismic waves.
An earthquake, where the epicenter (the point on the surface placed on the vertical dell’ipocentro) is in the midst of the sea then it will result in a tsunami. Many of these are caused by a sudden vertical movement of the seafloor and form of the waves on the surface (like when you throw a rock) very large, which can travel at speeds of 500 to 1000 km per hour. When such waves arrive near the coast rise (because it decreases the depth of the sea) up to heights of 40 meters and beyond. The tsunami generated by an earthquake in Chile in 1960, as well as destroying all the villages along 800 km of coastline, covered 17.000 km of Pacific Ocean and arrived in Japan after about 22 hours and caused considerable damage.
So to conclude earthquakes can be considered as the instrument by which the earth changes shape and evolves, the mountains rise and rise, the valleys open, short, earthquakes demonstrate that our planet is “alive” and constantly changing (see the flow of heat).
In the picture above, a pattern of propagation of waves caused by a tsunami. The lower the sea depth and over the crest of the wave rises.