Earthquakes: Underground motion

Before beginning to describe what an earthquake is, we must know about the theory of tectonic plates. It originated in Wegener’s continental drift theory, which states that during millions of years, the plates that form the Earth’s crust have been slowly, yet continuously moving. These blocks, which are found in the lithosphere (Earth’s most superficial, rigid and cold layer), they float over a layer of hot, flexible rock called the asthenosphere. Moreover, they converge (get together) or diverge (stray away) along areas of great seismic and volcanic activity.

There are three types of plates, depending on the kind of crust that makes up its surface.

– Oceanic Plates: they are only covered by a thin oceanic crust and are completely submerged. A few examples of this kind of plate are the Pacific Plate, Nazca Plate, the Cocos Plate and the Philippine Plate.

– Continental Plates: in theory, it is possible this type of plate only exists in convergence and collision phases of continental fragments. The two continental plates are the South American Plate and the Eurasian Plate.

– Mixed Plates: they are plates that are partly covered by continental crust and partly by oceanic crust. Most of the plates that currently exist are of this nature.

The way plates interact depends of the type of crust it has on its borders and its movement. This happens because the currents of the asthenosphere are different. In accordance with the aforementioned facts, the following phenomena are generated:

– Subduction zones or closing in: this happens when a plate slides beneath another one. The submerged plate transforms until it fuses within the Earth. They are zones of intense volcanic and seismic activity.

– Obduction zones or separation: the magma comes out and spreads over the ocean floor, where it cools and solidifies. It forms a new ground, causing the plates to stray from the areas they used to be in.

– Lateral motion: a partial motion is produced between two plates with a slight brushing up at the fault (the border between each plate). This friction can take place at both the oceanic and continental plates. An example of this type of motion is the San Andreas Fault, which separates the North American Plate form the Pacific Plate. The rubbing produced between the plates causes seismic activity.

The Earth moves

Earthquakes are unpredictable vibratory movements that originate within the Earth and spread in the form of waves. There are two known types of earthquake: earthquakes and tsunamis or tidal waves.

However, in some regions of America, the word tremor is used to indicate minor seismic movements.

There are three causes to an earthquake:

– Tectonic: it is the unforeseen release of energy from within the Earth due to its settling.
This placement is produced by the collision between tectonic plates and the zone where this type of movement is produced, which is known as geological fault. The hypocenter is usually located 10 to 25 kilometers deep.

– Volcanic: the ascension of magma towards the surface through the pipe of a volcano is so strong and violent, it can cause an earthquake. They have a low intensity and one cannot feel them just a short distance form the erupting volcano.
– Deep earthquake: although its origin is not totally clear, it is characterized by its hypocenter, which is located at huge depths (300 to 700km), beyond the borders of the lithosphere.

Types of earthquakes

As we mentioned before, there are two types of earthquakes, earthquakes and tsunamis. The first are motions of the Earth caused by the sudden release of energy accumulated in the Earth’s rust for a long time. The duration of a seismic movement can go from a few seconds to one or two minutes tops. However, one must separate total duration from sensitive duration; total duration is the time it takes for all of the seismic waves to pass, but only the most intense ones are felt, because the others are only picked up by seismographic instruments. Sensitive duration only last a few seconds and is comprised of the motion felt by people.

When their intensity is high, these seismic movements can bring down buildings and bridges, knock out phone and power lines, cause fires, explosions, landslides and the death of thousands of people.

A tsunami or tidal wave is a wave or several waves of great energy that travel long distances over the water until they break on coastal areas. The energy discharged by a tsunami depends on the height and speed of the wave or waves. This type of undulation removes a vastly superior amount of water form the sea than normal waves, which are produced by the wind.
Although a tsunami can take place in any ocean – as has occurred in the Atlantic and Indian- it is most frequent for them to happen in the Pacific Ocean, because the planet’s most active zone is there, the “Ring of Fire”. In addition, the type of fault that occurs between the Nazca and South American  plates is favorable to the  deformity of the ocean floor.

The main cause tsunamis are earthquakes. This telluric motion abruptly moves the ocean floor vertically in such a way that the ocean is thrust out of its habitual dynamic. Vertical motion can be of mere centimeters, but if it takes place at a deep enough spot, speed will be very high and the energy transmitted by the wave will be huge. When this immense mass of water recovers its trajectory, waves are produced.

It is worth pointing out that not all Earthquakes cause tsunamis, only the ones of great magnitude do, the ones that occur beneath the ocean floor and are capable of deforming it.

Other factors that cause tsunamis, although less frequently, are volcanic eruptions, submarine explosions, landslides and meteorites.

These causes bring on a high energy wave, yet of little depth and speed, reason for which it loses strength during its trajectory towards the shore. In addition, it is usually produced in relatively shallow waters.
Of all the factors, the most common one are volcanic eruptions, because they can sink islands or mountains in the sea in a matter of minutes.

On the subject of meteorites, there are no records of a tsunami being produced by an impact, but if it were to happen, the expansive wave it would cause upon entering the ocean or the impact at the bottom of the sea would cause waves of great amplitude in the beginning, but they would be very superficial, reason for which their effects might not be so large over great distances. Only the impact areas would be affected.

Measuring earthquakes

The intensity of earthquakes is measured with an instrument called a seismograph, which records the Earth’s vibration on a piece of paper.

Earthquakes have two measuring scales that mark their magnitude and intensity.

– Richter Scale: it is a scale used to measure the magnitude of an earthquake. Through it, one can learn the energy released at the hypocenter. According to this scale, an earthquake as a single score, that goes from 1 to 9 degrees.
Less than 3.5: the motion is barely felt, but it is recorded.
3.5- 5.4: are almost always felt, but they only cause minor damages.
5.5-6.0: slightly damages buildings.
6.1- 6.9: can cause great destruction in highly populated areas.
7.0- 7.9: causes severe damages and destruction.
8 or higher: total destruction.

– Mercalli Scale: represents the violence with which an earthquake is felt in the affected zone. It measures the intensity or damages on constructions and the impact caused to people. It is determined according to a scale that goes from 1 to 12 degrees. For example, the first degree corresponds to movements that are only perceived by seismic instruments and the 12th degree corresponds to the most catastrophic vibrations.

Effects of earthquakes

The damages caused by a seismic motion can vary depending on its magnitude.

– Destruction of houses and buildings: it is considered the most serious effect because the social cost is very high.
– Destruction of infrastructure (roads and bridges): causes an important impact on the economy because it prevents the affected area from receiving goods, like food and basic services.
– Diverse soil damage: among the most serious are fractures and liquefaction (the land behaves like quicksand or is muddy). For example, the Arena volcanoes formed due to the earthquake of Limon, in 1991 (Costa Rica).
– Landslides or avalanches: they cause serious damage to nature, housing, buildings, roads, bridges, power lines, etc.