DEFINITION:
An earthquake is a sudden Earth (in capitals, since we are referring to the planet), born from the sudden release of energy accumulated during a long time.
In general, the term earthquake is associated with the seismic movements of considerable size, although its entomology rigorously means “Earth movement”. Tremor caused by rock breaking and movement underneath the earth’s surface. Earthquakes may cause building and bridge collapse, downed telephone and electrical lines and may result in fires, explosions and landslides. Earthquakes may also cause large waves in the ocean called tsunamis and seaquakes that travel long distances on the water until crashing in coastal areas.
The following information includes general guidelines for safety and preparation in case of an earthquake. Since injury prevention techniques may vary from one state to another, it is advised to get in touch with your local emergency office, health department or Red Cross chapter.
TECTONIC PLATES:
The Earth’s cortex is formed by a dozen plates of an approximate density of 70 km, each with different physical and chemical characteristics. These (“tectonic”) plates are adjusting in a process that has been taking place for millions of years and have shaped what we now know as the surface of our planet, originating the continents and geographic reliefs in a process that is far from being completed. Usually these movements are slow and imperceptible, but in some cases these plates collide with each other like gigantic icebergs on an ocean of magma at the depths of the Earth, preventing their displacement. Then a plate begins moving over or under another one, causing slow changes in the topography. But if the movement is hindered, tension energy begins accumulating that will be released at some point and one of the plates will move suddenly against the other, breaking it and liberating a variable amount of energy that will originate the Earthquake.
What is a tectonic plate?
The term “tectonic plate” makes reference to the structures that make out planet. In geological terms, a plate is a rigid sleet of solid rock that makes the surface of the Earth (lithosphere), floating over igneous and melted rock that makes the center of the planet (asthenosphere). The lithosphere has a density between 15 and 200 km, the thickest being at the bottom of the sea.
Why does this tectonic plate float of it is so heavy?
Because compared with the metals that conform the core it is relatively lighter (it is made of mostly quartz and silicates).
The Earth, 225 million years ago (let’s remember that the Earth was born Although this theory was already proposed in 1596 by the Dutch cartographer Abraham Ortelius ratified by the German meteorologist Alfred Lothar Wegener in 1912 when noticing the similarity of shapes between South America and Africa. In the last 30 years, as a result of scientific developments, this theory has acquired enough support to revolutionize the understanding of many geologic phenomenons, including Earthquakes.
Earth before and after continent separation.
What are the findings that confirmed Wegener’s theory?
Fundamentally 4:
• More knowledge of the ocean floor by using the ecodoppler, sonar and other technology. It has determined that the bottom of the Atlantic was much thinner that we thought, that a submarine mountain range over 50,000 km long going around the entire Earth (Mid-Atlantic ridge), etc.
• The discovery of the “Magnetic List” of the ocean floor, which corresponds to magnetic minerals (magnetite) formed by the cooling of core magma and disposed in strips of inverse polarity among each other.
• Sea crust dispersion and recycling. The result of oil explorations has gathered samples of the ocean floor that show areas with different geological age: Some crests and reefs have been found to be younger and trenches or deep canyons have been determined to be older. This disposition coincides with the mountain range and the alternate polarity mentioned above. According to the scientists Harry H. Hess and Robert S. Dietz, the Atlantic’s lithosphere is expanding and the Pacific is shrinking. The old areas are sinking in the “trenches” and younger areas appear in the reefs, producing a sort of “recycling” of the ocean floor.
• More tremor occurrence in the crest and trench areas.
The phenomenon could be summarized by saying that the plates are in contact with each other, as large icebergs that come together or apart, causing geological changes (and tremors) at the plates’ borders.
The explanation of the reason they move is still unclear, but it could be explained by the convection, which refers to the influence that the core magma exerts on the different minerals, causing the hottest to float and sinking the coldest, similar to boiling water in a pot. The heat would come from the radioactive decanting isotopes such as uranium, torio and potassium (phenomenon that releases energy) such as from the residual heat still present from the formation of the Earth.
There are four fundamental types of plate boundaries: boundaries):
Divergent boundaries: Where a new crust generates, filling the gap of the plates when they separate.
The best known case of divergent boundary is this mid-Atlantic ridge we referred to before and that extends from the Arctic Ocean to the South of Africa. This boundary is separating the North American and Eurasian plates at a speed of 2.5 cm per year.
Convergent boundaries: Where the crust is destroyed when a plate sinks under another (subduction).
The best known example is the Nazca Plate, which is sinking under the South American in front of the coasts of Peru and Chile, originating one of the most seismic areas of the planet.
The plates may converge in the continent and originating mountain ranges such as the Himalayas.
They may also converge in the oceans, as it is happening in front of the Mariana Islands, near the Philippines, originating trenches that may reach 11,000 m deep or submarine volcanoes.
Transform boundaries: Where the crust is not destroyed or produced and the plates only slide horizontally amongst themselves.
An example of this type of boundary is the famous San Andreas Fault, in California.
Boundary zones of the plates: This is a wide belt where the boundaries are not well defined and the plate interaction effect is unclear.
TECTONIC PLATE STRUCTURE AND HOW THEY INFLUENCE GEOLOGICAL PHENOMENON
Transversal section of the earth crust illustrating tectonic plate types.
Illustration by José F. Vigil de "This Dynamic Earth", Mural Map produced jointly by the U.S. Geological Survey, el Smithsonian Institution, and the U.S. Naval Research Laboratory.
Current disposition of the different Tectonic Plates
This map of the earth’s surface shows the distribution of the world’s plates. The boundaries between each plate constitute the “faults”, which is the place where the earthquakes will be produced more often.
We can see that the west coast of America, North and South, is located over one of these boundaries, which explains the frequency of earthquakes in countries such as Chile, Peru, Mexico and the West of the United States. Something similar happens in Japan, Persian Gulf and the Philippines.
Countries located at the center of the plates and not on the edges (such as Brazil, Uruguay, Africa in general and most of Northern Europe) are at a notoriously lower risk.
FAULTS: Areas where the plates exert this force among them are called faults and are, therefore, the areas where seismic phenomenon are most likely to occur. Only 10% of earthquakes take place away from the limits of these plates.
OTHER CAUSES OF EARTHQUAKES: Underground activity originated by a volcano in process of eruption may originate a similar phenomenon. An extrinsic force caused by man has been estimated to be able to unleash an earthquake, probably a place where a geological fault was already present. This is why it has been assumed that nuclear experimentation or the force of a million tons of water accumulated in dams or artificial lakes could produce said phenomenon.
HYPOCENTER (OR FOCUS)
It is the point in the depth of the Earth where an earthquake’s energy is released. When it takes place at the cortex (up to 70 km deep) it is denominated superficial. If it takes place between 70 and 300 km it is denominated intermediate and if it is deeper: Deep (remember that the Earth’s core depth located at around 6,370 km).
EPICENTER
It is the point of the Earth’s surface directly over the hypocenter. It is, usually, the location of the earth’s surface where the intensity of the earthquake is greater. The characteristics of the fault, however, can make the point of greater intensity is away from the epicenter.
