You earthquakes are natural phenomena from the internal pressures of the planet, which make the tectonic plates move, releasing such pressures. Thus, the surface feels this release in the form of a tremor, which can cause serious damage.
Read too: Why are there so many earthquakes in Chile?
Causes of earthquakes
Seismic aftershocks or earthquakes usually occur when rocks are under great pressure, coming from inside the planet. This pressure exerts a force on the rocks (tectonic plates) and looks for some way to exhaust himself. The geological faults present in the contact zones between the plates are highly favorable for the occurrence of this dissipation.
Most earthquakes occur in the contact zones between the plates, as they are areas of rocky movement and with large geological faults. However, there may also be flaws inside the plates, which allow the occurrence of concussions in interior areas, and not just at the tectonic edges.
This pressure coming from inside the planet and the tectonic movement cause destructive seismic vibrations, quite common in plate boundaries. When a rocky block collides with another, there is a tremor, whether in the ocean or on dry land.
The point where this tremor begins is called the epicenter, that is, it is the geological point on the surface directly above the focus, that is, if an epicenter is located 50 kilometers from a region, means that the focus of the tremor is at a distance of 50 kilometers and at a depth that can vary between 2 and 20 kilometers, depending on the intensity of the tremor.
This depth of up to 20 kilometers occurs in continental areas, because below that there is a very high temperature, which makes the rocky shock difficult. already the oceanic tectonic plates have a more resistant composition, being able to have epicenters of up to 690 kilometers.
Earthquakes and tsunamis
As we saw, earthquakes are natural phenomena, which can occur in any area of the planet, but with greater frequency and intensity in the contact areas of tectonic plates.
When talking about this theme, it is useful to make a distinction, because, depending on where they occur, the nomenclature of these phenomena changes.
Seismic earthquakes that occur in terrestrial, continental areas, are called earthquakes. Seismic shocks that occurred at the bottom of the oceans, on the other hand, are called tidal waves. To learn more about this phenomenon that occurs with oceanic tectonic plates, read: seaquake.
earthquake intensity
To measure the intensity of these quakes, geologists use the seismographs, devices capable of accurately measuring geological faults, which contributes to the identification of areas prone to the occurrence of earthquakes.
Scattered around the world, seismographs are capable of analyze three types of movements of the ground:
north-south horizontal;
east-west horizontal;
vertical top-down.
In addition, they are able to measure the intensity of the tremor in order to improve studies in these areas. However, still we didn't reach the ability to predict earthquakes, but rather to study which areas (geological faults) are more subject to the occurrence of shocks in the long term.
The intensity of an earthquake is measured using two scales: a Richter and Mercali. The first was created by Charles Richter in 1935. This scale measures how strong the tremor is, which presents a variation on the scale from 1st to 10th. The higher the jolt value, the more damage it does to the surface.
According to Richter, the concussions vary by a factor of 10: Scale 2 earthquake is 10 times more intense than scale 1 earthquake. A scale 6 earthquake is 100 times stronger than a scale 4 earthquake, and so on.
already the scalemercali measures the destructive power of shocks in society, ranging from I to XII. The closer to XII, the more destructive.
Associating one scale with the other, we can have earthquakes of 7º on the Richter Scale, but II on the Mercali Scale. This is because these shocks can occur in desert areas, for example, causing little or no harm to human societies.
See too: Why is the United States so hit by hurricanes and tornadoes?
consequences of earthquakes
The earthquakes, for the most part, can cause serious harm to society, given the size and intensity of each. Furthermore, tsunamis can generate giant waves of 20 meters on average, sweeping coastal areas in the blink of an eye. These waves are called tsunamis. Such waves can reach the coast at a speed of up to 800 km/hour.
Destroyed houses and buildings, bridges with compromised structures, fatalities and/or trapped under rubble are some of the consequences of these tremors in continental areas. O phenomenon itself does not necessarily cause any surface damage., as many earthquakes can occur in areas of low population density, changing the landscape in a non-significant way.
The big consequences are linked to the collapse of civil constructions in consequence of:
landslides;
fires caused by outages in electrical wiring;
dam breakage;
floods caused by tsunamis.
Earthquakes in Brazil
Brazil, being located in an area far from the contact between tectonic plates, does not have major catastrophic events related to earthquakes. However, over time, small earthquakes have already happened in our territory, but with a low grade on the Richter Scale compared to those that occur in the contact zones of tectonic plates.
In 2010 an earthquake happened in Acre and became the biggest of all earthquakes in Brazil, with 6.5º on the Richter Scale. It is worth noting that the North region feels tremors with greater intensity, but that occur in neighboring countries, such as the one that hit Peru in 2011, with 7th on the Richter Scale. As a precaution, at the time buildings and other buildings were evacuated in cities in Acre and Amazonas.
In general, the earthquakes that occur in Brazil do not exceed 5th in the ANDRichter scale, which is considered a moderate tremor, but it does not cause serious damage.
In July 2020, an earthquake of 3.5º on the Richter Scale was felt in the lsouth coast of Bahia. One month later, a 4.6° tremor was felt in the same state and lasted about 20 seconds. It may seem like a short time, but 20 seconds with a 6th or more earthquake is enough for immeasurable damage. To learn more about these earthquakes in our territory, read: earthquakes in Brazil.
Earthquakes in the world
Earthquakes are events that can bring huge losses, both from a financial and a human point of view, with a significant number of victims. See some of the major earthquakes over the past few years.
Chile, 1960: on March 22, there was the biggest earthquake ever recorded in history, a 9.5º earthquake on the Richter Scale. An estimated 1600 people died and more than 2 million were left homeless. The earthquake caused tsunamis, volcanic eruptions and many areas sank and others rose, altering the country's coastal geography.
California, 1989: In October 1989, an earthquake measuring 7.1 on the Richter Scale left 60 people dead and more than 7 billion dollars in financial damage.
Japan, 1995: With a 6.9º on the Richter scale, this earthquake, which occurred in Kobe, caused a geological shallow fault of 9 kilometers, in addition to more than 5,000 dead and 310,000 homeless.
India, 2000: An earthquake measuring 8 on the Richter Scale caused the death of 20,000 people.
Japan, 2011: A 9.1º Pacific Ocean tremor on the Richter Scale caused the emergence of tsunamis almost 20 m high. Both the tremor and the tsunami were felt on the east coast of Japan, causing the deaths of 16 thousand people, in addition to the explosion of the Fukushima Nuclear Power Plant, which ended up releasing radiation in the area.
Nepal, 2015: In April, a tremor measuring 7.8 on the Richter scale hit the region of India, Pakistan, Bangladesh, China and Nepal — the latter being the hardest hit country. According to data from the authorities, the death toll exceeded 5,000 and affected more than 8 million people in the aforementioned countries.
Mexico, 2017: two tremors were felt in the country, one of 8.5º and the other of 7.1º, all in September. Despite the high magnitude, they brought little damage and just over 100 dead, a relatively low number compared to earthquakes of the same intensity on the Richter Scale. The epicenter of these tremors occurred in sparsely inhabited places, hence the low degree of destruction and death.
Also access: Lisbon earthquake of 1755 – earthquake that caused great destruction in the Portuguese capital
solved exercises
Question 1 - (FGV-RIO) Earthquakes, volcanoes and mountain formation are extremely important geological activities that occur on Earth. Look at the location of the seismic zones and the main volcanoes on the map.
IBGE. School Geographic Atlas. IBGE: Rio de Janeiro, 2010, p. 103.
Based on this map and your knowledge, what is RIGHT to say?
A) Only the movement of separating tectonic plates causes earthquakes.
B) Only the movement of separation of the tectonic plates causes volcanism.
C) Most of the seismic zones and volcanoes are located in the center of the tectonic plates.
D) Most of the zones of intense seismic activity and volcanoes are located on the edges of tectonic plates.
E) Zones of intense seismic activity are distributed randomly, with no evident relationship with the movement of tectonic plates.
Resolution
Alternative D. Most seismic zones are located in contact areas of tectonic plates, which occur on the edges of these rock blocks.
Question 2 - (Univale) Look at the image:
Tick the true alternative to the formation of a tsunami similar to the one that hit Southeast Asia in late 2004.
A) The origin of the phenomenon is associated with events of a tectonic order.
B) The formation of accentuated typhoons and areas of high atmospheric pressure generate such a phenomenon.
C) The formation of tsunamis is necessarily associated with the coral bottom of the Indian Ocean.
D) The effect of global warming is mainly responsible for the above-normal occurrence of this type of tsunami in recent years.
E) The melting of glaciers in the Indian Ocean region is responsible for the phenomenon indicated in the engraving.
Resolution
Alternative A. Giant waves, also known as tsunamis, are caused by the movements of tectonic plates in the oceans, causing severe damage to coastal areas.
By Attila Matthias
Geography teacher