Understanding earthquakes: A history of seismology

Wind, electric charges and even divine wrath were some explanations put forward over the centuries

A staff member of the Seismology Center points to a chart showing the earthquake activity detected by the central Weather Bureau in Taipei on April 20, 2015.
AFP
A staff member of the Seismology Center points to a chart showing the earthquake activity detected by the central Weather Bureau in Taipei on April 20, 2015.
font

Understanding earthquakes: A history of seismology

Beirut: The violent earthquake that struck southern Turkey and northern Syria at dawn on 6 February, and the strong aftershock that followed hours later, represent two of the 70 most severe tremors that the region has witnessed since the beginning of the 20th century.

Turkey and its surrounding areas are one of the most seismically active parts of the globe. There are two main faults, the Eastern Anatolian Fault of about 500-530 kilometres, and the Northern Anatolian Fault of about 1,350-1,500 kilometres.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

A post shared by Sky News (@skynews)

The last two earthquakes occurred in the Eastern Anatolian Fault and left thousands of people dead and tens of thousands injured in both countries, reaching 7.8 and 7.5 on the Richter scale of magnitude.

The biggest earthquake beforehand struck the north of Turkey in 1999 along the Northern Anatolian Fault, with a magnitude of 7.6. It killed 17,000 people.

What causes an earthquake?

These disasters are caused by a build-up of pressure over long periods of time in the Earth’s crust, or lithosphere, which is released suddenly and without clear warning signs, often resulting in devastating and tragic consequences .

The lithosphere consists of seven primary plates and many secondary plates. Under pressure from the heat of the planet’s interior, these plates converge, diverge, and run against each other, accumulating vast pressure over long periods that explodes in waves of energy sent in just moments across large areas.

The surface of our planet is made up of seven tectonic plates, a term rooted in its literal ‘structural’ meaning. The names of the plates — African, Antarctic, Eurasian, Indo-Australian, North American, Pacific, and South American — are related to the parts of the globe where they are located.

Fault lines, like the ones in Turkey and Syria, separate the primary and secondary plates.

Other associated causes

As well as unavoidable natural earthquakes in such areas, some are caused by human activity, such as tunnelling, damming, and projects like geothermal heat extraction and water-fracking to get shale oil and gas.

Many earthquakes are also associated with volcanic activity. But these types are usually less intense than tectonic earthquakes and don’t reach as far. When volcanoes release lava — molten rocks located at great depths where temperatures are very high — these eruptions can start seismic tremors.

There is also a type of earthquake caused by the sinking of the ground due to the erosion of limestone rocks or the collapse of mines.

Seismic pressure around Turkey is exerted as the Arabian Plate slowly moves northward, colliding with the Eurasian Plate, with the country located at the southern edge of the second plate.

As Michael Steckler of Columbia University’s Lamont-Doherty Earth Observatory told the National Public Radio (NPR): “Arabia has slowly been moving north and has been colliding with Turkey, and Turkey is moving out of the way to the west.”

Arabia has slowly been moving north and has been colliding with Turkey, and Turkey is moving out of the way to the west.

 

Michael Steckler, Columbia University's Lamont-Doherty Earth Observatory

This movement is responsible for the many earthquakes along the Eastern Anatolian Fault between the two plates for thousands of years, including an earthquake that destroyed the Syrian city of Aleppo in 1138.

Unlike its northern counterpart, the eastern fault was less active for a century until the 6 February earthquake struck.

Measuring magnitude

The Richter scale is a logarithmic scale developed by Charles Francis Richter (1900–1985), an American seismologist. Logarithmic means that each whole number is 10 times the previous one, so an earthquake of magnitude two is 10 times strong as an earthquake of magnitude one. 

The strongest earthquake known to date was the one that struck Chile in 1960 with a magnitude of 9.5. 

AFP
View 05 of the remains of Corral harbour, in the province of Valdivia, Chile, after the earthquake and the tidal waves that struck the area 22 May 1960.

The latest incident to hit southern Turkey and northern Syria was not far off the record and felt far away by people in the rest of Syria, Lebanon, Iraq, Cyprus and Egypt.

Aftershocks are an inevitable consequence of a major earthquake, as the first instance of pressure being released alters the smaller faults around the area.

Since the magnitude of the earthquake is related to the length of the fault on which it occurs, Turkey and Syria are highly unlikely to witness an earthquake stronger than the one that struck Chile in 1960. 

The US National Earthquake Information Centre estimates that about 55 earthquakes strike different parts of the world every day, most of which are too weak to be felt by people.

History of seismology

Ancient civilisations explained earthquakes, and other natural disasters, as divine wrath for human sins; the idea remained current in European science until about the mid-18th century. 

The Greeks, especially Aristotle, attributed earthquakes to winds blowing beneath the surface of the ground, while the Chinese thought they were caused by blockages that affect the ground's basic substance. 

At the beginning of European modernity, thinkers attributed earthquakes to explosions that occur in the ground or in the sky, and when European scientists discovered electric charges in the 18th century, they interpreted earthquakes as the result of the release of huge electric charges.

When Lisbon was struck by a devastating earthquake in 1755, sending sea waves that reached most of Europe's coasts, people thought it was caused by the waves. 

Modern seismology

But the emergence of modern science in the 19th century made seismology an independent field, and its new specialists relied on collecting seismic data in order to find certain patterns that could explain what is going on. 

Research focused on trying to find a link between earthquakes and astronomical and climatic phenomena. The authorities did not hesitate to form committees to study the causes of earthquakes after they occurred. 

Soon attention was drawn to geological phenomena in the quest for the causes of earthquakes. 

In the 1780s, the imperial government of Japan, a country prone to frequent earthquakes, commissioned local and foreign scientists to answer the question: How do earthquakes arise, and can they be avoided?

Modern seismology dates back to 1880 when British scientist John Milne, a professor at Japan's Imperial College of Engineering, founded the Seismological Society of Japan following an earthquake that struck the country on 22 February that year. 

Modern seismology dates back to 1880 when British scientist John Milne, a professor at Japan's Imperial College of Engineering, founded the Seismological Society of Japan following an earthquake that struck the country on 22 February that year. He laid the foundations of the mathematical data approach to seismic monitoring.

He laid the foundations of the mathematical data approach to seismic monitoring.

Japan's Seikei Sekiya became the world's first professor of seismology in 1886; he based his research on mathematical data issued by a newly established meteorological agency in his country, which monitored seismic disturbances along with weather conditions. 

The mathematical approach to earthquake measurement quickly spread around the world, overcoming the previous descriptive approach in this regard, although the measuring devices were primitive compared to what we have today and could only detect strong earthquakes.

Despite the setback suffered by science in general and seismology in particular due to the two world wars, the International Seismological Association, founded in 1904, was able to establish scientific seismic records made available to any researcher around the world. The association is today part of the International Union of Geodesy and Geophysics.

Plate tectonic theory

In 1967, the American geophysicist Jason Morgan came up with the plate tectonic theory — the idea that solid plates form the Earth's surface, move toward each other, and drag continents and oceans with them. 

In January of that year, sitting in his office at Princeton University, Morgan read a new article in Science magazine written by the American geologist H. William Menard, who mapped long fissures called "fracture zones" (faults) at the bottom of the Pacific Ocean. 

Morgan (now 87) was able to calculate that fractures of the Pacific Ocean's floor are all curved around a point just north of Siberia.

Thus, was born the theory of tectonic plates, which explained not only the main cause of earthquakes, but also why the coasts of the continents look like pieces in a jigsaw puzzle: They can be joined together to form a single land mass.

Morgan was able to calculate that fractures of the Pacific Ocean's floor are all curved around a point just north of Siberia. Thus, was born the theory of tectonic plates, which explained not only the main cause of earthquakes, but also why the coasts of the continents look like pieces in a jigsaw puzzle.

Continental drift

The idea that continents moved over time dates back to before the 20th century. 
Alfred Wegener, a German climatologist, changed the scientific debate by publishing two articles on a concept called continental drift in 1912.

He suggested that a supercontinent called Pangea began to disintegrate 200 million years ago, that its parts moved apart from each other, and that the continents we see today are parts of that supercontinent. 

To support his theory, Wegener pointed to the conformity of rock formations and similar fossils in Brazil and West Africa. In addition, South America and Africa seemed to be joinable.

Although the theory was initially rejected, it gained momentum in the 1950s and 1960s when new data began to support the idea of continental drift. Maps of the ocean floor showed a huge mountain range beneath the surface of the oceans that extends to almost the entire Earth. 

The American geologist Harry Hess suggested that these mountains were the result of molten rock that emerged from the asthenosphere, a stratum of Earth 100 to 200 kilometres below the Earth's crust that is considered its weakest layer. 

When the rocks reached the surface, they cooled, forming a new crust that pushed ocean floors away from the underwater mountains. The expansion of the ocean floor drives continental drift.

This may support preliminary data on the geological impact of the February earthquake, cited by Italian seismologist Carlo Doglioni to the online news site Italy24. 

He said 225 kilometres of the fault line separating the Eastern Anatolian Plate and the Arab Plate collapsed, meaning Turkey moved at least three meters away from Syria and could have moved by as much as five to six meters.

He expected more accurate information from satellites to be available in the coming days. 

There is agreement among experts, particularly at the US Geological Survey, that no one can predict the timing of an earthquake.

The most they can do is link the likelihood of earthquakes to faults witnessing remarkable seismic activity, but the odds extend over many years.

-Abdel-Rahman Ayas is a Lebanese journalist specialising in economic and scientific affairs 

font change
Related Articles