Understanding Earthquake

by Dr Afroz Ahmad Shah.

Earthquakes continuously pose unremitting threats to our young, 10,000-year-old, civilization. There are ample evidences to map a complete profile of vulnerabilities which we have been facing in the recent past. The devastations of Acehnese and Thai coasts (2004), Kashmir and New Orleans (2005), southwest Java (2006), Sumatra (2007), western Sichuan and Myanmar (2008), Haiti (2010), Japan, New Zealand and Turkey (2011), comprise an unremitting event of death and destruction. These disasters remind us of a desperate need to understand the basics of Earthquake Science and to implement various methods of mitigation and simultaneously, think of various ways in which prediction can be made possible.

Historical beliefs of earthquakes
History tells us that earthquakes have always haunted mankind and because of their horror and destruction, people have associated these events with some supernatural forces. In Kamchatka, Russia, it was believed that earthquakes occurred when a mighty dog, Kozei, shook freshly fallen snow from his coat. Similarly, the Chinese thought that the entire Earth rode on the back of a giant fox, while the Mongolians replaced the fox with a frog. It was said that whenever these animals moved, it created earthquakes. For Japanese, a giant catfish, also called ‘The Earthquake Nmazu’, was thought to be guilty for earthquakes. It has to be controlled by God and if God looked away or left for some ceremony, it will cause a quake.

In India, which has a long history of earthquakes and cultural diversity, the beliefs were multiple. Some believed that there were seven serpents, who were guardians of the seven sections of the lowest heaven. They were also responsible for holding the Earth and when they changed shifts, the Earth shook. There is yet another story, which maintained that Earth was held up by four elephants, these were standing on the back of a giant turtle, which in turn stood on a cobra. When these animals moved, it caused earthquakes.

In Scandinavia, the god Loki was held responsible for earthquakes. It was believed that Loki killed his brother and as punishment, he was taken into an underground cave and tied to a rock. There, a serpent would drip poison down on him. However, his sister would collect this poison into a vessel and when it was full, she would change it. This may be dangerous for Loki, therefore, to avoid the poison, he would twist and turn, thereby causing earthquakes.

The Greeks believed that the temper-tantrums of Poseidon, the god of the sea, were the cause of earthquakes. The Mexican people also blamed God for earthquakes. The region around and including Mexico City, was the homeland of the Aztec Empire 500 years ago. The earthquakes and volcanic eruptions were thought to be divine punishment for their wrongdoings.

The Native Americans (Chickasaw) also had some interesting stories about earthquakes. They claimed that one earthquake was because The Great Spirit was angry with a young chief named Reelfoot (twisted feet). It is said that the chief was in love with a Choctaw princess but his father was against the marriage and therefore he, along with his warriors kidnapped the princess and secretly married her. This angered the Great Spirit and stomped His foot. The ground shook and caused the Mississippi to overflow its banks and drown the entire wedding party. This, therefore, caused the formation of the Reelfoot Lake, which stretches along the western boundary of Tennessee. The Reelfoot Lake was formed when Mississippi River changed its course following the New Madrid Earthquake of 1811–1812.

Rise of the scientific researches on earthquakes
The Lisbon earthquake of November 1, 1775 was a wake-up call for earthquake scientists and it basically stimulated the developments of the modern seismology. This occurred on All Saint’s Day, when most of the 250,000 inhabitants of Lisbon were in Church. The buildings were shaken violently and collapsed on civilians. It was followed by a tsunami, which drowned the victims, mostly those who rushed to safety from the collapsing buildings. The incidents of fire were also reported. This devastation is still fresh in Europe where a quarter of Lisbon’s population was wiped out.

This devastation influenced scientists to understand the cause of earthquakes and various scientific investigations were carried out. It was Robert Mallet, an Irish engineer with an interest in earthquakes, who studied the 1857 Neapolitan earthquakes and laid the foundations of modern observational seismology. Mallet (1859) stated that earthquakes could be caused by “the breaking up and grinding over each other of rock beds”. However, it was Edward Suess (1875), who was close to understanding the actual causes and he stated: “Earthquakes occur along lines of tectonic movement in a mountain system”. In the 1880s, an American geologist, G. K. Gilbert studied the fault scarps of the 1872 Owens Valley earthquake and stated that “when an earthquake occurs, a part of the foot slope goes up with the mountain and another part goes down relatively with the valley. It is thus divided and a little cliff marks the line of division…This little cliff is in geologic parlance, a fault scarp”.

The 1891 Mino-Owari earthquake (Japan) and 1906 San Francisco earthquake (United States) are the two events which were investigated to record the ground truth. The latter event was studied in detail by prominent scientist H. F. Reid, a professor of Geology at Johns Hopkins University. His studies laid the foundations of the Elastic Rebound Theory, which until now remains the most valid explanation for the cause of earthquakes.

He concluded that the earthquake must have involved an “elastic rebound” of previously stored elastic stress. It is like a stretched rubber band, which if broken or cut, releases elastic energy stored in the rubber band during the stretching. Similarly, the earth’s crust can gradually store elastic stress that is released suddenly during an earthquake (Figure A). This accumulation and release of stress and strain is now referred to as the “elastic rebound theory” of earthquakes. Most earthquakes are the result of the sudden elastic rebound of previously stored energy.

Figure B illustrates the process of an earthquake build-up. A straight fence is built across an unknown fault. As the crust starts to deform, it slowly distorts the fence (Stage 2). As time passes, more energy is accumulated, which is finally released during an earthquake along a fault (a fault is a fracture, where appreciable movement can be measured, Stage 3). This releases the distortion and the two parts of the fence are again straight but now there is an offset, which can be measured and the sense of motion along the fault can be also be found. In this example, it is a right lateral strike-slip fault.

This is the first of a series of articles on geosciences. In the next issue, Dr Shah will write about the plate tectonics theory.

About the Author:

Dr Afroz Ahmad Shah is a research fellow at the Earth Observatory Sciences (EOS), Nanyang Technological University, Singapore. He did his PhD in 2010 with Prof. Tim Bell in Structural and Metamorphic Research Institute (SAMRI), School of Earth & Environmental Sciences James Cook University, Townsville, Australia. He worked on tectono-metamorphic evolution of Precambrian rocks that lie in the foothills of Colorado Rocky Mountains, USA. He obtained an M.Tech. degree in engineering geosciences (2006) from IIT Kanpur India, where he worked with Prof. JN Malik on Active Tectonics of Himalayan foot hills, Nanital. Dr. Shah joined the EOS in 2010, and currently working with Prof. Kerry Edward Sieh on earthquake geology of New Guinea. He can be contacted at afroz[at]ntu.edu.sg.

Find out more about his research in the Project Collab section (link).