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Building in Potential Seismic Zones

  • A retaining wall made of sand and gravel

A retaining wall made of sand and gravel (within inexpensive polymer compartments) that can withstand earthquakes, built at IISc's Earthquake Geotechnical Engineering Lab.
Credit: Prof Gali Madhavi Latha

Earthquakes have long been one of the most devastating natural calamities to property and life. On closer inspection, however, we see that it’s not the quake itself that causes the damage, but the falling structures and debris and the landslides and tsunamis triggered by the shaking that pose a threat. It is imperative to build structures that can withstand these vibrations of the earth, especially in areas prone to earthquakes, called “potential seismic zones”.

The Earth’s crust is made up of huge slabs called tectonic plates which are constantly moving. This movement not only enables the formation of the mountains and valleys we see, but also creates fractures in the crust called “fault lines”. The area around a fault line is generally prone to earthquakes.

“There are three types of plate boundaries”, explains Prof. G. Madhavi Latha, an Associate Professor at IISc who is currently researching earthquake resistant structures. “When two land masses slide past each other laterally, it is called a Transform Plate Boundary. This type causes some of the most devastating earthquakes, like the San Andreas Fault in California. When two land masses move away from each other, it’s called a Divergent Plate Boundary. This could lead to the formation of volcanoes and ridges. And finally, when two land masses collide pushing the land upward forming mountains; it’s called a Convergent Plate Boundary. The Mt. Everest was formed and is still being pushed upward by this type of movement of the crust”.

Forming geological features aside, plate movement also results in fracturing of rock. When the earth's crust moves, it causes strain in rocks; when the strain exceeds the strength of the rock; it fractures, causing an enormous outburst of energy. The energy released from the fracture travels in the form of a concentric waves, called seismic waves. When these waves travel through the body and surface of the earth, they cause violent shaking of the ground, called an earthquake.

Once we have understood the process, we can move on to building structures that can withstand the vibrations caused by seismic waves. According to Prof. Latha, “We currently have three methods of earthquake-proofing structures. Firstly, we could reduce the impact the quake has on the structure by modifying the base of the structure to absorb most of the energy. This method is called 'Decreasing Demand'. It can be achieved by either using simple techniques like compacting and densifying the ground, or using more expensive methods like isolating the structure from its base by using materials like rubber and steel springs to absorb the shock”.

“Secondly, we could build the structure with materials able to withstand the shaking. This method is called 'Increasing Capacity'. Again, we could use inexpensive and ductile materials like wood to build. Although Japan faces constant earthquakes, it does not cause much loss of property and life because they build most of their homes and buildings out of wood. One could also use shape memory alloys or inexpensive polymers to build these kinds of structures”.

The third method, called “Reducing Response” through sensors and controllers within the structures to actively counter the vibrations caused by the quake, she added. “Many big structures, like hotels and apartments use this method. Hollywood studios which stand on the California Faults use this kind of technique”, she said.

Technology has enabled us to endure harsh conditions and overcome natural disasters. But the idea of predicting earthquakes days or even weeks before remains remote. “Currently, we can predict an earthquake based on the strength of the rock and the amount of strain built up, but the prediction could be off by, sometimes, decades. Like the Himalayan earthquake which, according to predictions, should have occurred some 20 years ago”, explains Prof. Latha.

Unless we improve our forecasting methods, building faith with the civilians and governments will continue to remain a challenge bigger than building earthquake resistant structures. However, we can still reduce or even prevent the damage caused by quakes using technology that already exists. Using Tsunami warning systems, retaining walls to stop landslides and using simple tricks to earthquake-proof our structures could avert the disastrous outcome of this natural calamity.

About the expert

Madhavi Latha is an Associate Professor at the Department of Civil Engineering, IISc, Bangalore. http://civil.iisc.ernet.in/~madhavi/