Are We Prepared For The Next Big Earthquake?

Early morning on December 26, 2004, in my third-floor apartment in Chennai, I felt the tremors that caused the Asian tsunami. It was not a big shake, and I lazed back to sleep. Nearly two hours later, news started coming in from different parts of the southern Indian coast, Indonesia, and Sri Lanka of giant waves and the loss of thousands of lives.

It was the first tsunami experience for people of my generation. Even the seniors around us could not remember a similar lived experience. Flash forward to April 11, 2012, another major earthquake was reported from Indonesia. At Chennai, we braced ourselves for another tsunami. But it did not happen.

Why did the 2004 earthquake cause a tsunami, whereas the 2012 one did not? Even though they are unpredictable, are there any signs that can foretell an earthquake? What does the study of past earthquakes tell us about our geological origins? Geologist C.P. Rajendran and seismologist Kusala Rajendran try to answer these questions in their recent book The Rumbling Earth: The Story of Indian Earthquakes.

They write about the seismic event that caused the 2004 tsunami: “The earthquake of magnitude 9.2 originated off the west coast of Sumatra. Here the eastern part of the Indian plate slides beneath Southeast Asia. In plate tectonics jargon, such regions are called subduction zones. The earthquake occurred at a depth of 15-20 km, rupturing more than 1,200 km of plate boundary and displacing trillions of tons of rocks under the sea. The movement displaced many more trillion tons of water and generated a huge tsunami. The killer waves that radiated from the rupture zone slammed into the coastline of 11 countries from East Africa to Thailand, resulting in about 227,898 fatalities.”

If so, why didn’t the 2012 event cause similar swell waves despite a tsunami warning? The scientific explanation was that this was a tearing earthquake where the seafloor moved horizontally rather than vertically as in the 2004 event. And this, in turn, did not cause a similar upthrust of the seawater.

While the 2004 earthquake was caused by one geological plate slipping below the other, thereby causing an upward movement in the plate on the top, the 2012 earthquake was caused by a tear within a geological plate. If the 2004 event was like a book sliding below another, thereby causing a disruption, the 2012 event was like a page being torn from within one of the books.

It is like water cooling in a pot

As the authors explain, understanding continental plates moving and crashing against or separating from each other – known broadly as plate tectonics – is new to science. When Charles Darwin, famous for his theory on the evolution of life on Earth, sailed on HMS Beagle in the mid-19th century, he collected rock samples from all the places he visited. He was surprised to find a fossilised forest 2,100 metres above sea level in Argentina. Darwin and other geologists had to wait for answers to these puzzles till 1966-68 when the plate tectonics theory was developed and published.

There was also the puzzle of continent shapes fitting into each other, like the coins in a jigsaw puzzle. The authors write about the German meteorologist Alfred Wegener: “Across continents, he saw an apparent continuity in the shape of some mountain ranges, the composition of rocks, and other geological structures. The existence of identical species of plants and animals on different continents was key biological evidence.” Even though Wegener hypothesised that the continents had drifted away from each other, he could not explain what powered the continents to move.

For continents to move apart, the seafloor needs to spread, and the discovery that the seafloor was spreading was a spillover impact of an entirely different exercise – a military one, a magnetic survey executed after the Second World War to identify sunk submarines and ships. While some seabed rocks aligned with the current magnetic field polarity, others were aligned differently. American geophysicist Harry H. Hess proposed that this resulted from mid-ocean ridges (mountain ranges under the sea water) pushing land further away and this caused misalignment in seabed rocks.

In the mid-1960s, multiple research teams from different institutions came to an understanding of plate tectonics. The drifting of continents was powered by the energy generated as part of the earth’s cooling process, which also generated earthquakes.

“The mid-oceanic ridges where magma is coming out were marked by shallow (around 10-15 km deep) earthquakes,” write C.P. and Kusala Rajendran. “Their mechanism showed extension, consistent with the bulging of the magma chamber beneath the crust. On the contrary, trenches were dominated by compressional tectonics, consistent with the collision of two plates.”

In simpler terms, the earth, which started life as a ball of fire 4.6 billion years ago, is cooling convectionally – similar to how a pot of hot water would cool. Just as how hot water molecules would rise to the top as bubbles and spread over the surface to cool, hot magma comes out through the ridges, pushing the plates (on which the continents rest) away while the plates push and go under each other at the lines where they collide with each other.

At times, mountains rise along the lines where plates push against each other. The world’s youngest mountain chain, the Himalayas, rose around 50 million years ago as the Indian plate, that had broken off from the African plate, drifted across the sea and crashed against the Eurasian plate.

“The grey limestone deposited on the continental shelf off northern India, long before it began its northward journey, can be found in the Himalayas,” write the authors. “These sedimentary rocks, formed in the Tethys Sea, are rich in fossils such as reefs and bivalves and indicate abundant and diverse tropical marine fauna.”

Growth stress

The Indian plate continues to push northwards, and the Himalayas continue to grow despite the high erosional stress caused by human activities. This means that tectonic stress is building along the line where the two plates meet. These stresses are released, occasionally as major earthquakes and more often as smaller ones.

In the context of the Himalayas, the authors discuss the 1897 and 1950 Assam earthquakes, the Bihar earthquake of 1934, and the Kangra earthquake of 1905. They also refer to the Central Seismic Gap – the part of the Himalayan belt between the epicentres of the 1905 Kangra earthquake and the 1934 Bihar earthquake, which has not had a major earthquake in the recent past. This gap could be holding back seismic stress that could be expressed as a major earthquake. This seismic gap includes the mountains of Garhwal and Kumaon.

Though the authors do not mention it, many hydroelectric dams, including the 260.5-metre-high, rock- and earth-filled Tehri dam stand on the Central Seismic Gap. The seismic danger to this structure caused a long-running environmental controversy in the 1980s and 1990s.

The Rumbling Earth also deals with some of the earthquakes in central India. Even though the Killari (Latur) earthquake of 1993 was only of M 6.0 intensity, it caused disproportionate death and destruction. “One reason is the time of the event in the early morning when people were fast asleep,” explain the authors. “The non-engineered style of construction of the village adobes was another important factor… The walls and heavy roofs collapsed during the earthquake, crushing the people inside.”

More of a puzzle, even for experts, was the fact that there was an earthquake in an area that was otherwise considered tectonically stable. “Much of the state of Maharashtra sits on thick deposits of volcanic rocks that were deposited more than 60 million years ago. The sheets of dark-coloured rocks that solidified from molten lava are called basalt and vary in thickness from a few tens of metres to almost 2 km in some locations. The lava was deposited over the rigid and tectonically stable peninsular shield formed 3.8 to 2.5 billion years ago.”

The scientific consensus, the authors state, is that the 1993 earthquake could have happened over faults that had been inactive for thousands of years. The Killari earthquake of 1993, the Koyna earthquake of 1967, and the Jabalpur earthquake of 1997 show that earthquakes can strike even in areas otherwise considered geologically stable.

Concepts explained

The Rumbling Earth is a useful science book for popular reading because it explains geological and seismological concepts in detail (with illustrations). These concepts are juxtaposed with examples of historical events, thereby giving the reader an idea of what could have caused those events. The book is a pioneering effort.

A human lifespan is a nanosecond when compared to geological age. But for each of us, all our learning happens in this lifespan accorded to us. C.P. and Kusala Rajendran have used their life as the backbone of the narrative, building out from their personal milestones as they dived deep into the world of geology and seismology. It is commendable that the authors decided to articulate their lifelong learnings into this book so that even the uninitiated can be introduced to India’s seismic past.

The future is more important than the past, especially when dealing with unpredictable and destructive events such as earthquakes. Of highest concern for the authors is the Central Seismic Gap in the Himalayas, where a mega earthquake could strike in the unpredictable future. Because of thoughtless construction and development, the Himalayan slopes are becoming further vulnerable to cascading destruction.

“Like any natural process, earthquakes are bound to happen, But the aftermath would depend on our preparedness,” the authors conclude.

Unfortunately, our collective failure lies in our preparedness. When an event happens, we cry, shout, and point fingers on television talk shows and social media. Then, we return to our lives and forget.

(Published under Creative Commons from Mongabay-India. Read the original article here)