Earthquakes, volcanoes and Sri Lanka’s vulnerability

By Prof. Kapila Dahanayake

A spate of natural disasters has assailed countries across the globe in quick succession in recent times. For Sri Lanka, with the death and destruction in the wake of the 2004 inundations of coastal areas, set off by a tsunamigenic earthquake seared into its collective memory, any natural disaster causes anxiety.

Here we delve not only into the twin disasters of earthquakes and volcanoes but also look at Sri Lanka's vulnerability. Earthquakes and volcanoes are as old as the earth which was formed more than 4 billion years ago. There can be thousands of earthquakes and several volcanic eruptions in a given year. Only stronger earthquakes and tremors that cause widespread loss of life and property come to our attention with those of low magnitude going unnoticed.

So what is an earthquake?

Earthquakes refer to vibrations that occur suddenly on earth, when seismic waves travel inside and outside the earth at speeds of 15,000 to 20,000km per hour causing the ground to shake. These waves originate due to a fault or fracturing of rocks within the earth. The location of fracturing is known as the focus of the earthquake. During a fault, two blocks of rock move relative to each other. This movement can be vertical, horizontal or at an angle. The focus can be located at depths of several hundreds of kilometres from the earth's surface or ocean bed. The epicentre of an earthquake is identified at a point on the earth's surface or ocean bed directly above the focus.

An earthquake may generate a tsunami if its focus is located at a point less than 10km below the ocean bed; the fault movement is vertical and its magnitude-M is generally greater than 6 or 6.5. A layman on the beach feeling a strong earthquake should immediately observe the sea level and if it gradually starts to rise should go to high ground for safety.

The earth trembles in varying severities in different geographical locations depending on the magnitude of the earthquake and the underground geological conditions. The magnitude of the earthquake is calculated using data from seismometer networks located in different parts of the world. The magnitude as measured on the Richter scale developed in 1935 can have negative or positive values. Earthquakes (tremors) of magnitudes less than 3.4 may not be felt by all people while those above 6 can be categorized as dangerous. Those above 6.5 may develop tsunamis that can kill vast populations living in coastal areas sometimes thousands of kilometres from the epicentre like the December 26, 2004 Boxing Day earthquake which registered a magnitude of 9.3. In 1988, there were 99 earthquakes worldwide above the magnitude 6.0 with many of them causing extensive damage.

When taking a closer look at volcanoes, it is evident that on average about 55 of the earth's 550 or so historically active volcanoes erupt each year.

Earthquakes and volcanoes are not randomly scattered over the earth's surface but concentrated along edges of certain continents (eg. western margins of the Americas). According to the theory of plate tectonics, we know that the earth is a very dynamic planet, with its outermost shell (lithosphere) made up of a patchwork of a dozen or so (12 or 13) large, cool rigid slabs called tectonic or lithospheric plates. They are identified as either oceanic or continental plates and move horizontally relative to one another at a speed varying from less than 1 to 10cm per year.

The thickness of the plates average about 80 kilometres and are composed primarily of the earth's thin surface layer (crust) and the topmost layer of the earth's interior (part of the mantle). The plates are thicker under the continents than under the oceans.

These plates slide and collide with each other in very slow motion on top of the asthenosphere -- a 200km thick layer of the mantle that is hot, weak and capable of viscous flow characterized by convection currents thereby providing a mechanism for the plates to glide on top.

The stress that builds up where the plates grind against each other is relieved periodically through earthquakes that occur when rocks break along faults. Volcanoes, meanwhile, are formed when molten rock material or magma rises to the surface near a plate boundary and erupts along fissures or vents.
The theory of plate tectonics provides a tool to explain the distribution of earthquakes and volcanoes.

They are related to boundaries between tectonic plates and geologists recognize three principal types:

Divergent plate boundaries - these occur where the oceanic plates move away from each other. E.g mid oceanic ridges are impressive submarine mountain ranges identified in all ocean floors. The ocean floor spreads apart at these ridges and hot magma from the earth's mantle is injected to form new oceanic crust. Almost all of the world's divergent plate boundaries and associated abundant volcanoes are hidden by the oceans. The exception is Iceland where the mid-ocean ridge is exposed above sea level and active volcanoes are found on land. Earthquake activity along mid-ocean ridges is low and earthquakes occur at shallow depths.

Convergent plate boundaries - these boundaries occur when two similar or different plates collide. Most boundaries formed due to two different plates with different densities -- oceanic and continental -- generate subduction zones because one of the plates sinks or is subducted beneath the other, forming a deep trench in the ocean floor.

Collision of continental plates had formed the Himalaya mountains and such a convergence produces shallow and intermediate depth earthquakes but little volcanic activity. Earthquakes occur at the converging junction of two plates producing over 75% of the world's earthquakes at these boundaries.
About two thirds of the world's sub-aerial historically active volcanoes occur at convergent plate boundaries and typically erupt explosively like the one at Krakatowa in Indonesia.

Transform plate boundaries - these boundaries form when separate plates slide horizontally past one another. The friction between plates is so great that very large strains can build up in the rocks before they are periodically removed by large earthquakes. Transform boundaries produce earthquakes rather than volcanic activity.

One of the best examples is the San Andreas fault zone in California, United States, which is the boundary between the Pacific and North American plates. The San Andreas fault zone serves as a field laboratory for the United States Geological Survey for its extensive earthquake research studies.

Earthquakes: Looking back at Sri Lanka’s history

Stories of ancient earthquakes in Sri Lanka abound in historical texts. An earthquake had killed more than 2,000 people near Colombo Fort in 1615 while in the last few decades more than 60 earthquakes (with magnitudes between 5 and 6 on the Richter scale) have been reported in and around the island of Sri Lanka.

Tsunami 2004: Aerial view of destruction in Hambantota

Although there had been a few tsunamigenic earthquakes reported, not much damage had occurred except on December 26, 2004 in which 35,000 lost their lives and the coastal areas suffered extensive damage to property.

The tsunami recorded in ancient texts about 2,300 years ago during the reign of King Kelanitissa resulted in the sacrifice of his daughter, Viharamahadevi to stop the ill effects of the disastrous event.

About 3,000 years ago there had been subsidence of land in coastal areas due to earthquakes.

Earthquake events occurring thousands of kilometres from Sri Lanka could affect the country by way of tsunamis as it happened in 2004 and also by landslides following the onset of heavy rains at locations where ground shaking was felt.

Old volcanoes have not been reported in Sri Lanka and past tectonic events do not indicate such possibilities.

As such volcanism can be ruled out but the occurrence of hot water wells in the eastern part of the country warrant in-depth studies. However, volcanism in the Java-Sumatra Indonesian region has slightly affected Sri Lanka by way of mild tsunamis.

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