Natural disasters- Earthquakes:

Seismology and its role

Continued from last week...

one of the world's most devastating natural disasters, earthquakes are generally in the news because they occur frequently, at least on a minor scale, in some parts of the world.

Last week we enlightened you about a new theory put forward by scientists with regard to earthquakes and also featured some interesting information about these natural disasters which leave a trail of destruction wherever they take place.

Even though we have very little control over these natural phenomena, it is always good to know as much as we can about them because we never know when we might have to face one of them. So, today we thought of enlightening you about seismology and some other educational facts connected with earthquakes, in this series on natural disasters.

We have learnt so much about earthquakes and other natural disasters today thanks to the tireless efforts put in by those researching into such phenomena. There are various fields of study involved and the study of earthquakes and seismic waves that move through and around our planet Earth is known as seismology. A geologist who specialises in this field is known as a seismologist. There are thousands of seismologists in the world (there are some even in Sri Lanka) and they are constantly studying earthquakes that take place around the world so that by monitoring areas that are most prone to earthquakes, the destruction and death toll due to earthquakes could be minimised in the future.

However, up until the 20th century, the knowledge they had about the distribution of earthquakes around the world was limited; they basically had a mixed or warped picture because their knowledge was restricted mostly to earthquakes which occurred in the continents they lived in. There was a strong need to have a network linking the observations of seismologists in various parts of the world. This became a reality with the technological advances made in the 20th century. The setting up of a global seismograph network depended on the universal cooperation by seismologists of all nations to exchange travel times of seismic waves, during both war and peace. And in 1990, approximately 3300 seismographic observatories participated in international data exchange.

Today many seismographic stations have been set up around the world and each station sends readings of earthquakes or underground explosions by electronic communication to the National Earthquake Information Centre (NEIC) in Goldeu, Colarado, USA. Once the information reaches NEIC, using these readings, the officials rapidly compute the locations and magnitude of the earthquakes around the world. The readings are also sent to the International Seismological Centre (ICS) in the United Kingdom. Do you know that the ICS prints catalogues giving all this information so that they could be used for future research? In fact, it is these catalogues that provide the necessary information to carry out studies of the tectonic deformations of the earth and also calculate earthquake hazards around the world.

It is due to such studies of the global distribution of earthquakes that it has been possible to obtain crucial evidence of the present geodynamics and deformations of the whole earth.

According to research done based on the information sent from the thousands of seismographic stations around the world, about 90 per cent (and 81 per cent of the largest) earthquakes in the world take place in the horseshoe-shaped zone known as the circum-Pacific seismic belt. It is also called the Pacific Ring of Fire which is about 40,000 km long. Most part of this area bounds the Pacific Plate.

However, it is not the only area where massive earthquakes occur; they take place along other plate boundaries too such as along the Himalayan Mountains.In recent years, a decrease in the number of earthquakes taking place around the world has been detected, but seismologists and scientists point out that this decrease has nothing to do with a systematic trend. Instead they attribute it to a statistical fluctuation.

Siesmologists warn that the rapid growth of mega-cities such as Tokyo, Mexico City and Teheran in high seismic areas could result in huge death tolls exceeding even three million, in the event of a single major quake.

In general, as we explained in last week's article too, minor earthquakes take place constantly around the world in places like California, Alaska, (US) Gautamala, Indonesia, Chile, Peru, Iran, Pakistan, Turkey, New Zealand, Greece, Japan and Azoresin Portugal. But, earthquakes can occur anywhere in the world - even in New York city or England. May be even in our little island but let's certainly hope not.

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World's worst earthquakes

The world's worst earthquakes are generally measured by the death toll. And in terms of this the worst have occurred in China. The deadliest earthquake happened in China in the 16th century killing a little over 800,000 people. In recorded human history, it became the most catastrophic loss of life in the world. In addition to lying along the earthquake prone Ring of Fire, China has had a historically high density in population and this means there will be a high rate of casualties. The fact that China has had a long line of efficient bureaucracies able to document the high number of casualties from such a catastrophy as far back as the 1500s has been of great significance.

The second worst earthquake, though not as catastrophic as the one that took place in ancient China happened relatively recently. Even though we are all familiar with it as the 2004 tsunami, it was the result of an earthquake which took place in the Indian Ocean, causing well over 200,000 deaths.

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On the trail of past quakes

* The earliest reported earthquake in California was felt in 1769 by the exploring expedition of Gaspar de Portola while the group was camping about 48 kilometers (30 miles) southeast of Los Angeles.

* Before electronics allowed recordings of large earthquakes, scientists built large spring-pendulum seismometers in an attempt to record the long-period motion produced by such quakes. The largest one weighed about 15 tons. There is a medium-sized one three stories high in Mexico City that is still in operation.

*Seismoscope" to measure the shaking of the ground during an earthquake was developed in 1751, and it wasn't until 1855 that faults were recognized as the source of earthquakes.

* The core of the Earth was the first internal structural element to be identified. In 1906 R.D. Oldham discovered it from his studies of earthquake records. The inner core is solid, and the outer core is liquid and so does not transmit the shear wave energy released during an earthquake.

* The world's deadliest recorded earthquake occurred in 1556 in central China. It struck a region where most people lived in caves carved from soft rock. These dwellings collapsed during the earthquake, killing an estimated 830,000 people. In 1976 another deadly earthquake struck in Tangshan, China, where more than 250,000 people were killed.

* The earliest recorded evidence of an earthquake has been traced back to 1831 BC in the Shandong province of China, but there is a fairly complete record starting in 780 BC during the Zhou Dynasty in China.

* It was recognised as early as 350 BC by the Greek scientist Aristotle that soft ground shakes more than hard rock in an earthquake.

* The cause of earthquakes was stated correctly in 1760 by British engineer John Michell, one of the first fathers of seismology, in a memoir where he wrote that earthquakes and the waves of energy that they make are caused by "shifting masses of rock miles below the surface".

* In 1663 the European settlers experienced their first earthquake in America.

What are Seismic waves? Seismic waves are the waves of energy caused by the sudden breaking of rock within the Earth or an explosion. They are the energy that travels through the Earth and is recorded on seismographs.

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Types of Seismic waves

There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves.

Body waves can travel through the Earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves.

Body waves

Travelling through the interior of the Earth, body waves arrive before the surface waves emitted by an earthquake. These waves are of a higher frequency than surface waves.

P. waves or Primary waves

The first kind of body wave is the P wave. This is the fastest kind of seismic wave, and, consequently, the first to 'arrive' at a seismic station.

The P wave can move through solid rock and fluids, like water or the liquid layers of the Earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air.

Have you ever heard a big clap of thunder and heard the windows rattle at the same time? The windows rattle because the sound waves were pushing and pulling on the window glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake.

Dogs, for instance, commonly begin barking hysterically just before an earthquake 'hits' (or more specifically, before the surface waves arrive). Usually people can only feel the bump and rattle of these waves.

P Waves are also known as compressional waves, because of the pushing and pulling they do.

Subjected to a P wave, particles move in the same direction that the wave is moving in, which is the direction that the energy is travelling in, and is sometimes called the 'direction of wave propagation'.

S waves or secondary waves

The second type of body wave is the S wave, which is the second wave you feel in an earthquake.

An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth's outer core is a liquid. S waves move rock particles up and down, or side-to-side-perpendicular to the direction that the wave is travelling in (the direction of wave propagation).

Surface waves

Travelling only through the crust, Surface waves are of a lower frequency than body waves, and are easily distinguished on a seismogram as a result. Though they arrive after body waves, it is surface waves that are almost entirely responsible for the damage and destruction associated with earthquakes. This damage and the strength of the surface waves are reduced in deeper earthquakes.

Love waves

The first kind of Surface wave is called a Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion.

Rayleigh waves

The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885.

A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving.

Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

Concluded.