Structure of The Earth

Detecting an earthquake is much easier than predicting one. At its most simple level, a powerful earthquake can be felt by people in the area, and the damage it causes can be seen. The Mercalli scale is based on direct observations of this type. During a series of earthquakes in the Mississippi Valley, USA during 1811 and 1812 people were awakened by the rattling of ornaments and walls, and church bells in Boston, almost 2000km away, rung unattended due to the vibrations. At a great distance from the focus, or when the earthquake is small, direct observations are far less reliable.

Mechanical devices have existed to detect earthquakes for hundreds of years. The ancient Chinese are known to have had a device which consisted of four dragon heads, each facing one of the cardinal points of the compass. In each mouth was an iron ball, and below the mouth was a container. When an earthquake struck, the ball would fall from the dragon's mouth into the container. The direction from which the shockwaves originated was shown by reference to which balls had fallen.

Today, vibrations caused by the shock waves are still used to detect any movement, but the equipment is far more sophisticated. Many modern examples are so delicate that they are installed underground away from other sources of vibration such as traffic and even footsteps.

These machines, known as seismometers, or seismographs, if they can record the movements automatically, work on a simple principle; that a heavy weight suspended on a wire will remain steady as the vibrations pass, whilst the frame holding it will be vibrated.

Some vibrations will be in the vertical plain and will move the ground up and down; others in the horizontal plain will move the ground from side to side. To record both movements, two seismographs are needed, one for each plain.

The diagram below shows a simple seismograph to detect horizontal movement. As the ground is vibrated, the whole frame moves with it, but due to its mass, the weight remains steady. The pen attached to the weight stays still whilst the chart below it moves, thus leaving a record of the event. This idea can be difficult to understand at first, since we are used to moving the pen, not the paper, when we write or draw.

Two such devices will usually be installed, on facing North - South and the other facing East - West. To detect vertical movements the weight is mounted on a hinged arm held horizontal by a spring. As the frame moves up and down the weight again remains still as the spring flexes.

Earthquakes often seem to strike without warning, sometimes inflicting massive damage and casualties. Often they occur in predictable areas such as at plate margins where earthquakes have occurred in the past, so why can't scientists give adequate warning before an earthquake occurs?

Despite years of research, the use of advanced seismic recording equipment, and the efforts of many dedicated scientists, the task of predicting earthquakes is still, largely, a matter of informed guess work.

In the past there have been many earthquakes that led to massive loss of life and property, and they all struck with little or no warning. Japan has probably the most advanced earthquake detection system in the world, yet was caught completely unawares by the Kobe Earthquake of 1995.

When we talk about predicting quakes we tend to mean the accurate prediction of earthquakes that will happen soon, rather than at some vague time during the next ten, fifty or hundred years.

Long term and general predictions can be made by looking at the earthquake history of an area. We can say for certain that there will be a minor earthquake in California this week, possibly even today, because we've looked at the records for the last few decades and seen that there are hundreds of tremors every year; thousands if we include the minor ones.We cannot say when the next major one will occur.

We can say that there is a good chance that there will be a major earthquake in Mexico during the next 50 years, and that Britain is most unlikely to be hit by a damaging quake in the next two or three hundred years. Such information is useful on a long term scale but of no help when trying to predict the immediate future and save lives and property.

Accurate short term predictions are the goal of today's researchers. The Japanese and Americans have invested heavily in computer technology to constantly monitor and analyse seismic activity. Every year over 150,000 earthquakes are recorded world wide. As much detail as possible is gathered from the world wide network of seismometers, which, incidentally, were set up to monitor nuclear testing not earthquakes.

This data is processed at the Earthquake Data Centre, Boulder, Colorado, USA. By comparing data about the same earthquake, gathered at different locations, it is possible to calculate the exact location, magnitude and depth of each event.

Most of the prediction research is centered around the theory of Dilatancy. It has been noticed that when a rock is stressed it begins to expand; to dilate. This is caused by micro-cracks and fractures in the rock opening up and becoming larger. This only seems to start when a rock is roughly half way toward its breaking point.

Visually monitoring the size of a rock sample underground is not possible, but there are several indirect ways to gather the information. When a rock becomes stressed it begins to change physically. It transmits seismic waves at changing speeds, its magnetic properties can alter and its electrical resistance will also vary. The physical change in rock size may lead to a general uplifting of the ground surface or a change in the groundwater pressure and levels. Scientists monitor all these factors and are beginning to find generalized patterns of activity.

The hope is that patterns of activity can be identified which can be associated with the build up of significant earthquakes.
There has been some success with these methods, especially in relation to volcanic eruptions such as that of Mount St.Helens, USA, but accurate predictions are still not possible.

The Chinese have claimed considerable success with a totally different approach to the problem. It has long been known that animals, birds and insects seem to change their behaviour patterns before an earthquake.
In December 1974 Chinese scientists began to receive reports of snakes coming out of hibernation and freezing to death on the cold ground. This activity was followed by a series of minor tremors at the end of the month.

During January 1975 they received even more reports of strange animal behaviour. Much of this concerned larger animals such as cattle and horses which had become restless, refused to enter buildings or seemed frightened for no obvious reason.

In February that year a major earthquake struck. The epicentre was in Haicheng, the area from which most of the animal reports had been received.

It is known that some animals are very sensitive to sound, temperature, touch, light intensity and even magnetic fields, so it is perfectly possible that they can indeed detect the seismic activity which precedes an earthquake. Perhaps they are sensitive to the electrical and magnetic disturbances which form the basis of the Dilatancy theory .

There are other prediction methods, including religious beliefs and psychic powers. To date, neither of these methods has been able to demonstrate any more success than the more universally accepted scientific methods of observation and measurement. Indeed, many of the claimed successes of such techniques have been based on public ignorance and wide generalizations. As we mentioned earlier, anyone can safely predict an earthquake if you know where they occur, and give a wide enough time span. Indeed, whilst you have been reading this page, an earthquake has occurred somewhere on the planet!

The obvious answer may seem to be 'Yes, we do'. Sufficient warning would allow for the safe evacuation of the population, gas and water supplies could be cut before pipes were ruptured, and emergency services need never be caught by surprise again.

What about the other side effects though? It is unlikely that general predictions will ever be more accurate than within a year or two, perhaps ten. This time scale serves no purpose when it comes to evacuation and similar precautions. A city cannot be left deserted for years whilst the populace waits for a possible disaster. During that wait though, what would happen to the development of the area. Would business want to invest in something which might be destroyed? Would it be possible to get earthquake insurance when the insurance companies know they will have to pay out, and how many people would want to take out a mortgage on a home likely to be destroyed?

It is because of problems like these, and the fear of false alarms making the whole warning system a waste of time, that some people have suggested that warnings should never be released until a totally fail proof system has been developed.

1. Put yourself in the place of a scientist advising the Japanese government about issuing earthquake warnings. Produce a one page report outlining the advantages and disadvantages of having a system to warn the public about possible earthquakes.

Don't forget to consider the number of warnings you might need to make, the potential number of lives you could save, and the possible consequences of getting your warnings wrong.

3. If you wanted to detect earthquakes using a seismometer located at your school, where would you site it to ensure that you got the best readings? Make a decision about where you would put it, and explain why you selected that particular location.

4. We experience earthquakes in the UK, but when was the last time we had one? Write down your answer, then go to Earthquakes in the UK and see if you were correct.

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Physical Geography

Earth Science

Detecting And Predicting Earthquakes