Every earthquake is the result of strong movements that occur in the earth’s crust which cause tremors or seismic waves. The effects of a strong earthquake may be disastrous: the ground shakes and many buildings collapse causing many people to be buried under the rubble and lose their lives.

In a single year more than 80,000 earthquakes occur. Fortunately only a few take place in inhabited areas or are strong enough to cause serious damage.

As we have seen in the previous lesson the strength of an earthquake is measured by an instrument called seismograph. This draws a seismic trace of the earthquake called seismogram. The Richter scale is used to measure the strength and damage caused by an earthquake from grade 1 to 10. Each grade is 10 times stronger than the previous grade. This in fact means that an earthquake which measures grade 6 on the Richter scale is 10 times more powerful than one of grade 5, and 100 times stronger than one of grade 4.
Earthquakes cause most damage when they strike large cities. Strong earthquakes of grade 7 or over on the Richter scale may leave many dead, injured or homeless people. For example, this is what happened in 2005 in Pakistan when a grade 7.6 Richter scale earthquake killed 76,000 people and left millions of others homeless. Strong tremors can also cause extensive fires that burn for days on end. This is what happened in 1995 in Japan when an earthquake struck with such intensity that underground gas pipes and many electrical cables were broken. Where this happened many wooden houses caught fire and were reduced to cinder.

The video below shows how earthquakes can create various forms of damages including the destruction of settlements and the death of people.

Refer to the photos numbered 1 to 5 available in the resources section of this lesson. Download each photo and under each write down the effects caused by the earthquake in each image.

The effects of earthquakes are usually divided into two, primary and secondary.

Primary effects occur as a direct result from the violent shaking of the ground. This may result in the collapse of buildings and many people may be killed. For example when an earthquake occurs deep cracks may open in the ground. These damage or destroy roads, bridges, dams and other structures. In the 1995 Japan earthquake many people lost their lives when a flyover collapsed. During the same earthquake, a number of fast trains were derailed and many people lost their lives or were injured. The image below shows a road completely damaged by the earthquake.

Secondary effects occur as a result of the primary effect. In the days following an earthquake, the people suffer for lack of food, water, sleep and other necessities. Many people are injured and others remain homeless. Disease will strike due to lack of hygiene. Often, foreign governments and international voluntary organisations immediately send doctors and workers who can help the victims. Other material necessities such as blankets, camps, food, water and medicine are also dispatched. The image below shows many people homeless after an earthquake.

The video below highlights the primary and secondary effects of an earthquake.

The effects of an earthquake can also be classified into social, economic and environmental.

Examples of social impacts include people killed or injured, collapse of buildings, roads, bridges and railways damaged, water pipes may burst and water supplies contaminated, spread of disease, people may also need to be rehoused or live temporarily in camps.

An earthquake can have huge consequences on the economy of the area. Trade with other areas may be hindered due to damage in infrastructure such as roads and bridges. Shops, factories and businesses may be destroyed and the cost for rebuilding the damaged settlement is usually very high.

As a result of an earthquake we may also expect a number of environmental impacts. The built up area including important historical landmarks may be destroyed, As a result of gas leaks fires can spread damaging areas of woodland. Landslides may also occur and tsunamis often flood whole coastal areas.

Although we ae not able to predict and prevent earthquakes, much is being done to reduce the damage caused by them. In earthquake-prone areas engineers are planning structures that are anti-seismic, that is resistant to tremors. Such measures together with exercise, preparation and planning help in reducing the damage caused by earthquakes.

Some examples of building improvements resistant to earthquakes include:

• Purposely manufactured springs or rubber shock-absorbers placed in the foundations of high buildings in order to absorb the impact of the movement of the ground.
• Steel frames with cross-bracing that can sway during earth tremors.
• Computer controlled counter-weight on roofs which move to balance the effects of an earthquake.
• Lightweight roofs designed to reduce damage and injury.

The image below shows some of the main characteristics of an earthquake resistant building.

These two videos show some improvements designed by engineers constructed in earthquake prone zones to withstand strong earthquakes.

In earthquake prone areas like Japan and Italy earthquake drills in all public buildings such as schools and hospitals are carried out on a regular basis. Such drills help to reduce the impact of an earthquake and increase the chances for survival.

Work out the worksheet entitled Earthquakes worksheet available in the resources section of this lesson.