How do earthquakes happen, and why are some more destructive than others?
The earth consists of a series of solid and separate plates (tectonic plates) floating on a sea of magma. These plates move slowly because of currents within the magma. Faults occur along the margins of tectonic plates as they grind against each. The idea that the Earth consists of moving plates was first proposed by Alfred Wegener in 1915. His theory was considered controversial at the time and did not achieve widespread acceptance amongst geologists. However, as time passed increasing evidence was found to support the idea that the earth’s plates are in continual motion.
Not all plate movement cause earthquakes. If this was the case shocks and quakes would be continuous. Occasionally one of the plates will ride up on top of another. In this way mountains are formed. Earthquakes occur when the plates snag. This may cause a temporary halt to the slipping process. Eventually the pent-up energy is released in an explosive rapid movement. Once this occurs a great deal of energy is released. This energy moves through the earth’s crust in rapid waves. The place where the movement begins is called the focus or hypocentre.
The hypocentre can occur throughout the crust. The region of the earth’s surface above the hypocentre is termed the epicentre. The epicentre is usually the region that receives the brunt of the earth’s power and therefore suffers the most damage. Shallow earthquakes cause the most damage. Quakes which occur many miles beneath the earth often dissipate their energy before they reach the earth’s surface. The damage caused is also influenced by the ground make up. Solid bedrock is resistant to deformation while loose sediment and sand will move the most; the greater the movement the greater potential for damage. Earthquakes at sea can create huge waves, also known as Tsunamis. Tsunamis can be destructive to coastal areas. The Japanese earthquake and Tsunami of March 2010 resulted in the death of over 19,000 people.
Measuring Earthquakes
The power of an earthquake is measured according to magnitude. Although the scale is linear, the difference between the numbers does not relate to a simple linear progression in energy. For instance, a magnitude 4 earthquake is 30 times less powerful than a magnitude 5. A magnitude 6 earthquake releases 900 times more energy than a magnitude 4.
New Zealandand Earthquakes
My adopted country of New Zealand is very prone to earthquakes and experiences thousands every year. The majority are very small and cause no damage. Only about 150 are large enough to be felt, although sensitive seismic instrumentation will detect the rest. The reason for this seismic unrest is because New Zealand straddles the margins between the Australian and Pacific plates. Because New Zealand is subject to a great deal of seismic movement it has developed a sophisticated detection and data processing network. Data from earthquakes, and other seismic episodes, such as volcanic eruptions, are passed to ‘nerve centres’ where the risk is rapidly assessed. If need be this information can be quickly passed on to government agencies and the emergency services.
Earthquake Readiness
Earthquakes are just another potential environmental hazard humans must live with. Even with modern technology they can be difficult to predict. Not all regions of the earth are earthquake prone, although few places are totally immune. Inhabitants living in regions of known seismic activity would be wise to prepare and plan for a major event as emergency services are likely to be temporarily overwhelmed. At first, at least, outside help may be slow in coming. Earthquake education should be an essential item on the school curriculum and everyone should be encouraged to lay down an emergency cache of survival items such as food, water and first aid items. Major earthquakes may be rare events. However, when they occur the consequences, in terms of loss of life and property damage, are devastating. There are simple precautions anyone can put in place that will help them ‘get through’ and survive during and after an earthquake.