Causes and characteristics of Earthquakes
Earthquakes are vibrations in the Earth's crust that cause shaking at the surface. They are highly unpredictable and often occur suddenly without warning. As yet, we have no way of fully and accurately predicting when an earthquake will occur. Earthquakes are common events on the Earth, with around 50,000 occurring every year around the globe, mainly on the plate margins (see a map of latest British Isles Earthquakes). A lot of these earthquakes will be imperceptible to people and will be only picked up on the most sensitive of seismometers. We do know where most earthquakes will occur, and they tend to coincide with destructive, conservative, collisional and constructive plate margins.
Reasons for Earthquakes happening
Earthquakes occur because stresses build up between the plates as one plate passes another. As the plates move past one another they don't do so smoothly, rather, they snag and grind, allowing energy to build up. When the plates eventually move again this energy is released as shock or seismic waves through the Earth's crust. The point at which this slippage occurs is called the FOCUS, whilst the point on the ground surface above the earthquake FOCUS is called the EPICENTRE. Seismic shock waves will emanate radially outwards from these points and their energy will reduce with distance. This is typical of destructive margins (which account for 90% of the World's earthquakes) where the Oceanic plate grinds under a Continental plate (as on the East coast of Japan -see Kobe case study). They also occur at conservative margins, such as the San Andreas Fault line, where the North American plate and Pacific plate are grinding past one another
Earthquakes can also occur at constructive plate margins. Here, the earthquake is the result of magmas forcing its way between the plates, causing the earth to tremble. Collisional margins, where continental crust meets continental crust, can also have earthquakes as a result of the pressures generated by collision.
BODY WAVES - The first waves in an earthquake will shake the ground UP then down in a longitudinal movement. These waves are called P or PRIMARY waves they travel fastest, and can also cause back and forth movement. These waves are relatively weak and cause the surface to move in a back and forth motion. The next waves to arrive are S or Secondary waves, which travel slower through the crust. These waves cause the crust to move from side to side at right angles to the outward motion of the main wave. They are also called TRANSVERSE waves and are known to cause the most damage.
SURFACE WAVES – L waves –Love waves and Rayleigh waves travel along the surface only
Earthquakes can be measured using 2 scales -the Richter scale or the Mercalli scale. The Mercalli scale measures the effects of the earthquake and runs from 1 to 12. The higher up the scale the more damage is experienced by people and building structures. The Richter scale is different in that it measures the energy of an earthquake. The scale is logarithmic, which means that for every jump up the scale you get a tenfold increase in power of an earthquake. Therefore a magnitude 6 is 10 times more powerful than a magnitude 5, and 100 times more powerful than a magnitude 4. The higher the magnitude of an earthquake the less frequent it’s occurrence. The largest ever recorded was in Valvidia in Chile in 1960 and recorded 9.5 on the scale.
Reasons why Earthquake damage varies
Population Density - the more densely populated an area the more potential there is for loss of life and damage to property. One of the reasons for the huge extent of the damage caused by the earthquake which hit Kobe was the fact that the area is very densely populated.
Earthquake depth - generally, the deeper the focus of the earthquake in the Earth's crust the less damage that is caused. This is because the waves lose energy as they travel through the crust, so by the time they reach the surface the damage can be minimised.
Building design - Buildings can be designed to withstand the shaking of the earth and to limit the loss of life and damage caused. Some examples of such modifications can be seen below;
Earthquake strength - the stronger the earthquake the more damage would be caused. This is explained in how we measure earthquakes above.
Geology - the rock type of the area in which the earthquake occurs. If the area is solid rock there is generally less damage than on sands and clays. On clays, liquefaction can occur, where water penetrates between the clay particles creating quick sand like substance into which buildings can sink.