The focus is the point within a geological fault that is rupturing where the earthquake begins. The epicenter is the point on the surface of the earth that is directly above the focus. When the energy is released, the earthquake waves start at the focus, and then radiate out from there along the part of the fault that has ruptured. (Reference: http://earthquakesandplates.files.wordpress.com/2008/05/eqfocus.gif ) Earthquakes are most commonly found on or near plates of the earth's crust. They occur mostly along the edge of the oceanic as well as continental plates within areas of the crust where there are fractures called faults (long cracks in Earth) along which two crustal blocks have moved or slipped against each other. The picture below shows the major fault lines in Earth. (reference: https://wikis.glowscotland.org.uk/@api/deki/files/34976/=Tectonic_Plates.gif ) The earthquake depth range of 0 - 700 km is divided into three zones: shallow, intermediate, and deep. According to the zones, they have different impacts on places. 0 and 70 km deep- Shallow earthquakes 70 - 300 km deep- Intermediate earthquakes 300 - 700 km deep- Deep earthquakes Global earthquake epicenters for 1980–1996
The shallower an earthquake, the more damage it can produce at the surface; intermediate and deep-focus earthquakes are rarely destructive. This is because at greater depths, the rocks are very hot and under high pressure so they deform by flowing rather than breaking and faulting, hence having lesser impact usually.
As we all know, earthquakes causes many problems like collapsing structures. This can lead to economical damage, and even the loss of many lives! Many countries affected by earthquakes have used different methods to better manage them. One way is to implement guidelines on the location of new infrastructure and set limits to the height of buildings. An example of this method is in Tokyo, Japan, where the buildings all have a height limit so they are not too tall, so the chances of them collapsing in an earthquake is lowered.
However, there are limitations to this method. The control of land in developing countries is very difficult, where there is high rural-urban migration and the country does not have sufficient resources to improve the situation. Another method is to design earthquake-resistant infrastructures.
One such example is the Transamerica Pyramid in San Francisco. Here are some specifications of it:
The four-storey base of the building contains a total of 16,000 cu yd (12,000 m3) of concrete and over 300 mi (480 km) of steel re bar.
The building's foundation is 9 feet (2.7 m) thick and was the result of a 3 day, 24-hour continuous concrete pour
The building has a pyramid shape, with a wide base.
These are some measures used to protect the Transamerica Pyramid from earthquakes.
The disadvantages of this method is that poorer nations are unable to afford such infrastructure designs, as they require many resources. The Transamerica Pyramid cost S$75 million to build!
One other method is to educate people about earthquakes and prepare them by conducting drills regularly. This will familiarise people on what to do in the event of an earthquake. Japan is one such country that conducts drills often. Here is a video of a Japanese earthquake drill:
Most governments of earthquake-prone countries also use posters and signs to warn people of areas that are extremely prone to eartquakes. (e.g. coastal areas where eartquake can trigger tsunamis.)
However, a downside to this method is that people tend to get complacent and may not see the importance of earthquake education and drills, especially if earthquakes have not happened in that region for a while. For example, a study showed that people living in Tokyo are less prepared for earthquakes than people living in other regions of Japan as the last earthquake occurred around 90 years ago, in 1923.
Earthquakes can kill many people and destroy settlements. Although probably the most important, immediate shaking effects are not the only hazards and effects tied in to earthquakes. In fact, the aftershocks (smaller earthquakes that occur in the wake of the initial one and other tremors resulting from it) can cause more damage to the affected area. Firstly, on the smaller scale, earthquakes cause structural damage and fires. Shaking on the surface caused by seismic waves can cause buildings to collapse, resulting in people being buried underneath. Sometimes, if the earthquake is severe, gas mains may break, triggering fires. Aftershocks may occur as well, which are smaller earthquakes that occur in the wake of the initial earthquake. They can cause more damage as they can be destructive to the already damaged and weakened structures.
Secondly, the ground motions may trigger landslides and liquefaction. Earthquakes often cause landslides, especially in steep river valleys and areas of weak rocks (hills and mountains). If severe enough, they can bury a whole village under it, which can make rescue difficult. In other cases, liquefaction may occur. Liquefaction,in which water‐soaked sediment moves down-slope like a slurry. Buildings that were built on solid sediment may sink if liquefaction occurs. Furthermore, seismic sea waves, or tsunamis may occur if the sea floor suddenly shifts upward or downward, resulting in the sudden displacement of water. Tsunamis can be disastrous as the water does not quickly withdraw from the coast after the tsunami breaks. The water continues to rise for up to ten minutes until the long wavelength has passed through, resulting in widespread coastal damage.
Lastly, Disease and famine - fresh water supplies are often cut off causing typhoid and cholera. There will be a lack of shelter and food. People crowd in refugee camps, making it easier for disease to spread.
