• EARTHQUAKES •
Earthquakes are caused by the motion of tectonic plates - individual sections that make up the
earth's surface like panels on a football. Immense strain accumulates along fault lines where adjacent plates meet. When the rocks separating the plates give way, the ground suddenly shakes. Of course, if we want to know why earthquakes happen, we need to dig a little deeper.
The centre of the Earth is made up of three main layers:
•The core is at the centre of the Earth
•The mantle is a mobile semi-molten layer around the core
•The outer-shell of the Earth is called the crust. Scientists call this the lithosphere - it's the part we're on now. The crust is made up of 12 individual tectonic plates. Below the sea, they can measure three to six miles (4km-9.6km) thick and under land this increases to 20-44 miles (32km-70.8km).
Below the crust, radiation from the Earth's core heats the semi-molten mantle to temperatures of over 5000°C. All fluids when heated - even molten rock - are affected by a process called convection. The makes hot liquid rise to displace cooler liquid, creating a current. Tectonic plates effectively float on the mantle, like croutons in a bowl of super-heated soup. But these plates are constantly moving due to the convection current.
Of course, they creep along very slowly - roughly the same speed your fingernails grow.
Even at this sub snail-pace, the effects can be devastating. The annual force of earthquakes is equal to 100,000 times the power of the atomic bombs that flattened Hiroshima.
The point where the seismic activity occurs is the epicenter, where the earthquake is strongest.
But it doesn't always end there; seismic waves travel out from the epicenter, sometimes creating widespread destruction as they pass.
The magnitude of an earthquake is measured by a Richter scale. A digit on the Richter scale is 30 times greater that the previous digit. This proves that an earthquake of 6 Richter scale can produce energy equivalent to the Hiroshima atom bomb.
Factors that contribute to vulnerability of people to earthquakes:
•Location of settlements in the earthquake prone areas especially on alluvial or wind blown soil deposits on areas prone to geological faults.
•Dense collection of weak buildings with high occupancy.
•Buildings traditionally constructed using earth, rubble, bricks by masons, normally with heavy roofs and poor quality of maintenance of buildings. Heavy buildings are more dangerous than light wooden or flexible buildings.
•Weak or flexible storey intending for parking purposes.
•Physical damage: damage or loss of buildings. Fire, floods due to dam failures, landslides could occur. Theses are physical damages.
•Casualties: often high, near to the epicenter and in places where the population density is high and buildings are not resistant to earthquake forces.
•Underwater earthquakes can also cause a tsunami.
•Water supply: severe problems due to failures of water supply distribution network and storage reservoirs.
•Transport network: severely affected due to the destruction of roads etc.
•Electricity and communication: all links affected. Transmission towers etc. are affected badly.
•Bridges are at great risk during an earthquake. The waves move out from the epicenter. Often there are minor tremors before an earth quake.