• Causes of earthquakes

• Effects of earthquakes

Natural cause:

Earthquakes are the result of slow-moving processes that operate within Earth.

Earth was hot when it formed, and has been cooling ever since (near the surface, for each km into Earth, the temperature rises by about 30deg. Celsius).

Earth's cooling causes the portions of Earth to move, and that movement is what we call an earthquake.

Some of the human decisions

that have induced earthquakes:

• Dams and reservoirs:

It's just water, but, water is heavy. Large reservoirs of water created by dams have a long history of inducing earthquakes. The 2008 earthquake in Sichuan, China, that killed nearly 70,000 people was one of the most devastating in recent memory, and some scientists think it was triggered by the construction of the Zipingpu Dam nearby.

• Groundwater extraction:Taking water out of ground, which causes the water table to drop, can also destabilize an existing fault.

• Geothermal power plants:As the geothermal field operations have ramped up, seismic activity has increasedthere.

• Fracking and injection wells:

When waste fracking fluid is injected back underground into deep wells. The fluid can seep out and lubricate faults, causing them to slip more easily.

• Skyscrapers:

It is about putting too much pressure on the soft sedimentary rock below. This stress is due to all the extra steel and concrete used to make the

skyscraper solid enough

to withstand earthquakes.

The destructive effects of an earthquake can be classified into primary and secondary effects.

Primary effects:

They are the immediate damage caused by the quake, such as collapsing buildings, roads and bridges, which may kill many people. Those lucky enough to survive can suffer badly from shock and panic.

Secondary effects:

They are the after-effects of the earthquake.

Secondary effects of earthquakes

TsunamisSoil

liquefactionFire

Disease and famine

Landslides

• Fire: earthquakes destroy gas pipes and electric cables, causing fires to spread. Broken water mains prevent the Fires being extinguished. Fires spread very quickly in cities, especially in poor-quality housing areas where wooden buildings are common.

• Tsunamis: an earthquake on the sea floor or close to the coast may cause huge waves.

• Landslides: earthquakes

often cause landslides,

especially in steep river

valleys and areas of

weak rocks.

• Disease and famine: fresh water supplies are often cut off causing typhoid and cholera. Lack of shelter and food causes much suffering.

• Soil liquefaction: when soils with a high water content are violently shaken they lose their mechanical strength and behave like a fluid and sobuildings can literally sink.