Earthquakes
An earthquake is caused by a sudden slip on a fault.A fault is a fracture of the earth along which rocks on one side have moved relative to those on the other side.Stresses in the earth's outer layer push the sides of the fault together.After the stresses accumulate over time,the rocks slips suddenly,releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.Earthquake tend to be concentrated at the boundaries of the plates.There are 7 major crustal plates on earth,about 80 km(50 miles)thick,all in constant motion relative to one another.They move at between 10mm and 130mm per year.It is estimated that there are several million earthquakes in the world each year.Many of these eqrthquakes go undetected because they occur in remote areas or have very small magnitudes.On average,about 60 earthquakes per year are classified as significant,with 19 classified as major.A significant earthquakes is one of magnitude 6.5 or higher or one of lesser magnitude that causes casualties or considerable damage.Major earthquakes have a magnitude larger than 7.0.The Great Tangshan Earthquake of July 28,1976 is one of the largest earthquakes.Many sources list it as 8.2 on the Richter scale,but Chinese Government's official sources state 7.8 or 7.6.The 2004 Indian Ocean Earthquake was an undersea earthquakes that occurred on December 26.2004.The earthquake generated a tsunami that was among the deadliest disasters in modern history.At magnitude of 9.0,it was the largest earthquake sice 9.2 magnitude Great Alaska Earthquake in 1964.The earthquake originated in the Indian Ocean,off the western coast of northern Sumatra,Indonesia.The resulting tsunami devastated the shores of Indonesia,Sri Lanka,South India,Thailand and other countries with waves up to 30 m high.It caused serious damage and deaths as far as the east coast of Africa,with the furthest recorded death due to the tsunami occuring at Port Elizabeth in South Africa,8 000 km away from the epicentre.Anywhere from 228 000 to 310 000 people are thought to have died as a result of the tsunami,and the count is not yet complete.