The events which occurred in the Australia city of Newcastle on December 28, 1989, surprised almost all Australians. An earthquake - in Australia? Until that day, Australians thought of earthquakes as something that the rest of the world worried about. Indeed, in the same year, on October 17, an earthquake struck California in the United States and caused widespread havoc. But the Newcastle earthquake put the whole issue of earthquakes into the minds of Australians - if Newcastle could be hit, who could tell if their home was next?
An earthquake is basically a sudden shock on the Earth's surface. The resulting movement of the Earth's surface causes massive damage to the buildings built on top of the ground, as the buildings were designed with a solid foundation on rest on in mind. The earthquake sends out several shock waves through the Earth's crust when it occurs. These waves are called P, S, and L waves, and are what is detected by seismographs (a recording device used to detect earthquakes and other seismic activity. Seismologists (scientists who study seismic activity) can tell the distance of the earthquake by the time lapse between the detection of P and S waves. To detect the direction of the earthquake, the readings of two seismographs are compared, and circles' showing the distance the earthquake was detected at. The earthquake occurs at the point where the two circles meet. This location is called the epicentre of the earthquake.
Scientists believe that earthquakes are caused by the contact between various continental plates (refer to Continent Drift for a more detailed discussion of continental plates). At these points, called faults, the forces between the plates can become strong enough to cause the rocks at that point to break and move. The result of this is an earthquake.
Scientists measure the magnitude of earthquakes using the Richter scale, named after the American seismologist Charles F. Richter. The scale was developed by Richter in 1935. Each number on the Richter scale represents an earthquake 10 times as strong as the number below it, so an earthquake reaching 5 on the Richter scale is ten times the power of an earthquake reaching 4 on the same scale. Scientists consider earthquakes below 5 on the Richter scale to be relatively minor. An earthquake reaching over 7 is a serious disaster, and could easily cause massive loss of life if one occurred in a heavily populated area.