In mountains the valley gets deeper due to downward erosion and attains the shape of alphabet 'V'. This is called Vshapped valley.
Earthquakes are the vibrations caused by rocks breaking under stress. The underground surface along which the rock breaks and moves is called a fault plane.
The size or magnitude of earthquakes is determined by measuring the amplitude of the seismic waves recorded on a seismograph and the distance of the seismograph from the earthquake. These are put into a formula which converts them to a magnitude, which is a measure of the energy released by the earthquake. For every unit increase in magnitude, there is roughly a thirty-fold increase in the energy released. For instance, a magnitude 6.0 earthquake releases approximately 30 times more energy than a magnitude 5.0 earthquake, while a magnitude 7.0 earthquake releases approximately 900 times (30x30) more energy than a magnitude 5.0.
A magnitude 8.6 earthquake releases energy equivalent to about 10 000 atomic bombs of the type developed in World War II. Fortunately, smaller earthquakes occur much more frequently than large ones and most cause little or no damage.
Earthquake magnitude was traditionally measured on the Richter scale. It is often now calculated from seismic moment, which is proportional to the fault area multiplied by the average displacement on the fault.
The focus of an earthquake is the point where it originated within the Earth. The point on the Earth's surface directly above the focus is called the earthquake epicentre.
Recording earthquakes
Geoscience Australia monitors, analyses and reports on significant earthquakes to alert the Australian Government, State and Territory Governments and the public about earthquakes in Australia and overseas.
Earthquakes are detected by scientific instruments called seismometers. The word seismo originates from the Greek word seismos which means to shake or move violently and was later applied to the science and equipment associated with earthquakes. Seismographs, such as the Teledyne Geotech Helicorder pictured, were used in the past to detect earthquake activity and relied on a mechanical system to record the seismic energy in the Earth onto paper. In contrast, modern seismometers detect and convert any small movement in the Earth into an electrical signal for use in computer systems, as shown in the digital seismogram image of five seismic sensors which detected the magnitude 5.4 earthquake near Moe in Victoria on 19 June 2012.