What is an Earthquake?
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.
Earthquakes in Australia are usually caused by movements along faults as a result of compression in the Earth’s crust.
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 earthquake epicentre is the point on the Earth’s surface directly above the focus.
The effects of an earthquake
The amplitude of the shaking caused by an earthquake depends on many factors, such as the magnitude, distance from the epicentre, depth of focus, topography, and the local ground conditions.
Earthquake effects, as noted by people, are rated using the Modified Mercalli (MM) intensity scale, which ranges from I (imperceptible) up to XII (total destruction).
For the very shallow earthquakes common in many parts of southern Australia, with a focal depth of less than 10km, people who are near the epicentre and on average ground will usually experience the maximum MM intensities in the Table below.
If the people or buildings are on soft ground such as old river sediments, the MM intensity experienced may be one to two units higher; if on solid rock, it may be one unit lower. The intensity with which the earthquake is felt may also be higher on hilltops.
In Australia, earthquakes with magnitudes of less than 3.5 seldom cause damage, and the smallest magnitude earthquake known to have caused fatalities is the magnitude 5.6 Newcastle earthquake in 1989. However, magnitude 4.0 earthquakes occasionally topple chimneys or result in other damage which could potentially cause injuries or fatalities.
Apart from causing shaking, earthquakes of magnitude 4.0 or greater may also trigger landslides which can cause casualties. The larger the magnitude of the earthquake, the bigger the area over which landslides may occur.
In areas underlain by water-saturated sediments, large earthquakes, usually magnitude 6.0 or greater, may cause liquefaction. The shaking causes the wet sediment to become quicksand and flow. Subsidence from this may cause buildings to topple, and the sediment may erupt at the surface from craters and fountains.
Undersea earthquakes can cause a tsunami, or a series of waves which can cross an ocean and cause extensive damage to coastal regions.
The destruction from strong earthquake shaking can be worsened by fires caused by downed power lines and ruptured gas mains.