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.
