Measuring the danger
Instruments detect about 13 000 earthquakes every year around the globe. Most of these are very low in magnitude and are not felt by people. Earthquakes of magnitude 5 or more are potentially damaging. Australia’s most costly earthquake occurred in Newcastle on 28 December 1989. A magnitude 5.4 earthquake claimed 13 lives and damaged more than 50 000 buildings.
The 1989 Newcastle earthquake was Australia’s most damaging, but the largest measured earthquake was a magnitude 6.6 tremor at Tennant Creek in the Northern Territory. This earthquake released more than 30 times the energy of that of the Newcastle earthquake, but only damaged a gas pipeline. This highlights the fact that the location of an earthquake is a major factor in its impact.
Earthquake magnitude was first measured with seismographs on the Richter Scale. It is determined using the logarithm of amplitude waves. Moment Magnitude is based on physical properties, such as the distance of movement and rock affected. Each increase of one unit in magnitude represents 32 times more energy release. Learn more from the USGS.
Result of ground motion
An earthquake causes sudden movement of the ground. This motion alone is not particularly dangerous, but its effect on buildings, hillsides and the ocean floor can be deadly.
Building collapse occurs for several reasons. The earthquake may be near the resonant frequency of the building, causing it to shake more violently than others. Or the materials may be rigid or prone to breakage, like masonry. Note: building materials that can bend and sway (wood, metal) are more likely to withstand ground movement.
Landslides occur when shaking destabilises materials that make up a slope. The earth materials move due to gravity. This may occur both on land and underwater. Underwater landslides may cause tsunamis like that in Palu in 2018.
Tsunami is the deadliest and most far-reaching hazard associated with earthquakes. The most devastating tsunami was that on Boxing Day 2004 near Sumatra. More than 230 000 people in 14 countries were killed and many more were injured or left homeless. The tsunami was caused by a magnitude 9.1 earthquake that moved the seabed several metres, displacing hundreds of kilometres of sea water. Wave height rose to 30 metres in shallow water near Aceh Indonesia. Effects were seen as far away as Antarctica.
Liquefaction occurs in water-saturated loose sediments. You can learn more in the AusEarthEd blog on liquefaction.
- Experiment with earthquake hazards at home as shown in the AusEarthEd videos about liquefaction and ground motion.
- Read about earthquakes and tsunamis on the AusEarthEd website.
- Nature Education has information and activities about structural hazards and landslides in earthquake-prone areas.
- TED-Ed explains how tsunamis work and Geoscience Australia shows how tsunamis are caused by underwater earthquakes.
- Model the storage of elastic energy in rocks and learn how this is released in earthquakes.
- Discover how we detect earthquakes in our blog and build your own seismometer as shown in the video.
- Read about the effect of materials and water on slope stability in our blogs.
- Investigate the effects of materials and water on slope stability as shown in the AusEarthEd videos.
- Surface stability is vital when building infrastructure. Explore the importance of geology in civil engineering in our series of blogs: 1, 2, 3 and 4.