Basics

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.

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.

The seismic waveform record, which is the displacement of the Earth at the seismometer location over time, for 5 seismographs

Digital seismogram image of five
seismic sensors which detected
the magnitude 5.4 earthquake
near Moe in Victoria on 19 June 2012.

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.

Determining the location of an earthquake

The accurate locations of seismometers are stored in a database accessible by an earthquake monitoring computer system. The system also has access to crustal velocity models which provide approximate information on how fast the various earthquake waves travel through the different layers which make up the Earth in the area between the earthquake and the seismometers. The times at which the differing seismic waves arrive at various seismometers are identified by Seismic Analysts or by a computer system. The arrival times of the seismic waves at the seismometers, together with the locations of the seismometers and the speed at which the seismic waves travel to the seismometers are all used to determine the location of the earthquake. This location is also known as its focus or hypocentre which is represented by the latitude, longitude and depth below the surface.

The Teledyne Geotech Helicorder is a drum with paper wrapped around it, a stylus to draw on the paper and seismic waves that records the seismic displacement of the Earth at the seismometer location over time.

Teledyne Geotech Helicorder
used in the past to detect
earthquake activity.

How Geoscience Australia monitors earthquakes

Geoscience Australia monitors seismic data from more than 60 stations on the Australian National Seismograph Network and in excess of 300 stations worldwide in near real-time, 24 hours a day, seven days a week. Most of the 40 samples per second data are delivered within 30 seconds of being recorded at the seismometer to Geoscience Australia’s central processing facility in Canberra through various digital satellite and broadband communication systems.

Seismic data are also provided by overseas Governments which have national seismic networks. Geoscience Australia uses data provided by the Governments of New Zealand, Indonesia, Malaysia, Singapore and China and has access to data from global seismic networks provided by the USA, Japan, Germany and France. The Comprehensive Nuclear Test Ban Treaty Organisation’s International Monitoring System also provides seismic data for tsunami warning purposes.

The seismic data are collected and analysed automatically and immediately reviewed by Geoscience Australia’s Duty Seismologist.

As part of the Joint Australian Tsunami Warning Centre (JATWC), Duty Seismologists also are responsible for analysing and reporting within 10 minutes of the origin time, on earthquakes which have the potential to generate a tsunami. An earthquake alert is then sent to Geoscience Australia’s partner in the JATWC, the Australian Bureau of Meteorology, to determine tsunami advice and publish tsunami bulletins.

The parameters of all other earthquakes with a magnitude greater than 3.5 are generally computed within 20 minutes. The analysis includes its magnitude, origin time and date of the earthquake and the location of its hypocentre. Smaller earthquakes that are not detected by many seismometers are difficult to locate in real-time and, consequently, are located by Seismic Analysts using computer programs.

Revisions to the magnitudes of Australia's historical earthquakes

In 2016, Geoscience Australia revised the magnitudes of some of Australia's historical earthquakes as part of an international project to reassess the magnitude estimates of earthquakes around the globe. This project aimed to revise historic earthquake measurements to more accurately reflect their true size based on modernised measuring techniques.

As custodians of Australia's earthquake data, Geoscience Australia has updated information related to Australia's historical earthquakes, resulting in significant changes to what were previously thought to be some of Australia's largest events ever recorded. The following table represents our largest recorded earthquakes* as presented before and after the 2016 revision.

Magnitude post-2016 revisions Magnitude pre-2016 revisions Location Date

6.6

6.7

Tennant Creek, NT

1988

6.5

6.9

Meckering, WA

1968

6.4

5.6

Simpson Desert, NT

1941

6.3

6.4

Tennant Creek, NT

1988

6.3

7.2

Meeberrie, WA

1941

6.2

6.3

Collier Bay, WA.

1997

6.2

6.3

Tennant Creek, NT

1988

6.1

6.2

Cadoux, WA

1979

6.1

N/A

Petermann Ranges, NT

2016

6.0

6.0

West of Lake Mackay, WA

1970


* The earthquakes listed above have epicentres on the Australian mainland or adjacent to the Australian coast.

More information about these historical events can be viewed through the earthquake storymap.

The International Seismological Centre led this project which reassessed the location and magnitude of approximately 20 000 historical earthquakes worldwide as part of an effort to extend and improve their database of seismic events.

Frequently asked questions

What does this information add to our understanding of Australian seismicity?

Importantly in the Australian context, this project has seen significant revisions to some of Australia's largest historic earthquakes. This helps us to understand that while the magnitude of some of these Australian earthquakes may not be large when compared with events around the world; they still have significant impact. For example, the Meeberrie Earthquake of 1941 has been downgraded significantly from magnitude 7.2 to 6.3 yet it was felt over a large part of Western Australia. Reports of the earthquake came from Port Hedland in the north, to Albany and Norseman in the south, and even caused minor non-structural damage in Perth - more than 500km from the epicentre.

The largest recorded earthquake is now magnitude 6.6. Is this a good indication of the largest size earthquake we can expect on Australian shores? Or can we experience larger earthquakes, say of magnitude 7 and above like those seen in Haiti and Nepal?

While the revisions mean Australia has not experienced a magnitude 7 earthquake since record keeping began in the late 1800's, there is evidence of multiple earthquakes above magnitude 7 occurring on major faults across the Australian continent over the past 100,000 years. Even then these large earthquakes over magnitude 7 were very rare and occurred thousands of years apart.

How will this information affect our building codes?

Revising the magnitude of historical earthquakes changes our understanding of hazard levels across the country. In other words, it might decrease our estimates of how often earthquakes of a certain magnitude occur in particular regions. Geoscience Australia summarises this information in products such as the Atlas of Seismic Hazard.

The Building Code uses this information as one of various inputs to determine how strong buildings in different parts of the country should be to be sufficiently resilient to likely earthquake events.

The effects of an earthquake

Railway track damage caused by the 1968 Meckering Earthquake

Railway track damage caused
by the 1968 Meckering Earthquake
Reproduced with permission from
Alice Snooke

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.

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 can 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 can cause buildings to topple, and the sediment might 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.

Earthquake effects, based on human observation, 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 outlined below.

Modified Mercalli (MM) Scale of earthquake intensity (after Eiby 1966)
Magnitude MM Intensity
< 1.2 I
1.2 II
2.0 III
3.0 IV
4.0 V-VI
5.0 VI-VII
6.0 VII-VIII
7.0 VIII-IX
> 7.0 X-XII

MM Intensity Human Observation
I Not felt by humans, except in especially favourable circumstances, but birds and animals may be disturbed. Reported mainly from the upper floors of buildings more than ten storeys high. Dizziness or nausea may be experienced. Branches of trees, chandeliers, doors, and other suspended systems of long natural period may be seen to move slowly. Water in ponds, lakes, reservoirs, etc., may be set into seiche oscillation.
II Felt by a few persons at rest indoors, especially by those on upper floors or otherwise favourably placed. The long-period effects listed under MMI may be more noticeable.
III Felt indoors, but not identified as an earthquake by everyone. Vibrations may be likened to the passing of light traffic. It may be possible to estimate the duration, but not the direction. Hanging objects may swing slightly. Standing motorcars may rock slightly.
IV Generally noticed indoors, but not outside. Very light sleepers may be awakened. Vibration may be likened to the passing of heavy traffic, or to the jolt of a heavy object falling or striking the building. Walls and frame of building are heard to creak. Doors and windows rattle. Glassware and crockery rattle. Liquids in open vessels may be slightly disturbed. Standing motorcars may rock, and the shock can be felt by their occupants.
V Generally felt outside, and by almost everyone indoors. Most sleepers awakened. A few people frightened. Direction of motion can be estimated. Small unstable objects are displaced or upset. Some glassware and crockery may be broken. Some windows crack. A few earthenware toilet fixtures crack. Hanging pictures move. Doors and shutters swing. Pendulum clocks stop, start, or change rate.
VI Felt by all. People and animals alarmed. Many run outside. Difficulty experienced in walking steadily. Slight damage to masonry D. Some plaster cracks or falls. Isolated cases of chimney damage. Windows and crockery broken. Objects fall from shelves, and pictures from walls. Heavy furniture moves. Unstable furniture overturns. Small school bells ring. Trees and bushes shake, or are heard to rustle. Material may be dislodged from existing slips, talus slopes, or slides.
VII eneral alarm. Difficulty experienced in standing. Noticed by drivers of motorcars. Trees and bushes strongly shaken. Large bells ring. Masonry D cracked and damaged. A few instances of damage to Masonry C. Loose brickwork and tiles dislodged. Unbraced parapets and architectural ornaments may fall. Stone walls crack. Weak chimneys break, usually at the roof-line. Domestic water tanks burst. Concrete irrigation ditches damaged. Waves seen on ponds and lakes. Water made turbid by stirred-up mud. Small slips, and caving-in of sand and gravel banks.
VIII Alarm may approach panic. Steering of motor cars affected. Masonry C damaged, with partial collapse. Masonry B damaged in some cases. Masonry A undamaged. Chimneys, factory stacks, monuments, towers, and elevated tanks twisted or brought down. Panel walls thrown out of frame structures. Some brick veneers damaged. Decayed wooden piles break. Frame houses not secured to the foundation may move. Cracks appear on steep slopes and in wet ground. Landslips in roadside cuttings and unsupported excavations.  Some tree branches may be broken off.
IX General panic. Masonry D destroyed. Masonry C heavily damaged, sometimes collapsing completely. Masonry B seriously damaged. Frame structures racked and distorted. Damage to foundations general. Frame houses not secured to the foundations shift off. Brick veneers fall and expose frames. Cracking of the ground conspicuous. Minor damage to paths and roadways. Sand and mud ejected in alluviated areas, with the formation of earthquake fountains and sand craters. Underground pipes broken. Serious damage to reservoirs.
X Most masonry structures destroyed, together with their foundations. Some well-built wooden buildings and bridges seriously damaged. Dams, dykes, and embankments seriously damaged. Railway lines slightly bent. Cement and asphalt roads and pavements badly cracked or thrown into waves. Large landslides on river banks and steep coasts. Sand and mud on beaches and flat land moved horizontally. Large and spectacular sand and mud fountains. Water from rivers, lakes, and canals thrown up on the banks.
XI Wooden frame structures destroyed. Great damage to railway lines. Great damage to underground pipes.
XII Damage virtually total. Practically all works of construction destroyed or greatly damaged. Large rock masses displaced. Lines of slight and level distorted. Visible wave-motion of the ground surface reported. Objects thrown upwards into the air.

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.

Categories of non-wooden construction
Category Construction
Masonry A Structures designed to resist lateral forces of about 0.1g, such as those satisfying the New Zealand Model Building By-law, 1955. Typical buildings of this kind are well reinforced by means of steel or ferro-concrete bands, or are wholly of ferro-concrete construction. All mortar is of good quality and the design and workmanship are good. Few buildings erected prior to 1935 can be regarded as Masonry A.
Masonry B Reinforced buildings of good workmanship and with sound mortar, but not designed in detail to resist lateral forces.
Masonry C Buildings of ordinary workmanship, with mortar of average quality. No extreme weakness, such as inadequate bonding of the corners, but neither designed nor reinforced to resist lateral forces.
Masonry D Buildings with low standards of workmanship, poor mortar, or constructed of weak materials like mud brick and rammed earth. Weak horizontally.

Where do earthquakes occur?

No part of Earth's surface is free from earthquakes, but some regions experience them more frequently. They are most common at tectonic plate boundaries where different plates meet. The largest events usually happen where two plates are colliding, or colliding and sliding past one another. They particularly occur around the edge of the Pacific Plate, for example in New Zealand, Vanuatu, the Solomon Islands, Papua New Guinea, Japan and the Americas, and in Indonesia, where the Indo-Australian Plate collides with the Eurasian Plate. The depths of focus in these collision zones can range from 0-700 km.

Large shallow earthquakes also happen where two plates are pulling apart with the creation of new oceanic crust along mid-ocean ridges and on the transform faults that intersect them.

Shallow intraplate earthquakes occur in the relatively stable interior of continents away from plate boundaries. They are less common and do not follow easily recognisable patterns. This type of earthquake generally originates at shallow depths.

Although Australia is not on the edge of a plate, the continent experiences earthquakes because the Indo-Australian plate is being pushed north and is colliding with the Eurasian, Philippine and Pacific plates. This causes the build up of mainly compressive stress in the interior of the Indo-Australian plate which is released during earthquakes.

Australia's largest recorded earthquake was in 1988 at Tennant Creek in the Northern Territory, with an estimated magnitude 6.6, but it occurred in a sparsely populated area. A magnitude 6.5 earthquake at Meckering in 1968 caused extensive damage to buildings and was felt over most of southern Western Australia. Earthquakes of magnitude 4.0 or more are relatively common in Western Australia, with one occurring approximately every five years in the Meckering region.

Pre-historic Australian Earthquakes

Australia's historic seismic record of 100-200 years is poorly suited for assessing maximum magnitude earthquakes and identifying earthquake regions prone to damaging earthquakes because of the long recurrence cycles of large earthquakes. Instead, Geoscience Australia uses palaeo-seismology techniques to improve and extend the historic records of earthquakes out to tens of thousands of years.

The standard way of obtaining data on the locations and recurrence intervals of large, destructive earthquakes is to find active faults. Once found, soil and rock layers displaced across the fault can be mapped and dated to obtain single event displacements (a proxy for earthquake magnitude) and the ages of large earthquakes. Undisturbed layers draping the fault can be dated to give the time since the last event. The length of the scarp is also a proxy for palaeo-earthquake magnitude.

Figure 2: Lake Johnston scarp. (a) Shuttle Radar Topography Mission DEM.Red arrow marks the scarp. (b) Trace of the scarp with tick marks on the high side

Figure 2: Lake Johnston scarp.
(a) Shuttle Radar Topography
Mission DEM. Red arrow marks
the scarp. (b) Trace of the scarp
with tick marks on the high side.

However, fault scarps can be only subtly defined and are often difficult to recognise in the landscape. The vastness of the Australian continent also limits the effectiveness of traditional methods to identify these features, such as aerial photograph reconnaissance. High-resolution digital elevation models (DEMs) have recently emerged as an important tool for finding fault scarps (see image below). DEMs are well suited to exploration over large or remote areas, and so are useful for defining and mapping areas that are likely to have an elevated earthquake hazard.

Geoscience Australia is undertaking this research to determine recurrence rates of large earthquakes associated with individual scarps to improve the certainty of seismic hazard assessment for short return periods. Furthermore, the identification of earthquake prone regions allows emergency managers and planners to educate the local community and develop earthquake effect mitigation and response strategies. The data also provides constraint for crustal strain models.

The Neotectonic Features database contains information on faults, folds and other features within Australia that are believed to relate to large earthquakes during the Neotectonic Era (e.g. the past 5-10 million years).

  • Earthquake vibrations travel very fast, up to 14 kilometres per second. The fastest seismic waves take less than 20 minutes to reach the other side of the earth, a distance of almost 13 000 kilometres!
  • There are on average 100 earthquakes of magnitude 3.0 or more in Australia each year. Earthquakes above magnitude 5.5, such as the 5.6 magnitude event in Newcastle in 1989, occur on average every two years. About every eight years, there is a potentially damaging earthquake of magnitude 6.0 or more.