What causes earthquakes and how are they measured

Earthquakes occur when rocks beneath the surface move suddenly, releasing stored strain energy as seismic waves. Most earthquakes are tectonic, produced by forces associated with the movement of Earth’s plates. Smaller events can result from volcanic activity, collapsing caves, mine collapses, or changes in groundwater pressure.

Scientists distinguish an earthquake’s magnitude from the shaking and damage experienced at a particular location. Magnitude is calculated from seismic recordings, while intensity depends on distance, depth, local geology, buildings, and the ground beneath the observer. Understanding both helps communities in Australia prepare for tremors.

How tectonic stress becomes a tremor

Along a fault, one block of rock is pushed against another while friction prevents movement. The rocks gradually deform and store energy, much like a stretched elastic band. When stress becomes strong enough to overcome friction, the fault slips and the accumulated strain is released suddenly.

The rupture begins at a point underground called the hypocentre or focus. The point directly above it on the surface is the epicentre. Movement sends compressional P waves and shear S waves through the Earth, while surface waves travel along the ground. Their different arrival speeds allow instruments to identify an event’s location and estimate its depth.

Not every earthquake begins with a large, sudden fault rupture. Volcanic movements, landslides, and collapsing underground cavities can also generate seismic waves. Human activity can contribute in some places, including reservoir-induced seismicity and mining. For broader background on the forces involved, seismic hazard explained can provide additional context.

Why Australia experiences earthquakes

Australia lies mostly within the interior of the Australian Plate rather than directly on a plate boundary. Nevertheless, stress travels through the plate from active boundaries to the north, east, and southwest. This intraplate stress can reactivate ancient faults deep within the continent, producing small to moderate earthquakes.

Active areas include southeastern Australia, particularly the Gippsland, New England, and Southern Highlands regions, as well as parts of South Australia and southwestern Western Australia. The 2021 magnitude 6.7 Petermann Ranges earthquake in the Northern Territory was felt across large parts of the country. The 1989 Newcastle earthquake also demonstrated that cities in eastern Australia can experience significant damage from relatively moderate events.

Local geology changes how strongly a place shakes. Soft sediments around parts of Newcastle and Sydney’s Cumberland Plain can amplify seismic waves, while faults beneath and around Adelaide have produced notable earthquakes. Melbourne is also close to active southeastern structures, and Perth lies within the tectonically active southwest of the continent. These features do not mean every building is unsafe; they show why engineers consider regional faulting and local soil types together.

How instruments locate and size an event

A seismograph records ground motion using a suspended mass, a fixed frame, and a recording system. A broad-band seismometer is especially useful for detecting distant earthquakes and identifying P and S waves. Accelerometers record stronger movement and are valuable for estimating how a particular structure will shake.

The difference between each station’s P-wave and S-wave arrival times helps determine the distance to the earthquake. Multiple stations then compare those distances and calculate an epicentre. Additional stations improve accuracy, while local depth readings help distinguish between a shallow crustal event and one occurring deeper within the Earth.

Magnitude is derived from the recorded waveform rather than from a visual description. The local Richter scale, or ML, was designed for earthquakes recorded within a particular region. Body-wave magnitude, Mb, uses the amplitude of seismic body waves, while moment magnitude, Mw, estimates the total size of the fault rupture and is preferred for larger events. Authorities may revise a magnitude after more data arrives.

Magnitude, intensity, and local ground effects

Magnitude describes the overall size of an earthquake at its source. Intensity describes what people feel and what damage occurs at a given place. A moderate earthquake can therefore produce lower intensity in Adelaide than in a closer city, even though both communities report the same event magnitude.

Moment magnitude is logarithmic. An increase of one whole unit generally means roughly ten times greater ground amplitude and about 31.6 times more released energy. Intensity is reported using a scale such as the Modified Mercalli Intensity scale, which ranges from barely perceptible shaking through levels associated with widespread destruction.

Measurement terms worth knowing

Event and instrument measurements

• Magnitude estimates the size of the earthquake from its seismic waves.
• Seismic moment describes the physical size of the rupture, including fault area, slip, and rock strength.
• Focal depth measures the distance from the surface to the hypocentre.
• Peak ground acceleration records the strongest movement experienced at an instrument, often expressed as a fraction of gravitational acceleration.

Ground and impact assessments

• Modified Mercalli Intensity rates observed shaking effects at a particular location.
• Epicentral distance measures the distance from the epicentre to an observer or building.
• Site amplification describes how soft sediment or fill can increase shaking.
• Observed damage helps specialists compare instrumental predictions with real-world effects.

After an event, agencies combine station data, fault information, field observations, and reports from residents. The resulting maps show estimated shaking, mark the epicentre, and identify areas where aftershocks may occur. These products guide emergency response and support later analysis of the event.

From rupture to aftershocks and secondary hazards

A main shock is often part of an earthquake sequence. Smaller events called foreshocks may precede it, while aftershocks follow as the fault adjusts to a new position. Foreshocks do not provide reliable advance warning because they can be indistinguishable from ordinary background seismicity.

Aftershocks usually become weaker and less frequent with time, although larger aftershocks can occur. Their number and location help scientists identify the affected fault and estimate stress transferred to nearby rocks. Emergency plans therefore need to account for repeated shaking, not only the first and strongest tremor.

Earthquakes can also trigger landslides, liquefaction, damaged gas lines, fires, and, in unusual circumstances, tsunamis. A tsunami is most likely when seafloor displacement is large and vertical. Australian emergency agencies provide relevant public information through official alert systems and participating news outlets. Preparation has the same practical logic seen in sports and preparation, where repeatable routines help people manage unpredictable events.

Warnings, building rules, and the local market

Public earthquake early warning is less extensive in Australia than in countries such as Japan or the United States. Some regions and agencies provide alerts or event information, but people should not assume every damaging earthquake will receive a useful early warning. A warning can help when it gives enough time to take cover, yet the immediate priority remains protecting yourself during the shaking.

Australian building design is guided by Australian Standard AS 1170.0, which sets earthquake actions for structures. The National Construction Code references the standard, while states and territories administer building regulation through their own legislation and authorities. Modern design and good construction can limit collapse and serious injury, although no building is completely immune to severe ground movement.

The Australian insurance and home-improvement markets also influence earthquake readiness. Many domestic policies cover earthquake damage, but excesses, exclusions, and claims conditions vary, so households should read their policies carefully. The wider market includes retrofit contractors, cabinet latches, flexible gas connectors, flexible water connections, raised furniture supports, and anchoring systems for television sets and water tanks. These products can help, but installation quality and adherence to engineering advice are essential.

Preparing for tremors in Australian daily life

The standard protective action is Drop, Cover, and Hold On. People should drop to their hands and knees, cover their head and neck, and hold onto sturdy furniture until the shaking stops. Running for a doorway or outside during strong shaking creates avoidable injury risks, particularly when glass, masonry, or street furniture is moving.

Secure heavy wardrobes, bookshelves, televisions, and water tanks before an emergency, and keep heavy objects low in cupboards. Households should maintain emergency supplies, keep shoes beside each bed, and agree on an out-of-area meeting point. Australian commuters and workers can include seismic procedures in workplace drills for schools, shops, depots, hospitals, and transport networks.

Local conditions shape everyday preparations. Residents of Melbourne, Adelaide, Perth, Sydney, Darwin, and regional towns should know how their council and state emergency service distribute alerts. Downloading relevant apps, keeping a battery-backed radio, and avoiding reliance on mobile data during damaged networks can improve access to official information. The approach resembles athlete warm-up practices, where early preparation reduces avoidable harm. Bookmark the official alert sources, secure one heavy object today, and rehearse Drop, Cover, and Hold On before the next tremor.