Measuring Earthquakes: Magnitude and the Richter Scale

Earthquakes are sudden releases of energy from the Earth's crust, and humans have spent more than a century trying to give them reliable numbers. The most famous system, the Richter scale, was developed in California in the 1930s and still shapes how we talk about earthquake size today, even though modern seismologists often use more advanced successors.

In Australia, earthquakes are less common than in places like Japan or New Zealand, but the continent still records thousands of small tremors each year. Geoscience Australia maintains a national seismic network that logs events from the snowy ranges of Tasmania to the outback near Alice Springs, giving students plenty of local data to study.

Understanding how magnitude works helps everyone, from high schoolers studying plate tectonics in Brisbane to engineers designing buildings in Perth. This article walks through the science of the Richter scale, its mathematical basis, its limitations, and how the numbers translate into real-world consequences for communities.

How Seismologists First Measured Ground Motion

Long before the Richter scale, observers described earthquakes with subjective terms such as "slight," "strong," or "devastating." These descriptions were unreliable because they depended on the observer's distance and the type of building they occupied. By the early twentieth century, instruments called seismographs could record ground motion as a wiggly line on paper, and later as digital signals.

The breakthrough came in 1935 when Charles Richter, working with Beno Gutenberg at the California Institute of Technology, devised a numerical scale based on the amplitude of waves recorded by a specific seismograph. Their aim was to give Californian newspapers an easy way to report earthquake sizes to readers familiar with weather maps and temperature charts.

To compute a magnitude, Richter measured the height of the largest wave on a Wood-Anderson seismogram, then took its logarithm after correcting for the distance between the epicentre and the recording station. The result was a single number that could, in theory, be compared across events. An earthquake registering magnitude 5 produced waves ten times larger than one of magnitude 4, and roughly thirty times more energy.

Understanding the Richter Scale Formula

The Richter magnitude, now technically called local magnitude (ML), follows a compact mathematical relationship. It is defined as the logarithm of the maximum trace amplitude recorded on a standard seismograph, multiplied by a distance-correction factor, plus a small constant to keep numbers positive.

Because the formula uses a logarithm base 10, each whole step on the scale corresponds to a tenfold increase in measured wave amplitude. A magnitude 6 quake shakes the ground ten times harder in amplitude than a magnitude 5, and one hundred times harder than a magnitude 4. This logarithmic property lets the scale represent both tiny tremors and catastrophic events without running out of room.

The constant in the original formula was chosen so that a magnitude 0 would sit near the detection limit of 1930s instruments. Modern seismographs are far more sensitive, and seismologists routinely record negative magnitudes for the smallest quakes. The formula also assumed a particular seismograph design, which is why the scale became known as "local magnitude."

Limitations of the Original Richter Approach

Although revolutionary for its time, the Richter scale was built around a single instrument type and assumptions that did not hold for very large earthquakes. For events above magnitude 7, the Wood-Anderson seismograph often saturated, meaning the recorded trace no longer grew even though the earthquake released more energy.

The scale was calibrated for Southern California geology. Waves travel differently through hard granite, soft sediment, or thick layers of weathered sandstone. A formula tuned for the rocks near Pasadena did not always give accurate readings when applied to the basalt plains near Melbourne or the Sydney Basin.

Because of these issues, scientists searched for a scale that captured the total energy released rather than only amplitude at one station. This search led to new magnitude definitions during the 1970s and beyond, each designed to extend accurate measurement across larger and more distant events.

Modern Magnitude Scales: Moment Magnitude

The scale now used most widely by seismologists is the moment magnitude scale (Mw), introduced by Hiroo Kanamori and Thomas Hanks in 1979. Moment magnitude calculates size from the seismic moment, which combines the area of the fault that slipped, how far it moved, and the rigidity of the rocks involved.

Unlike Richter's logarithmic amplitude, moment magnitude is based on the physical work done by the fault. Because of this, it does not saturate, and a single Mw value can be assigned to earthquakes of nearly any size, from micro-tremors recorded by sensitive arrays to the largest megathrust events ever measured.

In Australia, Geoscience Australia reports magnitudes on the Mw scale for almost all significant events, although older reports from before the 1990s may still quote local magnitudes. Students comparing historical and modern data should always check which scale is being used, because small numerical differences can hide large differences in real energy release. The same tectonic forces that drive these slips also generate other dramatic geological events, and studying why volcanoes erupt gives a fuller picture of what happens where plates meet.

Seismic Monitoring Across Australia

Australia sits in the middle of the Indo-Australian tectonic plate, far from active plate boundaries, which is why most of its earthquakes are of moderate size. Even so, the country experiences more tremors than many residents realise, with dozens of felt events recorded each year. The 1989 Newcastle earthquake, a magnitude 5.6 event, killed 13 people and caused widespread damage in New South Wales, proving that even modest magnitudes can be devastating in urban areas.

Geoscience Australia operates a network of seismometers across the continent, with stations in remote Western Australia, southern Victoria, and offshore territories. These stations feed real-time data to the National Earthquake Alert Centre in Canberra, which can issue bulletins within minutes of a felt event near population centres such as Sydney or Adelaide.

Schools from Perth to Cairns can access earthquake data through educational portals linked to the national network. Science teachers in Adelaide regularly use these datasets for Year 9 Earth Science units, allowing students to plot recent tremors, calculate distances using P and S wave arrival times, and compare a magnitude 3 in their region with historical events.

What Magnitude Numbers Mean in Practice

A useful mental model is to think of magnitude steps as energy jumps. Each step up represents roughly thirty-two times more energy release than the one below. A magnitude 7 releases around 32 times more energy than a magnitude 6, and roughly one thousand times more energy than a magnitude 5.

At magnitude 2 or below, earthquakes are usually felt only by sensitive instruments or by people at rest on upper floors of buildings. Magnitude 3 events may produce a slight rattle of windows, while magnitude 4 tremors can be felt across a city the size of Hobart. Magnitude 5 can damage poorly constructed buildings, and from magnitude 6 upward, structural damage becomes widespread in populated areas.

For Australian communities, the practical lesson is that even a "moderate" magnitude matters when population centres sit near active fault lines. Planners in Newcastle rebuilt strict building codes after 1989, and engineers in Melbourne now factor seismic loading into major infrastructure projects.

Earthquake Safety in Australian Schools

Although Australia has fewer earthquakes than its neighbour New Zealand, schools still prepare for them through regular drills and structural assessments. State education departments require older masonry buildings in higher-hazard areas to be retrofitted or replaced, particularly in parts of western Victoria and the Flinders Ranges.

Classroom preparedness includes the Drop, Cover, and Hold On procedure, practised during national ShakeOut drills each year. Students in Sydney and Brisbane participate alongside their peers in Wellington and Los Angeles, an international exercise coordinated by emergency management bodies to reinforce safe habits.

Beyond drills, students can explore the science behind the alerts they receive. Free educational resources from Geoscience Australia show how a single magnitude value emerges from thousands of data points, turning a complex geophysical signal into a simple number that anyone can understand. Just as medical researchers track nutrient absorption and folic acid intake, seismologists developed specialised scales to put a number on the shaking of the ground.

To put this knowledge into action, choose one geological event reported in the news this month and find the official magnitude from Geoscience Australia, then check which scale is being used. Plot the event on a map of Australia or the Pacific Rim, calculate its depth using available arrival times, and compare its energy release to other recent earthquakes. Share your findings with your class, and you will have done the work of a junior seismologist before the end of term.