Valdivia, Chile. May 22, 1960. Magnitude 9.5. 1,655 killed. Prince William Sound, Alaska. March 26, 1964. Magnitude 9.2. 128 killed. Sumatra, Indonesia. December 26, 2004. Magnitude 9.1. 227,898 killed. Tohoku, Japan, March 11, 2011. Magnitude 9.1. 15,700 killed. These are the four most powerful earthquakes in recorded history. If you keep up with the news, then you have likely heard of Earthquakes being measured and compared by magnitude – particularly using something called the Richter scale. But how does this scale actually work? How do seismologists gauge the strength of one of the most complicated geological phenomena known? Well, grab your emergency kit and take shelter under the nearest doorway as we dive into the fascinating science and history of measuring earthquakes.

Before we begin, it is important to note that while the Richter scale has become synonymous with earthquake measurement, the scale proper is actually no longer used, having been replaced in 1979 by the more accurate and versatile Moment Magnitude Scale. However, for moderate earthquake intensities the two scales are roughly similar, and to understand the current system it is necessary to understand the history and development of its famous predecessor.

The remote detection and measurement of earthquakes is a surprisingly ancient practice, with the first modern seismometer being built in China in 132 C.E. by the great Han Dynasty mathematician and scientist Chang Heng. This instrument consisted of a two-metre tall, egg-shaped bronze vessel around which were mounted eight sculpted dragons with hinged jaws holding a small metal ball. Below these sat eight bronze frogs with open mouths. Inside the vessel hung a heavy metal pendulum, which was linked to each of the dragons’ jaws. When the seismic waves from a distant earthquake passed through the instrument, the vessel moved while the pendulum stayed still via inertia. This, in turn caused the dragon facing the direction of the earthquake’s epicentre to release its ball into the frog’s mouth. The instrument proved remarkably effective, as Chang Heng later wrote:

“On one occasion one of the dragons let fall a ball from its mouth though no perceptible shock could be felt. All the scholars at the capital [Chang’an] were astonished at this strange effect occurring without any evidence of an earthquake to cause it. But several days later a messenger arrived bringing news of an earthquake in Lung-Hsi (400 miles away). Upon this everyone admitted the mysterious power of the instrument.”

While modern seismometers are more sensitive and include systems for accurately measuring and recording seismic waves, they all fundamentally work on the same basic principle as Chang Heng’s pioneering design, using a pendulum or a weight suspended on springs as the detection element.

However, such instruments would not be developed until the 1920s, meaning early scales for classifying earthquakes were based on their physical effects – AKA its intensity. One of the earliest was the Rossi-Forel scale, developed by seismologists Michele Stefano Conte de Rossi and François-Alphonse Forel in 1873. Divided into ten categories, like the Beaufort scale for measuring wind speed, the Rossi-Forel scale was based on qualitative observations of common earthquake effects. For example, an intensity 1 and 2 earthquake was unnoticeable by most people, an intensity 5 earthquake could shift furniture and ring church bells, an intensity 7 earthquake could crack walls and bring down chimneys, while an intensity 10 earthquake was the most catastrophic of all, capable of levelling entire cities and tearing open the ground. This scale remained in use until 1902, when Italian volcanologist Giuseppe Mercalli developed his eponymous Mercalli scale, which used 12 points instead of 10 – though intensity 1 still represented an earthquake undetectable by humans and intensity 12 a catastrophic disaster. In 1931, Mercalli’s scale was refined by American seismologists Harry Wood and Frank Neumann to produce the Modified Mercalli Scale, which is still used today.

But such intensity scales were highly subjective, and as truly reliable seismometers became available in the 1920, it soon became clear that the observed intensity of earthquakes on the ground did not match up neatly with its measured magnitude – that is, the actual energy released at its hypocentre. Indeed, it was found that the intensity of an earthquake – its effects on the surface – depends as much on the depth of the hypocentre as the actual magnitude of the earthquake. In other words, a shallower, less powerful earthquake can inflict more surface damage than a deeper, more powerful one. And in case you are wondering, the epicentre of an earthquake is its origin point on the earth’s surface, while the hypocentre lies below the epicentre where the tectonic plates actually collide.

To correct this mismatch and standardize earthquake measurement, in 1935 American seismologist Charles Francis Richter introduced his famous eponymous scale, which he initially referred to simply as the local magnitude scale or simply the magnitude scale. The scale was based on the then-standard Wood-Anderson torsion seismograph, and expressed earthquake intensities as the logarithm of the maximum amplitude or height of the recorded seismic waves, expressed in microns or micrometers. This means that the Richter scale is not linear; instead, each whole-number magnitude represents an earthquake ten times more powerful than the previous number. For example, a magnitude 4 earthquake is 10³ or 1,000 times more powerful than a magnitude 1. Meanwhile, the total energy released by an earthquake increases 31.7 times between whole number magnitude values.

Richter’s magnitudes were calibrated such that a magnitude 3 earthquake detected by a Wood-Anderson seismometer located 100 kilometres from the epicentre would produce a trace on the recording chart 1mm tall. As the measured intensity of seismic waves changes with distance, Richter also created a table of correction factors to account for the actual position of the seismograph and the effects of local geology. In practice, the magnitude of a particular earthquake was determined by averaging the readings of multiple seismographs, each corrected for their respective distance from the epicentre. While Richter’s name has become synonymous with this method, his was actually a refinement of a similar method developed by Japanese seismologist Kiyoo Wadati in 1931.

Richter originally developed his scale specifically for use in California, where earthquakes tend to be moderate in magnitude and seismometers are never more than 600 kilometres from the epicentre. Because of this – and the relatively low sensitivity of early seismometers – the weakest earthquakes registered as magnitude 1 and the strongest around magnitude 8. However, as more and more seismographs were installed around the world, seismologists discovered that the accuracy of the Richter scale fell apart at greater magnitudes and distances. At magnitudes above 6.5, values calculated using Richter’s method tend to cluster or “saturate” near one another, causing the total energy release to be underestimated. Furthermore, as seismometers became ever more sensitive, they became capable of detecting earthquakes at magnitudes lower than 1 – which, given the logarithmic nature of the Richter scale, translated into negative magnitude values.

To address these shortcomings, Richter and his colleague Beno Gutenberg developed the Body Wave Magnitude and Surface Wave Magnitude scales. The former measures the magnitude of the Primary or P waves and the Secondary or S seismic waves that travel through the earth’s interior; while the latter measures the so-called Love and Rayleigh waves that travel along the earth’s surface. As the Love and Rayleigh waves of earthquakes above magnitude 4.5 can travel all the way around the world, the Surface Wave Magnitude Scale has no distance restrictions and can be measured from anywhere on the earth’s surface. But while these scales were a marked improvement over Richter’s original local magnitude scale, magnitude values still tended to saturate at magnitudes above 8. And so, in 1979, Japanese seismologist Hiroo Kanamori and American seismologist Thomas C. Hanks introduced an entirely new measurement system: the Moment Magnitude Scale.

Instead of measuring the peak amplitude of seismic waves recorded on a seismometer, the Moment Magnitude Scale is instead based on an earthquake’s Seismic Moment – that is, the total displacement or slip of the tectonic fault across its entire surface during an earthquake multiplied by the force needed to move said fault. This method avoids the saturation problem, allowing earthquakes above magnitude 8 to be accurately measured. However, the scale still expresses magnitudes logarithmically, and at moderate values lines up almost exactly with the old Richter scale. Thus, even though all seismologists now use the Moment Magnitude Scale to report earthquake magnitude, using the term “Richter Scale” as news reporters sometimes do is still technically accurate in most cases.

But what do the numbers on the Moment Magnitude Scale actually mean? As the earth’s tectonic plates are constantly in motion, earthquakes are happening all the time somewhere in the world. The vast majority of these quakes, however, are minuscule and undetectable by humans. Millions of such micro-quakes occur every year, registering at magnitudes between 1.0 and 1.9. Going up the scale, magnitude 5.0-5.9 earthquakes are considered moderate, being readily felt by humans and capable of inflicting minor damage to buildings and other infrastructure. Around 1,000 to 1,500 of these earthquakes occur every year worldwide. Magnitude 7.0-7.9 earthquakes, on the other hand, are considered major, and can inflict major damage to buildings across a radius of 250 kilometres from the epicentre. Around 10-20 such quakes occur every year. And finally, at the other end of the scale, are extreme earthquakes of magnitude 9.0 or higher. Since regular seismic monitoring began in the early 20th century, only 5 earthquakes of this magnitude have ever been recorded.

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