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Read more: “Instant Expert: Earthquakes“

What is an earthquake? We have always been aware of the planet’s rumblings, but it has taken us centuries to grasp the true causes. And as for sizing them up, seismologists only settled on a reliable scale of measurement in the 1930s

What and where

Our awareness of earthquakes dates back to our earliest days as a sentient species, but for most of human history we have not understood their causes. It’s only in the past century that scientists have been able to answer the question: what exactly is an earthquake?

Earthquakes in the ancient world, including in the Mediterranean region and Middle East, occurred frequently enough to have been part of the cultural fabric of early civilisations. Legends ascribing geophysical unrest to the whims and fancies of spiritual beings are a recurring theme in early cultures. In more recent history, people began to seek physical explanations. The ancient Greeks in the shape of Aristotle and Pliny the Elder, for example, proposed that earthquakes were the result of underground winds.

The earliest scientific studies of earthquakes date back to the 18th century, sparked by an unusual series of five strong earthquakes in England in 1750 followed by the great Lisbon earthquake of 1755 in Portugal. Early investigations included cataloguing past earthquakes and trying to understand the seismic waves of energy generated during the events. These waves, which radiate from the earthquake’s source and cause the ground to heave, remained the focus of scientific efforts until the end of the 19th century. Indeed, the word “earthquake” is derived from the ancient Greek word for “shaking”, although when modern scientists say “earthquake” they are generally referring to the source, not the ground motion.

Following the 1891 Mino-Owari earthquake in Japan and the 1906 San Francisco earthquake, attention shifted to the mechanisms that give rise to these events. Using data from triangulation surveys – an early forerunner to GPS – conducted before and after the 1906 earthquake, geophysicist Harry Fielding Reid developed one of the basic tenets of earthquake science, the theory of “elastic rebound”. This describes how earthquakes occur due to the abrupt release of stored stress along a fault line (see diagram).

Another half-century elapsed before the plate tectonics revolution of the mid-20th century provided an explanation for the more fundamental question: what drives earthquakes? We now know that most earthquakes are caused by the build-up of stress along the planet’s active plate boundaries, where tectonic plates converge or slide past each other.

Other earthquake causes have also been identified, such as post-glacial rebound, when the crust returns to its non-depressed state over timescales of tens of thousands of years following the retreat of large ice sheets. Such processes, however, make up only a tiny percentage of the overall energy released by earthquakes due to plate tectonics.

Thus has modern science established the basic framework to understand where, how and why earthquakes happen. The devil continues to lurk in the details.

Sizing up

How do we measure earthquakes? By the early 20th century, geologists knew that some earthquakes create visible rips across the earth’s surface, which gives some indication of their force. But since most fault ruptures are entirely underground, we need other methods to size up and compare earthquakes.

The earliest scales were called intensity scales, which typically assign Roman numerals to the severity of shaking at a given location. Intensity scales remain in use today: well-calibrated intensity values determined from accounts of earthquake effects help us study historical earthquakes and their effects within densely populated areas, for example. Following an earthquake in Virginia in 2011, over 140,000 people reported their accounts to the US Geological Survey’s “Did You Feel It?” website.

To size up an earthquake directly, one needs to record and dissect the waves it generates. Today, this is done with seismometers employing digital recording, but it wasn’t always so. The first compact instrument capable of faithfully recording small local earthquakes was called a Wood-Anderson seismometer. When the ground shook, a mass suspended on a tense wire would rotate, directing a light onto photosensitive film. The image “drawn” by the light reflected the severity of the seismic waves passing through.

In the early 1930s, Charles Francis Richter used these seismometers to develop the first magnitude scale – borrowing the word “magnitude” from astronomy. Richter’s scale uses a logarithm to produce magnitude values that are easily tractable: each one unit increase in magnitude corresponds to a 30-fold increase in energy release. A magnitude 7 earthquake thus releases almost 1000 times more energy than a magnitude 5 earthquake.

Magnitude values are relative: no physical units are attached. Richter tuned the scale so that magnitude 0 (M0) was the smallest earthquake that he estimated could be recorded by a surface seismometer under ordinary conditions. Earthquakes with negative magnitudes are possible but thus unlikely to be recorded. The scale is open-ended, but Richter might have had an upper limit of M10 in mind: he also tuned the scale so that the largest recorded earthquakes in California/Nevada were around M7, and surmised that the 1906 San Francisco earthquake was probably around M8. (The largest earthquake recorded since then was in Chile in 1960, with an estimated magnitude of 9.5.)

Relationships have been developed since to relate the energy released by earthquakes to magnitude. In the 1960s, Keiitti Aki introduced a fundamentally different quantity: the “seismic moment”. This provides a full characterisation of the overall size of an earthquake and is the measure generally used in scientific analyses.

The so-called moment-magnitude scale was introduced to convert the seismic moment to an equivalent Richter magnitude. This figure is the one usually reported in the media. Strictly speaking this reported value is not “on the Richter scale”, because it is calculated differently to Richter’s formulation. Still, following Richter’s approach, moment-magnitude values have no physical units, and are useful for comparing earthquakes.

“Strictly speaking, earthquake magnitudes reported in the media are not on the Richter scale”