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This Word Means: Magnitude (of an earthquake)

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Magnitude is a quantitative measure of the scale of an earthquake.

A 7.7 magnitude earthquake with its epicentre close to the city of Mandalay devastated crucial infrastructure in Myanmar, with some impact felt in Thailand as well. The death toll in Myanmar crossed 1,600 on Sunday as rescuers worked to save those who may still be trapped under the debris.

For a long time, the scale of an earthquake was described in terms of numbers on the Richter scale, developed by American professor Charles Richter in 1935. But this is just one of the ways of detailing earthquakes’ characteristics and is actually no longer used much.

WHAT IS THE MAGNITUDE OF AN EARTHQUAKE?

Magnitude is a quantitative measure of the scale of an earthquake. This differs from intensity, a qualitative measure of the shaking at a given location.

Seismographs are the scientific instruments used to record the motion of the ground during an earthquake. According to the United States Geological Survey (USGS), the device is mounted onto the Earth’s surface so that when the ground shakes, the entire unit shakes with it, except for the mass on a spring. The recording device on the mass records the relative motion between itself and the rest of the instrument. This process of recording the shaking has been digitised today.

WHY DID THE RICHTER SCALE FALL INTO DISUSE?

The reading on the Richter scale is derived from a formula involving a logarithm with a base of 10, meaning the scale multiplies from one number to the next. So, an earthquake of magnitude 8.1 is 10 times larger than one of magnitude 7.1 and 100 times larger than one of magnitude 6.1. It takes into account seismic waves’ amplitude and variations in distance between various seismographs and the epicentre.

Explained | Why the sizes of earthquakes multiply rather than add up

Today, the scale is only used for measuring small earthquakes recorded locally in California, where it was developed, because of accuracy issues. USGS notes, “As more seismograph stations were installed around the world, it became apparent that the method developed by Richter was strictly valid only for certain frequency and distance ranges.”

SO HOW IS THE MAGNITUDE OF AN EARTHQUAKE MEASURED TODAY?

The modern scale measures a quantity called “moment magnitude” (denoted as Mw), which is based on variables such as the area of the rupture in a fault and the size of the seismic waves. A fault is a fracture between two blocks of rock, which allows for the movement of slabs – which sometimes leads to earthquakes.

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Moment is a physical quantity proportional to the slip on the fault and multiplied by the area of the fault surface that slips, according to the USGS. It is measured by devices, and standard formulas are then applied to convert it into a number similar to the Richter scale.

For measuring intensity, the Modified Mercalli Intensity scale and the MSK scale are used. Both classify earthquakes from I (least perceptible) to XII (most severe). These readings are based on factors such as how people perceive the shaking. The same earthquake will have different intensity readings at different places; the farther one moves away from the epicentre, the less intense the shaking.