Was the Nepal earthquake twice as big as we thought?

Indiscriminate destruction.
Indiscriminate destruction.
Image: Reuters/Navesh Chitrakar
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On April 25, Nepal was hit with the biggest earthquake in 80 years—but just how big was it?

Amidst the destruction, there was a spat on the issue between the US and China. The US Geological Survey (USGS), which monitors earthquakes worldwide, reported that the Nepal earthquake measured at a magnitude of 7.8. However, the China Earthquakes Network Center (CENC), which hopes to provide a similar service, measured the same earthquake at a magnitude of 8.1.

A difference of 0.3 in the magnitude of the seismic activity may not seem like much, but the apparently small differences in magnitudes of earthquakes reported by different agencies around the world are, in real-life, huge. Because if we are to believe the Chinese data, the Nepal earthquake may have been 2.8 times bigger than if we believe the US data.

This is because of how earthquakes are measured.

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Scientists use a type of logarithmic scale to ensure that they are able to measure both very small and very large events on a sensible scale. Charles Francis Richter, the seismologist, said such a scale was needed because “the range between the largest and smallest magnitudes [of earthquakes] seemed unmanageably large.” (The use of his famous Richter scale was discontinued in the 1970s because it was based on California’s geography. Its current use is almost always incorrect.)

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So who is correct? There isn’t an independent body that can verify which of the two data points we should believe. Also, the discrepancy may be due to using different parameters in measurement: USGS uses moment magnitude and CENC uses surface-wave magnitude.

According to Stephen Hicks, a seismologist at the University of Liverpool, surface-wave magnitude becomes less reliable if the earthquake is bigger than 7.5 in magnitude. “Anyway,” he continues, ”the USGS magnitude estimate is the most widely used since it is uniform for all global regions and can be used to directly compare between different earthquakes.”

Correction (Apr. 29): A previous version of this item incorrectly described and displayed the difference in size between earthquakes two steps apart on the magnitude scale. The larger earthquake is 1000 times more powerful, not 100 times.