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How can a 7.1 earthquake be 10 times stronger than a 6.4? Quake magnitudes explained

Video: How prepared or unprepared are you for an earthquake?

FEMA explains what you should do before an earthquake happens and when it occurs in an animated video called "When The Earth Shakes."
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FEMA explains what you should do before an earthquake happens and when it occurs in an animated video called "When The Earth Shakes."

The earthquake that rattled Californians on Friday night measured a 7.1 magnitude, which doesn’t sound much worse than the 6.4-magnitude quake that struck a day earlier.

But it was.

The small numerical difference masks the fact that Friday’s quake in the Mojave Desert was actually around 10 times larger than the first one, said Lucy Jones, a California Institute of Technology seismologist, according to the Los Angeles Times.

Wait, what?

Welcome to the moment magnitude scale, which measures the size of earthquakes, the U.S. Geological Survey says.

The moment magnitude scale is an extension of the Richter Scale developed by Caltech’s Charles F. Richter in 1935 to rate tremors, according to the USGS.

As time went on, seismologists discovered the Richter Scale really only worked well with Southern California earthquakes. Seismographs in other parts of the world generated different ranges of frequencies and distances, requiring a new scale.

The moment magnitude scale, now considered the most reliable, measures “the slip on the fault multiplied by the area of the fault surface that slips,” the USGS says.

On both scales, each whole number increase is a tenfold jump in the size of an earthquake, according to the U.S. Geological Survey.

The scale functions differently than those that measure other natural disasters.

Tornadoes are measured using the Fujita scale, which places a twister in one of six categories — 0 through 5 — primarly based on damage, which is then correlated to wind speed. Each category reflects increasing levels of destruction.

Hurricanes are categorized using the Saffir-Simpson wind scale. Sustained winds between 74 and 95 mph represent a Category 1. Category 2 winds are 96 to 11 mph, on up to Category 5, with sustained winds over 156 mph.

The moment magnitude scale is logarithmic, making huge jumps from number to number. An earthquake that registers an 8.7 is a whopping 794 times larger than a magnitude 5.8 earthquake, according to the U.S. Geological Survey.

“The magnitude scale is really comparing amplitudes of waves on a seismogram, not the STRENGTH (energy) of the quakes,” the USGS said.

The U.S. Geological Survey says that “as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.”

The survey’s “How much bigger…?” calculator can help you compare quake sizes.

One appeal of the scale is that it provides each quake with “an easy-to-remember and easy-to-interpret single-digit number,” Scientific American reports.

“A magnitude 3 is a tiny earthquake,” according to Scientific American. “A magnitude 6 is one that can cause substantial damage. A magnitude 9, like the one that caused December’s deadly Indian Ocean tsunami, is capable of causing severe devastation.”

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Jared Gilmour is a McClatchy national reporter based in San Francisco. He covers everything from health and science to politics and crime. He studied journalism at Northwestern University and grew up in North Dakota.
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