Here’s how earthquakes are measured.


Earthquake strength is registered on what is called the moment magnitude scale, which measures how much energy was released when the rocks along a fault moved during the quake. The magnitude is calculated using data from a network of instruments, called seismometers, that record the energy spreading from the quake’s origin in the form of waves of ground movement.

Until the 1970s, a different scale was used, one developed by an American seismologist, Charles Richter. That scale was found to be inaccurate for very large earthquakes.

The moment magnitude scale is logarithmic — that is, each whole number of magnitude represents about a 30-fold increase in energy released. So a 9.5-magnitude quake — such as the one that occurred in Chile in 1960, the strongest ever recorded — is more than 30,000 times more powerful than a 6.5-magnitude quake, which itself can be very destructive.

The amount of shaking a quake causes is measured on separate scales, such as the Modified Mercalli Scale. That value varies form place to place, and depends on the geography of the area along with the strength and depth of the quake. Using that scale, the U.S.G.S. reported a maximum shake value of just over 6 — or “strong” — near the epicenter on Friday, and that people felt shaking across the region.

Worldwide, the frequency of earthquakes has remained largely unchanged over decades. On average, there are about 1,500 quakes of magnitude 5.0 or higher every year. Of these, about 15 are magnitude 7 or higher. Theoretically a quake of magnitude 10 is possible, but the fault would have to be about 8,000 miles long, or one-third the Earth’s circumference.

Magnitude is far from a complete measure of a specific quake’s destructiveness. Other geological factors, like location and depth of the fault and type of rocks and soil can affect how much shaking and destruction an earthquake can cause. Construction methods and quality also play a major role in the number of casualties in a quake, as does the timing of the event and whether people are at home, work or out and about.

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