09/22/2017

(latimes.com) We know an earthquake is severe when it has a high magnitude number. The 1985 quake that devastated Mexico City was an 8. The quake that shook central Mexico this week was a 7.1.

But what exactly is magnitude?

Basically, magnitude is a number representing the total energy released in an earthquake, said seismologist Lucy Jones. The energy released is determined by how much rock moves and how far it moves.

Here’s an explainer for what magnitudes mean, and how that relates to the shaking we feel. This explains why an 8 could be so much more terrifying than a 6.

**How does magnitude relate to how much energy was produced in an earthquake?**

For each whole-number increase in magnitude, the seismic energy released increases by about 32 times. That means a magnitude 7 earthquake produces 32 times more energy — or is 32 times stronger — than a magnitude 6.

A magnitude 8 releases 1,000 times more energy than a magnitude 6, but it releases that energy over a larger area and for a longer time, Jones said.

Originally, the definition of magnitude related to seismograms, in which machines used an ink stylus to record rapid motions on a rolling drum of paper that would measure shaking. Magnitude was about how big the waves were on a seismogram at a particular distance from the epicenter. On the so-called Richter scale, a magnitude 8 on a seismogram was 10 times bigger than a magnitude 7.

But the Richter scale was eventually scrapped in favor of what is known as the moment magnitude scale. The moment magnitude scale measures the movement of rock along the fault, and accurately measures larger earthquakes, which can last for minutes and affect a much larger area; the Richter scale did not accurately record such quakes, Jones said.

The U.S. Geological Survey has a calculator that can help you make these calculations. So, for instance, a magnitude 8.2 — probably the strongest earthquake that could hit Southern California on the San Andreas fault — would produce an astonishing 178 times more energy than the magnitude 6.7 Northridge earthquake in 1994.

Read more here: Here’s what earthquake magnitudes mean—and why an 8 can be so much scarier than a 6