What Do Earthquake Magnitudes Mean?

Illustration for article titled What Do Earthquake Magnitudes Mean?

Whenever there's an earthquake you'll hear all over the media that it's "(blank number) on the Richter Scale." Surprise, the Richter Scale was abandoned years ago because it was inaccurate. Today we use the moment magnitude scale, or the MMS.


The MMS measures movement in three directions: north/south, east/west and, and vertical (up/down). Like the Richter Scale, it's logarithmic, which, in English means that a small numeric bump equates to a serious increase in the violence with which the Earth shakes. That's probably why most people (and many media outlets) haven't noticed that it's a whole new metric. It works in orders of magnitude where a 5.0 earthquake is not 20% stronger than a 4.0 quake, it is 10 times as strong One-thousand percent. There is a massive difference between a 6.0 and a 7.0 quake.

So, the earthquake that just struck Virginia was a 5.9. How does that *ahem* shake out in the pantheon of tectonic titans?

The March 11th, 2011 quake that took place off the coast of Japan and caused much devastation was a 9.0. Last year's earthquake in Haiti was a 7.0. The 1989 quake that rocked San Francisco (and screwed up the World Series) was a 7.1. The largest ever recorded was in Chile in 1960, scoring a (literally) earth-shattering 9.5 on the MMS.

So, compared to those, today's earthquake wasn't such a huge deal size-wise. However, anything in the 6.0 range can cause serious damage to roads, buildings, and infrastructure. What's seriously alarming, though, is that it's extremely rare to get earthquakes of that magnitude this far north and east in the US. In fact, the largest earthquake to ever hit Virginia before today (as far back as we have records) was also a 5.9. That was back in 1897.

Of course, there was no MMS back in 1897—hell, there was no Richter scale. There weren't even seismometers; scientists used pendulums to measure the movement of the Earth. No, the Richter Scale only made its debut in 1935, and was unseated by the MMS in 1979. Fun fact: The CalTech scientist who developed the MMS? His name was Tom Hanks. No relation.

Image credit: Shutterstock/Serr Novik

You can keep up with Brent Rose, the author of this post, on Google+ or Twitter.


Giz Explains is where we break down whatever science or tech questions are scratching at the backs of our noggins. Got questions of your own? Email them to us at explains@gizmodo.com and we'll see about answering them.



logorhythmic - Rhythms formed by words, such as in rap music (logos - word, Ancient Greek)?

logarithmic - Of, relating to, or expressed in terms of logarithms.