How big was that earthquake?

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

I have found that there’s something about a seismograph that invites people to jump up and down and then ask, “How big an earthquake did I just make?” As a seismologist, I also often get asked questions like, “What is the magnitude of a football stadium full of fans jumping to cheer for their teams?” Or, “What’s the biggest earthquake that could ever happen?”

Earthquakes are frightening, and people want to understand what they are all about. I think these questions are an expression of people trying to grasp the extent of the awesome power of nature that may result in tragic devastation and human suffering from earthquakes. Answering these kinds of questions can help people appreciate just how astoundingly huge earthquakes are, compared to just about anything else we typically experience.

So, how many people jumping 1 ft would generate the equivalent energy of, for example, a magnitude 5 earthquake? The surprising answer is: about 3.5 billion—or about half the population of the Earth. And, what about a magnitude 6? That would be about 14 times the population of Earth!

This table shows the number of people jumping 1 ft that would be the equivalent energy for earthquakes of a given magnitude. (Yes, magnitudes can be negative.)

Magnitude	Number of people jumping 1 ft
-1	4
0	110
1	3,481
2	110 thousand	Large football stadium
3	3.5 million
4	110 million
5	3.5 billion	Almost half the population of Earth
6	110 billion	14 times population of Earth
7	3.5 trillion	450 times population of Earth
8	110 trillion	14,000 times population of Earth
9	3.5 quadrillion	446,000 times population of Earth

No matter how many times I go through these calculations, I still find the results to be astounding and hard to believe. But earthquakes really are that big, which explains why they cause such devastation and human tragedy.

Hopefully though, through better understanding of the science of earthquakes, we can mitigate at least some of the tragedy caused by earthquakes by learning about where they are likely to occur, how big they are likely to be, and what effects they are likely to have on people. The more we learn about earthquakes, the better we will be able to prepare for them and improve our response to them when they happen.

Further reading:

T. Bravo, M. Hubenthal, and J. Taber, How Big of a Quake Can You Make?, IRIS Teachable Moment, www.iris.edu/hq/retm/event/6512

P. Bormann and D. Giacomo. The Moment Magnitude and the Energy Magnitude: Common Roots and Differences. Journal of Seismology, Springer Verlag, 2010, 15 (2), pp.411-427. 10.1007/s10950-010-9219-2.. .hal-00646919.

Energy of Earthquakes, www.volcanodiscovery.com/earthquakes/energy.html