If you prefer, you can watch this as a GIF or on YouTube.
The animation above shows all earthquakes with epicenters in the bounded area and magnitudes greater than 5.0. The first slide says “Richter scale” because that’s most familiar to most people, but the actual scale used was the Moment Magnitude Scale; it’s generally within a few decimals of the 1930s-era Richter.
The data is from IRIS (the Incorporated Research Institute for Seismology), the maps were produced using Python with Pandas, Matplotlib and Basemap, and the animation with GIMP and HTML5 conversion with gfycat.
The width of the circles is the result of a compromise relationship between magnitude scale number, circle area, perceived circle size difference and total energy release. I chose a scale that (a) was intermediate between the extremes of size every approach suggested, and (b) had a simple formula so that it would be, at the very least, transparent:
circle size = 20 pixels * (magnitude - 4.5) ^ 1.5
As you can see, it’s arbitrary, but there is no non-arbitrary scale that is not equally misleading in certain respects, which is probably why IRIS does not vary the circle size at all in their maps, instead indicating intensity with color (which has its own perceptual issues, unfortunately: pdf).
The main thing to note is that the circle size is somewhat related to the area on the surface affected by the earthquake, but the relationship is very fuzzy. Different geographical features affect the distance earthquakes travel; faults, for example, actually contain them in a smaller area. In the animation, the most important difference in scale to show is that earthquakes of magnitude 5.0, which can be felt and are alarming but generally cause little damage in areas prepared for them, are small circles, and the 42 quakes with magnitude 7 or above are quite perceptually different.
Feel free to disagree and comment. As usual, I don’t claim to have found the solution to a quandary, just a solution, and I’m sure there are better ideas out there.
As you watch the animation, you may want to keep an eye out for the following notable earthquakes (and you’ll also notice a lot of large earthquakes that are not notable, because thankfully the damage they caused was not in proportion to their magnitude).
|•||July 1976: The Tangshan Earthquake (magnitude 7.5) on the northern Chinese coast near Korea. The deadliest earthquake of the 20th century, killing between 250,000 and 650,000 people.|
|•||January 1995: 7.3 The Great Hanshin Earthquake (magnitude 7.3) near Kobe in southern Japan, caused $100 billion in damage, 2.5% of Japan’s GDP at the time|
|•||September 1999: The 9/21 Earthquake (magnitude 7.5) in Taiwan (at the very bottom edge of this map)|
|•||May 2008: The Great Sichuan Earthquake (magnitude 7.9) in central China, killed 70,000|
|•||March 2011: The Tōhoku earthquake (magnitude 9.0) and tsunami, the fifth largest earthquake in modern times; hundreds of huge aftershocks appear on the map all the way through December 2013.|