Check your local newspaper today, and the lead story is likely the magnitude 8.7 earthquake that occurred yesterday off the coast of Indonesia.
Or was that 8.2? Or 8.5, maybe? And what’s the difference, anyway?
These are big differences. (The scale is logarithmic, so the shaking from a quake measured at 8.0 is ten times more powerful than one measured at 7.0.) So, what gives?
There are several reasons for the differences, according to Stuart Sipkin of the US Geological Survey’s National Earthquake Information Center. First is the nature of earthquakes themselves. The energy released during a quake travels in several different kinds of waves, some faster than others. After any big earthquake, the USGS issues a preliminary calculation of the temblor based on readings from the faster-traveling waves, called body waves. In this case, it initially rated the quake an 8.2.
But for large earthquakes such as yesterday’s, the fastest waves don’t supply quite enough data to make fine distinctions. For that, the USGS has to wait for the longer, slower waves (called surface waves) to reach its seismographs. After analyzing those waves, the USGS issues a revised take on the magnitude, which is why it later upgraded yesterday’s quake to an 8.7.
Second, different services issue measurements based on different scales. The USGS calibrates its measurements to the Richter scale, a measurement of the severity of the shaking caused by the quake, which gives the familiar whole-number-and-decimal ratings. Other services calibrate their measurements to other scales, which, unfortunately, also use the whole-number-and-decimal formulation despite the fact that they’re not directly comparable.
Finally, the magnitude of an earthquake may appear to be different depending on where you’re measuring it from, and how many data points you have to compare. The more diverse the locations of the readings, the more points you have to triangulate from. (The widely reported 8.5 reading from the Japan Meteorological Agency came from stations solely within Japan; the National Earthquake Information Center has stations around the world).
All of these translate to different versions of “really big earthquake,” of course. But as with assertions about any topic, ranging from Social Security to basketball point spreads, it’s worth keeping in mind where your information is coming from.