 This is an iso-seismal map from Otto Netli's paper about the 1811-1812 sequence of three earthquakes that happened in New Madrid. An iso-seismal map is one in which similarly felt seismic intensities are contoured, so that you can see how far away different intensities of shaking were felt from the epicenter of the earthquake. It helps you figure out where the epicenter probably was, and it helps you figure out what the magnitude probably was. So these kinds of maps are normally used when we don't have the instrumentation to actually have determined where the epicenter was or the magnitude with a seismometer. In this case, the way Netli made this map is that he combed through old newspaper accounts of eyewitness reports of felt shaking from this earthquake, and then he put those on a scale of 1 to 12, one being hardly felt at all, or not at all, and 12 being absolute destruction hell on earth. So, for example, near New Madrid where the epicenter was, there are some reports of 10 to 11 far away in New England this earthquake was felt, but at a much smaller intensity, so there's an intensity of 2 or 3. Then after you collect a lot of this data and put it on a map, you can try to connect up similar intensities, and basically you end up drawing rings of intensity around your epicenter. So here is the contour for intensity 10, say. Out here is the contour that separates the fours from the threes, and so on. One thing to notice from this map is that there weren't a lot of people west of the Mississippi, so all the rings kind of die out here at the Mississippi. That's the best we can do. These maps are actually still used today, but they're usually used for ground-truthing as best we can historic earthquakes. For example, when the magnitude 5.8 earthquake happened in August of 2011, you could have logged into the USGS site, told them your zip code, told them how much shaking you thought you felt, and then they compiled the map, and this is the map they compiled, that shows how much shaking there were in different places around the country. So the folks in Richmond felt very strong shaking between intensity 6 and 7 estimated, and up into Canada people felt this earthquake too. So of course we know the actual magnitude of this earthquake because we have instrumentation now and we can calculate it. So what this is useful for is to take a map like this, and if you know you have historic earthquakes in this area, you can try to figure out how far away people felt that, and then guess about what magnitude it probably was compared to where people felt it today, where we do know what the magnitude was. So when you read Nutley's paper, keep in mind how these isosaisal maps are made and how much detective work it takes to actually do this.