English: Geology of the Reelfoot Rift. From the source: "Geologic structures related to earthquakes in the central Mississippi Valley region have been deeply buried over hundreds of millions of years by thick layers of sediment. Geophysical studies have revealed a major buried northeast-trending feature known as the Reelfoot Rift (here shown in cross section), which formed more than 500 million years ago when powerful geologic forces began to pull the Earth's crust apart. Most quakes in the central Mississippi Valley appear to be related to the major faults and large bodies of igneous rock within the rift." Thomas G. Hildenbrand,Victoria E. Langenheim, Eugene Schweig, Peter H. Stauffer, and James W. Hendley II (USGS)