#bootheel

The pattern of earthquakes clearly traces out a fault with 3 segments. This fault is not exposed at the surface; these earthquakes take place about 10 kilometers below the Earth’s surface on faults that are remnants of the continent’s ancient history. The story of the New Madrid Seismic Zone begins over 1.5 billion years ago. The continent that would eventually become North America was growing by adding volcanic arcs onto the core that is today found in Canada, expanding outwards a block at a time, when something changed. The center of the growing continent began to pull apart, forming a long rift valley. That valley is named the “Reelfoot Rift”. We don’t know exactly what all the plates were doing that long ago, but its clear that the continent started opening and things stalled. A comparison might be the East African Rift Zone today; the Arabian plate has fully pulled away from Africa to form the Red Sea, but East Africa itself is forming a deep, fault-filled basin loaded with volcanoes. If the East African rift shut down, it would eventually look a lot like the Reelfoot Rift. The continent bears many scars from this rifting. There are igneous rocks throughout the area formed between 1.5 and 1.3 billion years ago. during this rifting Measurements of the gravity and magnetic fields over the rift also illustrate its presence. The modern Mississippi River even generally follows this valley today as the ancient faults still allow enough movement to make the rift zone a lowland in the continent’s center. The faults formed during this rifting event don’t appear at the surface, they have been buried by sediments deposited by the Mississippi River system over the last 100 million years. The cities in the Central United States therefore sit mostly on top of fairly loose sediments that filled in these lowlands at the center of the continent. This is the area that in the early 1800s suffered a surprising series of disasters. Three of the largest earthquakes in U.S. history occurred in the area between Missouri, Arkansas, Tennessee, and Kentucky over a period of about 3 months starting on December 16, 1811. With earthquakes that occurred before modern seismic instruments were around to measure them it can be difficult to fully tell the story, but these events are important enough that scientists have assembled many details. See how there are 3 segments to the fault? You’ve already seen the reason why there were 3 quakes. The first quake took place on the southernmost segment and ruptured in a strike-slip motion. The second quake took place on the middle segment and ruptured a normal fault. The final quake took place on the northernmost segment and again had a strike-slip motion. This structure therefore looks like a piece of the rift, a normal fault segment with two large strike-slip faults on its edge. The quakes were extremely powerful; USGS estimates place their moment magnitudes at 7.5, 7.7, and 7.7; comparable in strength to the 7.8 Mw 1906 San Francisco Earthquake. Because the crust in the Eastern U.S. is older and colder than that in the west, the shaking transmitted over a greater distance; historical records report the earthquakes caused church bells to ring as far away as Boston. The quakes were a disaster for this area even though the population was sparse. The fault motion shifted the Mississippi River’s position, creating the modern Reelfoot Lake and also drowned and submerged many other areas. Sediments shifted and blew out of the ground across the region. Any structures present were likely destroyed, although there are very few remaining records. There was enough damage that a single landholder named John Hardeman Walker was able to cheaply buy up the affected land in the years following the quake as most of the inhabitants simply left. When Missouri became a state in 1818 he lobbied for inclusion of his land in Missouri at the expense of Arkansas, leading to the inclusion of 3 counties in Missouri as a “bootheel”. The New Madrid quakes therefore literally show up on the U.S. map. Although these events are huge, they’re very much an anomaly. We teach that most major earthquakes are associated with boundaries between plates; even if the earthquake is happening far inland of the plate boundary it tends to relate to plate tectonics. The New Madrid quakes are so far from any plate boundary it’s extremely hard to say what is driving the motion on the faults. There are ideas. We do know that plates can transmit stresses long distances as they move; the New Madrid area could be feeling the impact of stresses as far away as California. The New Madrid Area could also be responding to the change in mass on top of it from melting of the huge ice sheets 12,000 years ago. Finally, there are even proposals that a small mantle “hotspot” has interacted with the Central U.S. over the past few million years and that could contribute to stress on the New Madrid Faults. These big quakes aren’t the only things this fault zone has produced. Not only do we see that earthquakes continue to this day, but scientists have also found evidence for previous earthquakes in the centuries prior to historical documentation. When these quakes happen, loose sand in the soil bursts onto the surface like a geyser, a feature called a “sand blow” or a “sand boil” (sand blows covering the land were probably a big reason why it was cheap to buy after the 1812 quakes). Older sand blows have been found indicating several large quake sequences happened on this fault before historical records were recorded, with the most recent ones happening about 1350 and 900 a.d. Furthermore, seismic techniques have been used to image the subsurface throughout the Mississippi valley and found evidence of faults across a broad area that have been active over the past few million years - not just these exact faults, but a multitude of them throughout the valley. The sequence of prehistoric earthquakes and the ongoing smaller quakes is good evidence that this fault is still an active threat. If the smaller quakes were aftershocks of the 1812 sequence, there would be fewer of them with time; instead their rate is pretty much constant, suggesting they’re caused by continued stress on the fault. Although the fault most recently ruptured about 400-500 years apart, there’s no reason why it can’t go sooner next time. In fact, during the US Geological Survey’s most recent earthquake hazard assessment, they increased their assessment of risk from this fault system due to the mapping of the prehistoric earthquakes. If this fault system ruptures again, there are vastly more people in this area than last time. St. Louis, Memphis, and Nashville are all in the area that could see heavy shaking; smaller cities like Paducah, Jackson, Evansville would feel it as well, and smaller towns even closer to the epicenter could feel even stronger shaking. Several factors would likely increase the damage to these cities even beyond what is observed in major earthquakes elsewhere. Almost all the buildings in these areas sit on sediments deposited by the Mississippi River and loose sediments are extremely weak during earthquakes. When shaken, loose sediments break apart and lose all strength, a phenomenon known as liquefaction. Any buildings built atop those sediments will be at risk of severe damage or even collapse (http://tmblr.co/Zyv2Js1WTUw1o). These areas also have very little in the way of building codes that could limit damage. Building codes are hugely important during earthquakes as unprepared buildings tend to completely collapse while limited building codes can save huge numbers of lives. Some of the states in the area do have seismic building codes, but many local areas do not. Major commercial buildings tend to do pretty well during earthquakes if the ground does not liquefy, but only 10% of the local areas have seismic building codes covering residential homes. If another earthquake were to hit these areas, residences would be absolutely devastated and the losses would rival the recent hurricanes as the worst disasters in U.S. history. If you live in these areas, earthquake preparation is smart. Have an earthquake kit, including stored water (http://tmblr.co/Zyv2Js1pz9oUR). Make sure your family knows what to do if a quake starts. Practice the “Drop, cover, and hold on” techniques during the yearly shakeout drills. If you own property, see about a seismic retrofit – a few cheap upgrades can be the difference between no damage and a house being completely lost (http://tmblr.co/Zyv2Js1puIWDj). And, if you have any influence on the political processes in the area, keep pressure on decision-makers to be ready. Cities can and should practice earthquake emergency responses and building codes in this area need to be upgraded to reflect the seismic risk. This fault system is still there and active. It might be 300+ years before another major quake series strikes, or it could be much less. If a major quake does hit, this is not an area you want to be in given current preparation levels. -JBB Image credit: http://bit.ly/1CTZavp Read more (tons of references): https://www.usgs.gov/natural-hazards/earthquake-hazards/lists-maps-and-statistics http://dnr.mo.gov/geology/geosrv/geores/techbulletin1.htm http://www.new-madrid.mo.us/index.aspx?nid=132 http://s1.sos.mo.gov/archives/history/bootheel http://www.showme.net/~fkeller/quake/maps.htm http://pubs.usgs.gov/fs/2009/3071/pdf/FS09-3071.pdf http://pubs.usgs.gov/pp/1538e/report.pdf http://www.britannica.com/event/New-Madrid-earthquakes-of-1811-1812 http://www.reelfoot.com/new_madrid_earthquake.htm http://bit.ly/1OvtNsu http://www.shakeout.org/centralus/