Exploring the New Madrid Earthquakes

In the early morning of the 16th of December 1811, something strange happened in the United States. The ground began to shake violently. Here in Asheville in North Carolina, for instance, the shaking, although not severe enough to cause any significant damage or fatalities, woke much of the town. Most of the people living there had never experienced an earthquake, let alone such a strong one, and were understandably terrified. An eyewitness account of the events was published a few days later in a local paper. On its own, this seems unremarkable. What makes this interesting is how widespread this phenomenon was. Similar articles were published in newspapers all over the country, like in Natchez and Charleston to the south, and Washington, Pittsburgh, Detroit, and New York to the north, to only name a few. Places that, mind you, are as far away from each other as the UK is from Greece. It seems essentially the entire United States was shaken that night. And this wasn't the end of it. Strong earthquakes like this continued to occur regularly for almost two months until February the following year. Naturally, people were very eager to figure out where these tremors came from, where the epicenter of these earthquakes was. But because communication was slow in these days, this, of course, took a while. As the letters and newspapers from further inland slowly began to arrive in the large cities on the East Coast, It became clear pretty quickly that the further west they came from, the more dramatic they became. And the most terrifying accounts came from here, from the Mississippi Valley in the New Louisiana Territory. What the people of this region had experienced in the winter of 1811 and 1812 is without question the most remarkable but also horrifying series of earthquakes that took place in the United States. In total, over 2,000 earthquakes shook the region. of which some reached a magnitude of 7 and even 7.5 or greater. These were, by many measures, the strongest and most powerful earthquakes ever in the contiguous United States. And they happened, notably, in a very unusual place. In a place that you wouldn't normally think of when you think of earthquakes in the US. Not somewhere along the west coast, but instead, right in the center of it, near the town of New Madrid. But what's perhaps even more surprising is that even today, more than 200 years later, we still don't exactly know what caused them. This is a map of the strongest earthquakes that occurred in North America in the past decade. As you can see, they exclusively happened along the West Coast. This is, of course, no coincidence. Earthquakes are, as you probably know, the result of- plate tectonics. When the huge slabs that make up the Earth's crust bend and deform, stress builds up, slowly but steadily over years and decades. At some point, when the stress exceeds the strength of the rocks, they break and the plates snap back into a relaxed position, releasing the buildup stress spontaneously and violently all at once. The area where this deformation is most severe is, of course, the plate boundaries and boundary zones, where the plates collide, whereas the plate interiors remain mostly undeformed. In the US, these plate boundaries lie exclusively on the West Coast, and that's where seismic activity is concentrated. But if you now extend the time frame to include earthquakes from the 19th and 20th century as well, you can see that strong earthquakes can also occur in the interior of the North American plate, far away from these boundaries. While these so-called intraplate earthquakes are clearly more rare, they do happen, and understanding them is therefore important, not just for the people that live in these regions, but also because they seem to contradict what we believe to know about earthquakes and how they are triggered. Because of this, geologists have spent a lot of time in the past decades trying to figure out what exactly happened in New Madrid, and possibly more vitally, if it can happen again. If these were just some freak events, or if they happen, like earthquakes near plate boundaries, with a somewhat predictable regularity. Since the area began to be studied in more detail in the 80s and 90s, We have in fact recorded regular and continuous seismic activity here, dozens, hundreds of earthquakes every year. Most of them are too small to be felt, but they clearly show that something is happening here. And this is not the only place where this happens. Although certainly the most active and best studied seismic zone, zones of this localized seismic activity could also be identified in other places in the interior of the North American plate, like here, here, and here. And if history is any indication, which for earthquakes is usually the case, then these zones are very much capable of producing large and potentially devastating earthquakes comparable in magnitude to earthquakes near plate boundaries. In 2011, for instance, a 5.8 magnitude magnitude earthquake shook the Virginia seismic zone. While there were no reported fatalities and only a small number of injuries, the shaking did however cause between two to three hundred million dollars in damage, most of it uninsured. In 1886, the Charleston seismic zone was hit by a 7.3 magnitude earthquake. Sixty people were killed, thousands were injured, and much of Charleston was destroyed. And then of course, there was the series of major earthquakes that shook the region around New Madrid. in the Mississippi Valley in the winter of 1811 and 1812, the strongest of which were as strong as the 1906 San Francisco earthquake, the deadliest earthquake in U.S. history. So clearly, despite being far away from active plate boundaries, and despite the lack of an obvious mechanism that could explain the buildup of stresses in these zones, they nevertheless can and have produced devastating earthquakes. In New Madrid in particular, field research has revealed that strong earthquakes, comparable to the 1811 and 1812 series, have occurred here regularly for over 2,000 years, with average return periods of somewhere between 200 to 800 years. Based on this history, the USGS now puts the likelihood of a magnitude 6 or greater earthquake in this seismic zone in the next 50 years at about 25 to 40 percent, and the likelihood of a magnitude 7.5 or greater earthquake still at about 7 to 10 percent. This is frighteningly high, considering what an earthquake of that magnitude could mean for the region. and the eastern United States in general. But these are of course only very rough estimates based to a large degree on what happened here in the past. Our understanding of the processes that take place now, today, is still limited. Although we have made incredible progress in identifying what makes New Madrid geologically unique, we are still lacking a universally agreed upon model that fully explains the ongoing and continuous buildup and release of stresses here. And such a model is critical if we want to accurately predict the likelihood of future earthquakes and the hazard associated with them. At the time of the earthquakes, New Madrid and the land surrounding it had only been part of the US for a few years. Before that, the region was, as part of the Louisiana Territory, owned by, going backwards in time, the French, the Spanish, And again, the French. The town was established in 1789 as a Spanish colony composed mostly of American settlers that, as a condition to settle there, had to take the oath of allegiance to the Spanish king. That's why the town is called New Madrid, although it is pronounced differently today. The Spanish minister to the United States who had granted the colonists the land to settle on hoped that a colony of Americans loyal to the Spanish crown would prevent the unwanted expansion of the U.S. into Spanish Louisiana. Beyond that, the colony was also meant as a way to ensure Spanish control over the commerce on and beyond Mississippi. Despite early setbacks such as floods and food shortages, the liberal land policy of the Spanish authorities nevertheless stimulated westward immigration to the region, so that by the turn of the century, the population had risen from a few dozen to a few hundred. This made New Madrid the largest settlement between St. Louis and the Gulf Coast, and therefore the most important station on the upper Mississippi and the port of entry for all vessels going up and down the river, some 100 boats on a good day. The Spanish period came to an end in 1800 with the secret transfer of Spanish Louisiana, including New Orleans and St. Louis, to France. Only three years later, Napoleon, busy with his war in Europe and no longer interested in a French colonial empire in North America, And, to prevent it from falling into the hands of the British, in turn gave Louisiana to the United States for a mere $15 million, roughly $300 million in today's money. Through the Louisiana Purchase, the city of New Madrid and New Madrid County officially became U.S. territory in 1804, to the delight of its largely American population. But this joy was only short-lived, as only eight years later, the town was all but completely destroyed by the earthquakes we will now talk about. together with most of the other villages and towns that had sprung up around it up and down the Mississippi. This has roughly and summarized the historical backdrop. Let's now take a look at some of the eyewitness accounts from the people that were there when the earthquakes happened, so you can fully appreciate how devastating they were. William Pierce was part of a convoy of shallow-bottomed boats, or flatboats, taking goods from Pittsburgh down the Ohio and Mississippi River to New Orleans. When the first earthquakes hit, they were anchored for the night, a few days away from New Madrid. His account of what happened became one of the most quoted and reprinted accounts of the earthquakes in the following weeks and months. Precisely at 2 o'clock on Monday morning, we were all alarmed by the violent and convulsive agitation of the boats, accompanied by a noise similar to that which would have been produced by running over a sandbar. Every man was immediately roused and rushed upon deck. The idea of an earthquake then suggested itself to my mind, and this idea was confirmed by a second shock, and two others in immediate succession. These continued for the space of eight minutes. So complete and general had been the convulsion. that a tremulous motion was communicated to the very leaves on the surface of the earth. A few yards from the spot where we lay, the body of a large oak was snapped in two, and the falling part precipitated to the margin of the river. The trees in the forest shook like rushes. The alarming clattering of their branches may be compared to the effect which would be produced by a severe wind passing through a large cane break. Exposed to a most unpleasant alternative, we were compelled to remain where we were for the night. or subject ourselves to imminent hazard in navigating through the innumerable obstructions in the river. Considering the danger of running twofold, we concluded to remain. At the dawn of the day, we experienced a fifth shock more severe than either of the preceding. It was indeed most providential that we have started, for such was the strength of this last shock, that the bank on which we were but a few moments since attached was rent and fell into the river, whilst the trees rushed from the forests, precipitating themselves into the water with force sufficient to have dashed us into a thousand atoms. It was now light, and we had an opportunity of beholding in full extent all the horrors of our situation. During the first four shocks, tremendous and uninterrupted explosions resembling a discharge of artillery was heard from the opposite shore. At that time, I imputed them to the falling of the riverbanks. The fifth shock explained the real cause. Wherever the veins of the earthquake ran, there was a volcanic discharge of combustible matter. to great heights. An incessant rumbling was heard below, and the bed of the river was excessively agitated. Whilst the water assumed a turbid and boiling appearance near our boat, a spout of confined air breaking its way through the waters burst forth, and with a loud report, discharged mud, sticks, etc. from the river's bed at least thirty feet above the surface. These spoutings were frequent, and in many places appeared to rise to the very heavens. Large trees, which had lain for ages at the bottom of the river, were shot up in thousands of instances, some with their roots uppermost and their tops planted. Others were hurled into the air. Many again were only loosened and floated upon the surface. Never was a scene more replete with terrific threatenings of death. With the most lively sense of this awful crisis, we contemplated in mute astonishment a scene which completely beggars description, and of which the most glowing imagination is inadequate to form a picture. Here, the earth- river, etc., torn with furious convulsions opened in huge trenches, whose deep jaws were instantaneously closed. There, through a thousand vents, so furious streams gushed from its very bowels, leaving vast and almost unfathomable caverns. Everywhere, nature itself seemed tottering on the verge of disillusion. During the day, there was with very little intermission, a continued series of shocks, attended with innumerable explosions like the rolling of thunder. The bed of the river was incessantly disturbed, and the water boiled severely in every part. I consider ourselves as having been in the greatest danger from the numerous instances of boiling directly under our boat. Fortunately for us, however, they were not attended with eruptions. One of the spouts which we had seen raising under the boat would inevitably have sunk it, and probably have blown it into a thousand fragments. Our ears were constantly assailed with the crashing of timber. The banks were instantaneously crushed down. and fell with all their growth into the water. It was no less astonishing than alarming to behold the oldest trees of the forest, whose firm roots had withstood a thousand storms and weathered the sternest tempests, quivering and shaking with the violence of the shocks, whilst their heads were whipped together with a quick and rapid motion. Many were torn from their native soil and hurled with tremendous force into the river. One of these, whose huge trunk at least three feet in diameter, had been shattered, was thrown better than a hundred yards from the bank and planted into the bed of the river, there to stand a terror to future navigators. Several small islands have been annihilated, and from appearances many others must suffer the same fate. As we descended the river, everything was a scene of ruin and devastation. This horrifying description of the events on the 16th of December, was mirrored by many other people that lived and worked in the northern part of the Mississippi Valley at that moment in time. And this was, if you remember, only the beginning. Besides the aftershocks, which continued for months, years, even decades, there were two more major shocks as strong and even stronger as the first ones. This is another reason why this series of earthquakes is so unique, because it was a whole chain of major earthquakes that happened in relatively quick succession, triggered by a complex network of faults. The next main shock happened on January 23rd the following year, and the final on the 7th of February. The last one was also the strongest one, with an estimated magnitude of 7.5 to 7.8. It happened here, right in the center of the New Madrid seismic zone, between the two faults that had triggered the earlier earthquakes. Unlike the previous shocks, which were triggered by strike-slip falls that result mostly in horizontal offset, this final earthquake happened on a thrust fall which produces vertical offset. Unsurprisingly, the effects the earthquake on February the 7th had, particularly on the river, were much more severe. Of course, the shape of the Mississippi was very different back then. It looked more like this, although this is probably also not entirely accurate. East of this line, the land subsided. To the west, it was lifted up. The difference in elevation was about 5 to 6 meters or 20 feet. Because the fault cut through the rivers in two places, the uplift caused two huge rapids or waterfalls spanning the entire breadth of the Mississippi. It even temporarily blocked the flow of water and pushed it back, so that the whole river, the largest river in the US mind you, ran backwards against its natural current for almost an hour, carrying boats many miles upstream. This is the retelling of an account by a boatman who was on the river when that happened. moored about 13 miles above New Madrid. He was awakened by a tremendous roaring noise, felt his vessel violently shaken, and observed the trees over the bank falling in every direction. He immediately cut his cable and put off into the middle of the river, where he soon found the current changed, and the boat hurried up for about the space of a minute with the velocity of the swiftest horse. He was obliged to hold his hand to his head to keep his hat on. On the currents running its natural course, which it did gradually, he continued to proceed down the river, and at about daylight he came to a most terrific fall, which, he thinks, was at least six feet perpendicular, extending across the river, and about half a mile wide. The whirls and ripplings of this rapid were such that the vessel was altogether unmanageable, and destruction seemed inevitable. He and his men were constantly employed in pumping and bailing, by which, and the aid of providence, he says, he got safe through. On his arrival at New Madrid, he found that place a complete wreck, sunk about 12 feet below its level and entirely deserted. Its inhabitants, with those of the adjacent country who had fled there for refuge, were encamped in its neighborhood. He represents their cries as truly distressing. Of nearly 30 loaded boats, only this and one more escaped destruction. Several lives were lost among the boatmen. New Madrid was indeed most severely affected by the earthquake, being only 10 miles away from the epicenter. and a mile or two above where the fault cut the Mississippi a second time. Eliza Bryan, a schoolteacher and one of the few people that had not fled the region after the town's initial destruction two months earlier, gave us this vivid description of what the effects of the earthquakes looked like, including the last one of the main shocks. She writes, On the 7th, about 4 o'clock a.m., a concussion took place so much more violent than those that had preceded it. At first, the Mississippi seemed to recede from its banks. and its waters gathering up like a mountain, leaving for the moment many boats, which were here on their way to New Orleans, on bare sand, in which time the poor sailors made their escape from them. It then rising fifteen to twenty feet perpendicularly, and expanding, as it were, at the same moment, the banks were overflowed with a retrograde current, rapid as a torrent. The boats, which before had been left on the sand, were now torn from their moorings, and suddenly driven up a little creek, at the mouth of which they laid. to the distance, in some instances, of nearly a quarter of a mile. The river, falling immediately, as rapid as it had risen, receded in its banks again with such violence that it took with it whole groves of young cottonwood trees, which ledged its borders. They were broken off with such regularity, in some instances, that persons who had not witnessed the fact would be difficultly persuaded that it had not been the work of art. A great many fish were left on the banks, being unable to keep pace with the water. The river was literally covered with the wrecks of boats, and it's said that one was wrecked in which there was a lady and six children, all of whom were lost. In all the hard shocks mentioned, the earth was horribly torn to pieces. The surface of hundreds of acres was, from time to time, covered over in various depths by the sand which issued from the fissures, which were made in great numbers all over this country, some of which closed up immediately after they had vomited forth their sand and water. It is impossible to say what the depths of the fissures were. We have reason to believe that some of them are very deep. Aftershocks could be felt every 20 minutes for the next few days, and after that with decreasing frequency and strength for more than 10 years. It took about a week until the obstructions in the river were washed away to the point that the rapids were reasonably passable again. To the east, where the land had subsided, the river flooded the surrounding woodlands along its shores, creating huge swampy forests that would actually become the site of an important battle during the Civil War 50 years later, and a huge lake that still exists today. The trees that still rise from the water of this lake to this day are a stark reminder that this is earthquake territory. This should be enough to convey how powerful and devastating these earthquakes were. Of course the damage was most severe in the Mississippi Valley, an area relatively sparsely populated at the time, but as we have heard in the opening, they were felt over a much greater area, up to a thousand miles away in some cases. Even in Charleston, South Carolina, more than 500 miles away, they were still strong enough to damage a few buildings. And yet, despite this, despite all the chilling eyewitness accounts, despite how severe and widespread the effects were felt, and the impacts these quakes had in and beyond the Mississippi region, it even resulted in the very first disaster relief bill ever passed by Congress, which was a complete disaster in its own right, doing practically nothing for the people affected, and only resulting in huge amounts of speculation and fraud. Despite all this, This astonishing series of earthquakes was virtually forgotten only 50 years later. The history of these quakes was only slowly rediscovered in the second half of the 20th century. One question that is obviously impossible to avoid when talking about these historic earthquakes is what would happen if something similar happened today? What damage and devastation could or would that cause? Before we can answer this question, however, we need to understand two important factors about the geology of this region. Factors that alone, without even talking about the human element, like building codes, infrastructure, population size and all that, make earthquakes here much more dangerous. The first has to do with sand. The entire Mississippi floodplain is essentially one giant pit of sand, hundreds of meters deep in some places. The sand was deposited here by two forces. The first is, of course, the Mississippi itself. When rivers flow from the highlands and mountains where they originate, down into the lowlands, they carry sediments with them, sand, silt, and clay. Some of that is deposited on the floodplain as the river meanders across it over the centuries and millennia, and the rest makes its way into the ocean, usually. How much sediment rivers can transport is illustrated very well by the Mississippi River Delta. A stretch of land that today is inhabited by over a million people, most notably in New Orleans. But 4,000 years ago, when on the other side of the ocean the pyramids were being built, none of this land did exist. The Mississippi created these 8,000 square miles of land, an area larger than Northern Ireland, in only a few thousand years, simply by carrying sediment downstream. The other force responsible for the large quantities of sand in this region was the ocean. When we go even further back in time, 80 million years or so, We know that much of the southern United States was underwater, including the so-called Mississippi Embayment, the northern part of today's floodplain near Memphis and New Madrid. Why this is, is actually an important aspect of this story. Obviously, sea levels were much higher, so it's easy to assume that this region was simply flooded by the Gulf of Mexico as water levels rose. However, only about 10 to 20 million years earlier, this deep basin was actually the opposite. It was part of a huge mountain range. So clearly, something unusual has happened here. We'll talk about that later. After the formation of this basin, the Gulf of Mexico slowly filled it up with marine sediment from the south and the Proto-Mississippi with river sediment from the north, so that today the entire 1-2 kilometer deep hole is completely filled with sand and clay. That's why the Mississippi Embayment is essentially just one giant pit of sand. If you look at a cross section of the sediment layers below Memphis, the largest city in the region, for example, you can see that it is not underlain by rock, but by roughly 1 kilometer or 3,000 feet of unconsolidated loose waterlogged sediment. This has some benefits. Because these sand layers that sit between layers of much finer clay are essentially just giant underground lakes, they provide easy access to clean drinking water. In fact, the water from this aquifer supplies about a third of the people in Memphis with water. Side note, there was a few years ago actually quite a high-profile lawsuit regarding this aquifer, between the state of Mississippi and the city of Memphis. Because Memphis sits right at the border with Mississippi and big parts of the aquifer also lie below its territory, Mississippi alleged that the city of Memphis, specifically the Memphis Light, Gas, and Water Division, had, over multiple decades, unlawfully pumped hundreds of billions of gallons of groundwater that naturally occur under Mississippi out of the aquifer and sued the city for almost a billion dollars in damages. But both a U.S. district judge as well as the Supreme Court later rejected the lawsuit. Now, where this huge wet sandpit becomes a problem is when you have an earthquake in the crust below. Because loose soil consists of individual particles that can move independently from one another, it is affected by the earthquake's waves much more severely than solid rock is. It is particularly problematic when the soil is wet. You might have observed this when shaking a bag or box with loosely packed objects. Everything gets packed much more efficiently. It gets compressed, essentially. When this happens to wet sand, where the pockets between the particles are not filled with air but with water that can't immediately go anywhere, the pore pressure increases. This reduces the friction between the individual particles. It pushes them apart essentially, to the point that during strong earthquakes of magnitude 5.5 or higher, friction can actually reduce to zero, meaning the sand particles can move around freely and the soil essentially starts to behave like a viscous liquid. This is known as soil liquefaction, and it is one of the most destructive earthquake hazards. It essentially turns the ground into quicksand, which, needless to say, results in tremendous damage to things built on top of it. Liquefaction causes houses and other heavy structures to sink into the ground or tilt. Hollow structures like underground pipes and tanks to become buoyant, break and rise to the surface. It undermines the foundations of roads, bridges, and dams, causes landsliding and soil subsidence. That's why it is so dangerous. Because even if the structure on top is in itself strong enough to withstand the earthquake, the ground failure below can still render it unusable. Liquefaction has been observed during most major earthquakes around the world, but the new Madrid earthquakes still stand out simply because of the extent and the scale of the liquefaction they caused. One way we know this is through sandblows. If you remember, many eyewitness accounts mention eruptions of water and sand that shot up out of the ground. That's what sandblows essentially are. It's when the pressure below the solid upper soil layer becomes so high that the liquefied water-sand mix begins to rise through vents or fissures in the ground, and erupts or boils onto the surface, leaving behind a layer of sand or silt on the ground, often in the shape of a circular or elliptical cone. On photos of the 2011 Christchurch earthquake, you can see these sandblows a lot. They are a clear indication of severe soil liquefaction, which is the main reason why this earthquake, despite its relatively modest magnitude, caused such an extraordinary amount of damage. The liquefaction covered an area of roughly 30 to 50 square kilometers around the epicenter, which was unfortunately situated right under the city. This made it the most costly earthquake in New Zealand's history, with estimated damages of around 20 to 30 billion US dollars. But now, compare this to the New Madrid earthquakes, which induced severe soil liquefaction over an area of at least 10,000 square kilometers. And even beyond this primary region, There is still evidence for liquefaction, in some cases 250 kilometers away from the epicenter, near St. Louis and south of Memphis. Now, some of that is obviously explained by the larger magnitude. Each earthquake released somewhere between 25 to 100 times the amount of energy of the Christchurch earthquake. But it is also due to the unusual geology of the Mississippi Valley, which makes the region very susceptible to liquefaction. The thousands and thousands of sandblows that you can still find buried in the soil, or even see at the surface as light-colored patches in plowed fields all across the Mississippi Embayment, are clear evidence of this. Many are also unusually large. Normally, individual sandblows are only maybe a few meters in diameter at most. But in New Madrid, many are 40, 50, 60 meters in diameter, with vents that alone are already almost a meter thick. All this confirms that these must have been very powerful earthquakes. But there's more. This is a photo of a sand blow that was taken almost 100 years after the earthquakes. It still clearly shows what looks like an enormous explosion crater. We now know that in extreme cases when the pressure builds up too fast or the top soil layer is too strong, it can actually lead to so much hydraulic pressure underground that the sand blow will literally explode, throwing not just water and sand, but also everything that's in the ground, like stones, rocks, and in the case of the Mississippi floodplain, often a coal-like substance many meters into the air. According to the eyewitness accounts, this happened a lot during the New Madrid earthquakes. So this is the first problem, that the Mississippi Embayment is perhaps the best place, if you pardon the wording, for large-scale soil liquefaction in the world. One more side note. New Madrid is, of course, not the only place where this has happened, the explosion of liquefied sand. Similar craters formed, for example, during strong earthquakes in India and Yellowstone. And this picture from before is actually from southern Italy, which was hit by a not dissimilar series of unusual earthquakes in the year 1783, which killed over 30,000 people. A few years ago in Spain, the remnants of an ancient Roman city were unearthed just a few kilometers east of Madrid, the old Madrid, if you will. At its time, it was likely one of the largest cities in the region, until the 4th century, when it was, together with its neighboring towns, abruptly abandoned and rebuilt elsewhere, after what looks like a strong earthquake shook the region. One of the most notable earthquake features archaeologists uncovered were these distinct explosion craters right in the city, which seems to have formed during the earthquake as a result of the liquefaction of the river sediments that the city was built on. Many of these craters are at least 2 to 3 meters in diameter, and lie, in some cases, directly under the houses. Imagine for a minute what kind of terrifying experience this must have been, especially considering this was more than 500 years before any human-made explosion. Curiously, at the bottom of these craters, archaeologists have found dozens and dozens of tools such as knives, bill hooks, and axes. It looks like they were thrown into the craters after the explosions by the Romans, possibly as a means to appease the gods, who they undoubtedly blamed for this catastrophe. But back to the new Madrid. We already touched on this, but one of the most remarkable effects of the earthquakes was over how far of a distance they were felt. They walk up people in New York City and beyond, a thousand miles away from the epicenter. Why is best explained by this image. It shows the thickness of the North American plate. This large blue area, which includes basically everything east of the Rocky Mountains, is where the plate is the thickest, 200 kilometers or more. The west coast of the United States, on the other hand, only has a thickness of less than 100 kilometers. That's because it is much younger. It formed only during the last couple of hundred million years. While this part of the North American plate formed 1.5, 2, and to a large degree, even more than 2.5 billion years ago. As a result, the crust under the central and eastern United States is much colder, and most importantly, much denser than the crust below the western United States. Think of it that way. The western crust is like a piece of wood. not very dense with lots of air pockets that can absorb shock waves, while the eastern crust is more like a piece of metal that can transfer vibrations really, really well and over large distances. Let's now compare what happens when you have two equally strong earthquakes in these very different parts of the continent. This is a Did You Feel It map compiled by the USGS for a 5.7 magnitude earthquake in Greenville, California in 2013. It is essentially data collected from the population about where and how strong the earthquake was felt, which is then compiled into this color-coded map. Yellow and orange mark strong and very strong shaking that cause minor to moderate damage. This was mostly felt in the immediate epicenter region. Blues and greens mark zones with weak to light shaking that didn't cause any damage but can nevertheless be felt. This zone extended, for this particular earthquake, maybe 200 to 300 kilometers in every direction. Beyond that, the seismic waves became too weak to even be felt. Now, let's compare this with an equally strong earthquake in the eastern United States. This is the 2011 Virginia earthquake. The zoom factor is exactly the same. The difference is pretty obvious right off the bat. We have severe and violent shaking near the epicenter that can and has caused significant damage to houses in that region. But more importantly, for this comparison, we have a very large area of strong and very strong shaking extending all the way to Washington and Baltimore. Landslides were still triggered 250 kilometers away from the epicenter, 200 kilometers further than during any similar-sized earthquake on the west coast, and indeed elsewhere in the world. This increases the area where landslides are expected to occur in this region by 20 times. This increase is also mirrored in how far the earthquake was felt. If we zoom out, you can see that this area too is about 20 times larger. Here's the data for another earthquake of that magnitude in Oklahoma in 2016, which is a bit closer to the New Madrid seismic zone. These three earthquakes released the same amount of energy, and yet they were felt much, much further in the east, which shows that you can't really compare them, even if, or precisely because, they had the same magnitude. By now you should understand why, from a geological standpoint alone, this is a very bad spot for strong earthquakes. First, you have an old dense crust that can transfer seismic waves really, really well over very large distances, and then, on top of that, in particular in the epicentral region of the Mississippi Valley, you have a huge area of loose waterlogged sediments, which will shake like crazy. And if all of this wasn't already bad enough, the damage potential is of course even further amplified by the fact that this part of the country isn't even remotely as prepared for strong earthquakes as, say, California is, both from an engineering but also from a societal standpoint. We will talk about that in more detail in part 2. Now, one thing we also haven't touched on thus far is, of course, why earthquakes happen here. For that, we have to dive a bit deeper into the geology of the New Madrid seismic zone, which, because this video is already very long, we will also have to do next time. Hopefully, you will join us for this video too.

Most of the people living there had never experienced an earthquake, let alone such a strong one, and were understandably terrified. An eyewitness account of the events was published a few days later in a local paper. On its own, this seems unremarkable.

What makes this interesting is how widespread this phenomenon was. Similar articles were published in newspapers all over the country, like in Natchez and Charleston to the south, and Washington, Pittsburgh, Detroit, and New York to the north, to only name a few. Places that, mind you, are as far away from each other as the UK is from Greece.

It seems essentially the entire United States was shaken that night. And this wasn't the end of it. Strong earthquakes like this continued to occur regularly for almost two months until February the following year. Naturally, people were very eager to figure out where these tremors came from, where the epicenter of these earthquakes was. But because communication was slow in these days, this, of course, took a while.

As the letters and newspapers from further inland slowly began to arrive in the large cities on the East Coast, It became clear pretty quickly that the further west they came from, the more dramatic they became. And the most terrifying accounts came from here, from the Mississippi Valley in the New Louisiana Territory. What the people of this region had experienced in the winter of 1811 and 1812 is without question the most remarkable but also horrifying series of earthquakes that took place in the United States.

In total, over 2,000 earthquakes shook the region. of which some reached a magnitude of 7 and even 7.5 or greater. These were, by many measures, the strongest and most powerful earthquakes ever in the contiguous United States. And they happened, notably, in a very unusual place. In a place that you wouldn't normally think of when you think of earthquakes in the US.

Not somewhere along the west coast, but instead, right in the center of it, near the town of New Madrid. But what's perhaps even more surprising is that even today, more than 200 years later, we still don't exactly know what caused them. This is a map of the strongest earthquakes that occurred in North America in the past decade.

As you can see, they exclusively happened along the West Coast. This is, of course, no coincidence. Earthquakes are, as you probably know, the result of- plate tectonics.

When the huge slabs that make up the Earth's crust bend and deform, stress builds up, slowly but steadily over years and decades. At some point, when the stress exceeds the strength of the rocks, they break and the plates snap back into a relaxed position, releasing the buildup stress spontaneously and violently all at once. The area where this deformation is most severe is, of course, the plate boundaries and boundary zones, where the plates collide, whereas the plate interiors remain mostly undeformed.

In the US, these plate boundaries lie exclusively on the West Coast, and that's where seismic activity is concentrated. But if you now extend the time frame to include earthquakes from the 19th and 20th century as well, you can see that strong earthquakes can also occur in the interior of the North American plate, far away from these boundaries. While these so-called intraplate earthquakes are clearly more rare, they do happen, and understanding them is therefore important, not just for the people that live in these regions, but also because they seem to contradict what we believe to know about earthquakes and how they are triggered. Because of this, geologists have spent a lot of time in the past decades trying to figure out what exactly happened in New Madrid, and possibly more vitally, if it can happen again. If these were just some freak events, or if they happen, like earthquakes near plate boundaries, with a somewhat predictable regularity.

Since the area began to be studied in more detail in the 80s and 90s, We have in fact recorded regular and continuous seismic activity here, dozens, hundreds of earthquakes every year. Most of them are too small to be felt, but they clearly show that something is happening here. And this is not the only place where this happens.

Although certainly the most active and best studied seismic zone, zones of this localized seismic activity could also be identified in other places in the interior of the North American plate, like here, here, and here. And if history is any indication, which for earthquakes is usually the case, then these zones are very much capable of producing large and potentially devastating earthquakes comparable in magnitude to earthquakes near plate boundaries. In 2011, for instance, a 5.8 magnitude magnitude earthquake shook the Virginia seismic zone. While there were no reported fatalities and only a small number of injuries, the shaking did however cause between two to three hundred million dollars in damage, most of it uninsured.

In 1886, the Charleston seismic zone was hit by a 7.3 magnitude earthquake. Sixty people were killed, thousands were injured, and much of Charleston was destroyed. And then of course, there was the series of major earthquakes that shook the region around New Madrid. in the Mississippi Valley in the winter of 1811 and 1812, the strongest of which were as strong as the 1906 San Francisco earthquake, the deadliest earthquake in U.S. history.

So clearly, despite being far away from active plate boundaries, and despite the lack of an obvious mechanism that could explain the buildup of stresses in these zones, they nevertheless can and have produced devastating earthquakes. In New Madrid in particular, field research has revealed that strong earthquakes, comparable to the 1811 and 1812 series, have occurred here regularly for over 2,000 years, with average return periods of somewhere between 200 to 800 years. Based on this history, the USGS now puts the likelihood of a magnitude 6 or greater earthquake in this seismic zone in the next 50 years at about 25 to 40 percent, and the likelihood of a magnitude 7.5 or greater earthquake still at about 7 to 10 percent. This is frighteningly high, considering what an earthquake of that magnitude could mean for the region.

and the eastern United States in general. But these are of course only very rough estimates based to a large degree on what happened here in the past. Our understanding of the processes that take place now, today, is still limited. Although we have made incredible progress in identifying what makes New Madrid geologically unique, we are still lacking a universally agreed upon model that fully explains the ongoing and continuous buildup and release of stresses here.

And such a model is critical if we want to accurately predict the likelihood of future earthquakes and the hazard associated with them. At the time of the earthquakes, New Madrid and the land surrounding it had only been part of the US for a few years. Before that, the region was, as part of the Louisiana Territory, owned by, going backwards in time, the French, the Spanish, And again, the French.

The town was established in 1789 as a Spanish colony composed mostly of American settlers that, as a condition to settle there, had to take the oath of allegiance to the Spanish king. That's why the town is called New Madrid, although it is pronounced differently today. The Spanish minister to the United States who had granted the colonists the land to settle on hoped that a colony of Americans loyal to the Spanish crown would prevent the unwanted expansion of the U.S. into Spanish Louisiana. Beyond that, the colony was also meant as a way to ensure Spanish control over the commerce on and beyond Mississippi. Despite early setbacks such as floods and food shortages, the liberal land policy of the Spanish authorities nevertheless stimulated westward immigration to the region, so that by the turn of the century, the population had risen from a few dozen to a few hundred.

This made New Madrid the largest settlement between St. Louis and the Gulf Coast, and therefore the most important station on the upper Mississippi and the port of entry for all vessels going up and down the river, some 100 boats on a good day. The Spanish period came to an end in 1800 with the secret transfer of Spanish Louisiana, including New Orleans and St. Louis, to France. Only three years later, Napoleon, busy with his war in Europe and no longer interested in a French colonial empire in North America, And, to prevent it from falling into the hands of the British, in turn gave Louisiana to the United States for a mere $15 million, roughly $300 million in today's money.

Through the Louisiana Purchase, the city of New Madrid and New Madrid County officially became U.S. territory in 1804, to the delight of its largely American population. But this joy was only short-lived, as only eight years later, the town was all but completely destroyed by the earthquakes we will now talk about. together with most of the other villages and towns that had sprung up around it up and down the Mississippi. This has roughly and summarized the historical backdrop. Let's now take a look at some of the eyewitness accounts from the people that were there when the earthquakes happened, so you can fully appreciate how devastating they were.

William Pierce was part of a convoy of shallow-bottomed boats, or flatboats, taking goods from Pittsburgh down the Ohio and Mississippi River to New Orleans. When the first earthquakes hit, they were anchored for the night, a few days away from New Madrid. His account of what happened became one of the most quoted and reprinted accounts of the earthquakes in the following weeks and months. Precisely at 2 o'clock on Monday morning, we were all alarmed by the violent and convulsive agitation of the boats, accompanied by a noise similar to that which would have been produced by running over a sandbar.

Every man was immediately roused and rushed upon deck. The idea of an earthquake then suggested itself to my mind, and this idea was confirmed by a second shock, and two others in immediate succession. These continued for the space of eight minutes.

So complete and general had been the convulsion. that a tremulous motion was communicated to the very leaves on the surface of the earth. A few yards from the spot where we lay, the body of a large oak was snapped in two, and the falling part precipitated to the margin of the river. The trees in the forest shook like rushes.

The alarming clattering of their branches may be compared to the effect which would be produced by a severe wind passing through a large cane break. Exposed to a most unpleasant alternative, we were compelled to remain where we were for the night. or subject ourselves to imminent hazard in navigating through the innumerable obstructions in the river.

Considering the danger of running twofold, we concluded to remain. At the dawn of the day, we experienced a fifth shock more severe than either of the preceding. It was indeed most providential that we have started, for such was the strength of this last shock, that the bank on which we were but a few moments since attached was rent and fell into the river, whilst the trees rushed from the forests, precipitating themselves into the water with force sufficient to have dashed us into a thousand atoms.

It was now light, and we had an opportunity of beholding in full extent all the horrors of our situation. During the first four shocks, tremendous and uninterrupted explosions resembling a discharge of artillery was heard from the opposite shore. At that time, I imputed them to the falling of the riverbanks.

The fifth shock explained the real cause. Wherever the veins of the earthquake ran, there was a volcanic discharge of combustible matter. to great heights. An incessant rumbling was heard below, and the bed of the river was excessively agitated. Whilst the water assumed a turbid and boiling appearance near our boat, a spout of confined air breaking its way through the waters burst forth, and with a loud report, discharged mud, sticks, etc. from the river's bed at least thirty feet above the surface.

These spoutings were frequent, and in many places appeared to rise to the very heavens. Large trees, which had lain for ages at the bottom of the river, were shot up in thousands of instances, some with their roots uppermost and their tops planted. Others were hurled into the air. Many again were only loosened and floated upon the surface.

Never was a scene more replete with terrific threatenings of death. With the most lively sense of this awful crisis, we contemplated in mute astonishment a scene which completely beggars description, and of which the most glowing imagination is inadequate to form a picture. Here, the earth- river, etc., torn with furious convulsions opened in huge trenches, whose deep jaws were instantaneously closed. There, through a thousand vents, so furious streams gushed from its very bowels, leaving vast and almost unfathomable caverns. Everywhere, nature itself seemed tottering on the verge of disillusion.

During the day, there was with very little intermission, a continued series of shocks, attended with innumerable explosions like the rolling of thunder. The bed of the river was incessantly disturbed, and the water boiled severely in every part. I consider ourselves as having been in the greatest danger from the numerous instances of boiling directly under our boat.

Fortunately for us, however, they were not attended with eruptions. One of the spouts which we had seen raising under the boat would inevitably have sunk it, and probably have blown it into a thousand fragments. Our ears were constantly assailed with the crashing of timber. The banks were instantaneously crushed down. and fell with all their growth into the water.

It was no less astonishing than alarming to behold the oldest trees of the forest, whose firm roots had withstood a thousand storms and weathered the sternest tempests, quivering and shaking with the violence of the shocks, whilst their heads were whipped together with a quick and rapid motion. Many were torn from their native soil and hurled with tremendous force into the river. One of these, whose huge trunk at least three feet in diameter, had been shattered, was thrown better than a hundred yards from the bank and planted into the bed of the river, there to stand a terror to future navigators.

Several small islands have been annihilated, and from appearances many others must suffer the same fate. As we descended the river, everything was a scene of ruin and devastation. This horrifying description of the events on the 16th of December, was mirrored by many other people that lived and worked in the northern part of the Mississippi Valley at that moment in time.

And this was, if you remember, only the beginning. Besides the aftershocks, which continued for months, years, even decades, there were two more major shocks as strong and even stronger as the first ones. This is another reason why this series of earthquakes is so unique, because it was a whole chain of major earthquakes that happened in relatively quick succession, triggered by a complex network of faults. The next main shock happened on January 23rd the following year, and the final on the 7th of February. The last one was also the strongest one, with an estimated magnitude of 7.5 to 7.8.

It happened here, right in the center of the New Madrid seismic zone, between the two faults that had triggered the earlier earthquakes. Unlike the previous shocks, which were triggered by strike-slip falls that result mostly in horizontal offset, this final earthquake happened on a thrust fall which produces vertical offset. Unsurprisingly, the effects the earthquake on February the 7th had, particularly on the river, were much more severe. Of course, the shape of the Mississippi was very different back then.

It looked more like this, although this is probably also not entirely accurate. East of this line, the land subsided. To the west, it was lifted up. The difference in elevation was about 5 to 6 meters or 20 feet. Because the fault cut through the rivers in two places, the uplift caused two huge rapids or waterfalls spanning the entire breadth of the Mississippi.

It even temporarily blocked the flow of water and pushed it back, so that the whole river, the largest river in the US mind you, ran backwards against its natural current for almost an hour, carrying boats many miles upstream. This is the retelling of an account by a boatman who was on the river when that happened. moored about 13 miles above New Madrid. He was awakened by a tremendous roaring noise, felt his vessel violently shaken, and observed the trees over the bank falling in every direction.

He immediately cut his cable and put off into the middle of the river, where he soon found the current changed, and the boat hurried up for about the space of a minute with the velocity of the swiftest horse. He was obliged to hold his hand to his head to keep his hat on. On the currents running its natural course, which it did gradually, he continued to proceed down the river, and at about daylight he came to a most terrific fall, which, he thinks, was at least six feet perpendicular, extending across the river, and about half a mile wide.

The whirls and ripplings of this rapid were such that the vessel was altogether unmanageable, and destruction seemed inevitable. He and his men were constantly employed in pumping and bailing, by which, and the aid of providence, he says, he got safe through. On his arrival at New Madrid, he found that place a complete wreck, sunk about 12 feet below its level and entirely deserted.

Its inhabitants, with those of the adjacent country who had fled there for refuge, were encamped in its neighborhood. He represents their cries as truly distressing. Of nearly 30 loaded boats, only this and one more escaped destruction. Several lives were lost among the boatmen. New Madrid was indeed most severely affected by the earthquake, being only 10 miles away from the epicenter.

and a mile or two above where the fault cut the Mississippi a second time. Eliza Bryan, a schoolteacher and one of the few people that had not fled the region after the town's initial destruction two months earlier, gave us this vivid description of what the effects of the earthquakes looked like, including the last one of the main shocks. She writes, On the 7th, about 4 o'clock a.m., a concussion took place so much more violent than those that had preceded it. At first, the Mississippi seemed to recede from its banks. and its waters gathering up like a mountain, leaving for the moment many boats, which were here on their way to New Orleans, on bare sand, in which time the poor sailors made their escape from them.

It then rising fifteen to twenty feet perpendicularly, and expanding, as it were, at the same moment, the banks were overflowed with a retrograde current, rapid as a torrent. The boats, which before had been left on the sand, were now torn from their moorings, and suddenly driven up a little creek, at the mouth of which they laid. to the distance, in some instances, of nearly a quarter of a mile.

The river, falling immediately, as rapid as it had risen, receded in its banks again with such violence that it took with it whole groves of young cottonwood trees, which ledged its borders. They were broken off with such regularity, in some instances, that persons who had not witnessed the fact would be difficultly persuaded that it had not been the work of art. A great many fish were left on the banks, being unable to keep pace with the water. The river was literally covered with the wrecks of boats, and it's said that one was wrecked in which there was a lady and six children, all of whom were lost. In all the hard shocks mentioned, the earth was horribly torn to pieces.

The surface of hundreds of acres was, from time to time, covered over in various depths by the sand which issued from the fissures, which were made in great numbers all over this country, some of which closed up immediately after they had vomited forth their sand and water. It is impossible to say what the depths of the fissures were. We have reason to believe that some of them are very deep.

Aftershocks could be felt every 20 minutes for the next few days, and after that with decreasing frequency and strength for more than 10 years. It took about a week until the obstructions in the river were washed away to the point that the rapids were reasonably passable again. To the east, where the land had subsided, the river flooded the surrounding woodlands along its shores, creating huge swampy forests that would actually become the site of an important battle during the Civil War 50 years later, and a huge lake that still exists today. The trees that still rise from the water of this lake to this day are a stark reminder that this is earthquake territory.

This should be enough to convey how powerful and devastating these earthquakes were. Of course the damage was most severe in the Mississippi Valley, an area relatively sparsely populated at the time, but as we have heard in the opening, they were felt over a much greater area, up to a thousand miles away in some cases. Even in Charleston, South Carolina, more than 500 miles away, they were still strong enough to damage a few buildings. And yet, despite this, despite all the chilling eyewitness accounts, despite how severe and widespread the effects were felt, and the impacts these quakes had in and beyond the Mississippi region, it even resulted in the very first disaster relief bill ever passed by Congress, which was a complete disaster in its own right, doing practically nothing for the people affected, and only resulting in huge amounts of speculation and fraud.

Despite all this, This astonishing series of earthquakes was virtually forgotten only 50 years later. The history of these quakes was only slowly rediscovered in the second half of the 20th century. One question that is obviously impossible to avoid when talking about these historic earthquakes is what would happen if something similar happened today? What damage and devastation could or would that cause? Before we can answer this question, however, we need to understand two important factors about the geology of this region.

Factors that alone, without even talking about the human element, like building codes, infrastructure, population size and all that, make earthquakes here much more dangerous. The first has to do with sand. The entire Mississippi floodplain is essentially one giant pit of sand, hundreds of meters deep in some places.

The sand was deposited here by two forces. The first is, of course, the Mississippi itself. When rivers flow from the highlands and mountains where they originate, down into the lowlands, they carry sediments with them, sand, silt, and clay.

Some of that is deposited on the floodplain as the river meanders across it over the centuries and millennia, and the rest makes its way into the ocean, usually. How much sediment rivers can transport is illustrated very well by the Mississippi River Delta. A stretch of land that today is inhabited by over a million people, most notably in New Orleans. But 4,000 years ago, when on the other side of the ocean the pyramids were being built, none of this land did exist.

The Mississippi created these 8,000 square miles of land, an area larger than Northern Ireland, in only a few thousand years, simply by carrying sediment downstream. The other force responsible for the large quantities of sand in this region was the ocean. When we go even further back in time, 80 million years or so, We know that much of the southern United States was underwater, including the so-called Mississippi Embayment, the northern part of today's floodplain near Memphis and New Madrid. Why this is, is actually an important aspect of this story.

Obviously, sea levels were much higher, so it's easy to assume that this region was simply flooded by the Gulf of Mexico as water levels rose. However, only about 10 to 20 million years earlier, this deep basin was actually the opposite. It was part of a huge mountain range. So clearly, something unusual has happened here. We'll talk about that later.

After the formation of this basin, the Gulf of Mexico slowly filled it up with marine sediment from the south and the Proto-Mississippi with river sediment from the north, so that today the entire 1-2 kilometer deep hole is completely filled with sand and clay. That's why the Mississippi Embayment is essentially just one giant pit of sand. If you look at a cross section of the sediment layers below Memphis, the largest city in the region, for example, you can see that it is not underlain by rock, but by roughly 1 kilometer or 3,000 feet of unconsolidated loose waterlogged sediment.

This has some benefits. Because these sand layers that sit between layers of much finer clay are essentially just giant underground lakes, they provide easy access to clean drinking water. In fact, the water from this aquifer supplies about a third of the people in Memphis with water. Side note, there was a few years ago actually quite a high-profile lawsuit regarding this aquifer, between the state of Mississippi and the city of Memphis.

Because Memphis sits right at the border with Mississippi and big parts of the aquifer also lie below its territory, Mississippi alleged that the city of Memphis, specifically the Memphis Light, Gas, and Water Division, had, over multiple decades, unlawfully pumped hundreds of billions of gallons of groundwater that naturally occur under Mississippi out of the aquifer and sued the city for almost a billion dollars in damages. But both a U.S. district judge as well as the Supreme Court later rejected the lawsuit. Now, where this huge wet sandpit becomes a problem is when you have an earthquake in the crust below.

Because loose soil consists of individual particles that can move independently from one another, it is affected by the earthquake's waves much more severely than solid rock is. It is particularly problematic when the soil is wet. You might have observed this when shaking a bag or box with loosely packed objects.

Everything gets packed much more efficiently. It gets compressed, essentially. When this happens to wet sand, where the pockets between the particles are not filled with air but with water that can't immediately go anywhere, the pore pressure increases. This reduces the friction between the individual particles. It pushes them apart essentially, to the point that during strong earthquakes of magnitude 5.5 or higher, friction can actually reduce to zero, meaning the sand particles can move around freely and the soil essentially starts to behave like a viscous liquid.

This is known as soil liquefaction, and it is one of the most destructive earthquake hazards. It essentially turns the ground into quicksand, which, needless to say, results in tremendous damage to things built on top of it. Liquefaction causes houses and other heavy structures to sink into the ground or tilt.

Hollow structures like underground pipes and tanks to become buoyant, break and rise to the surface. It undermines the foundations of roads, bridges, and dams, causes landsliding and soil subsidence. That's why it is so dangerous.

Because even if the structure on top is in itself strong enough to withstand the earthquake, the ground failure below can still render it unusable. Liquefaction has been observed during most major earthquakes around the world, but the new Madrid earthquakes still stand out simply because of the extent and the scale of the liquefaction they caused. One way we know this is through sandblows. If you remember, many eyewitness accounts mention eruptions of water and sand that shot up out of the ground. That's what sandblows essentially are.

It's when the pressure below the solid upper soil layer becomes so high that the liquefied water-sand mix begins to rise through vents or fissures in the ground, and erupts or boils onto the surface, leaving behind a layer of sand or silt on the ground, often in the shape of a circular or elliptical cone. On photos of the 2011 Christchurch earthquake, you can see these sandblows a lot. They are a clear indication of severe soil liquefaction, which is the main reason why this earthquake, despite its relatively modest magnitude, caused such an extraordinary amount of damage. The liquefaction covered an area of roughly 30 to 50 square kilometers around the epicenter, which was unfortunately situated right under the city. This made it the most costly earthquake in New Zealand's history, with estimated damages of around 20 to 30 billion US dollars.

But now, compare this to the New Madrid earthquakes, which induced severe soil liquefaction over an area of at least 10,000 square kilometers. And even beyond this primary region, There is still evidence for liquefaction, in some cases 250 kilometers away from the epicenter, near St. Louis and south of Memphis. Now, some of that is obviously explained by the larger magnitude.

Each earthquake released somewhere between 25 to 100 times the amount of energy of the Christchurch earthquake. But it is also due to the unusual geology of the Mississippi Valley, which makes the region very susceptible to liquefaction. The thousands and thousands of sandblows that you can still find buried in the soil, or even see at the surface as light-colored patches in plowed fields all across the Mississippi Embayment, are clear evidence of this. Many are also unusually large. Normally, individual sandblows are only maybe a few meters in diameter at most.

But in New Madrid, many are 40, 50, 60 meters in diameter, with vents that alone are already almost a meter thick. All this confirms that these must have been very powerful earthquakes. But there's more.

This is a photo of a sand blow that was taken almost 100 years after the earthquakes. It still clearly shows what looks like an enormous explosion crater. We now know that in extreme cases when the pressure builds up too fast or the top soil layer is too strong, it can actually lead to so much hydraulic pressure underground that the sand blow will literally explode, throwing not just water and sand, but also everything that's in the ground, like stones, rocks, and in the case of the Mississippi floodplain, often a coal-like substance many meters into the air.

According to the eyewitness accounts, this happened a lot during the New Madrid earthquakes. So this is the first problem, that the Mississippi Embayment is perhaps the best place, if you pardon the wording, for large-scale soil liquefaction in the world. One more side note. New Madrid is, of course, not the only place where this has happened, the explosion of liquefied sand. Similar craters formed, for example, during strong earthquakes in India and Yellowstone.

And this picture from before is actually from southern Italy, which was hit by a not dissimilar series of unusual earthquakes in the year 1783, which killed over 30,000 people. A few years ago in Spain, the remnants of an ancient Roman city were unearthed just a few kilometers east of Madrid, the old Madrid, if you will. At its time, it was likely one of the largest cities in the region, until the 4th century, when it was, together with its neighboring towns, abruptly abandoned and rebuilt elsewhere, after what looks like a strong earthquake shook the region. One of the most notable earthquake features archaeologists uncovered were these distinct explosion craters right in the city, which seems to have formed during the earthquake as a result of the liquefaction of the river sediments that the city was built on.

Many of these craters are at least 2 to 3 meters in diameter, and lie, in some cases, directly under the houses. Imagine for a minute what kind of terrifying experience this must have been, especially considering this was more than 500 years before any human-made explosion. Curiously, at the bottom of these craters, archaeologists have found dozens and dozens of tools such as knives, bill hooks, and axes. It looks like they were thrown into the craters after the explosions by the Romans, possibly as a means to appease the gods, who they undoubtedly blamed for this catastrophe.

But back to the new Madrid. We already touched on this, but one of the most remarkable effects of the earthquakes was over how far of a distance they were felt. They walk up people in New York City and beyond, a thousand miles away from the epicenter.

Why is best explained by this image. It shows the thickness of the North American plate. This large blue area, which includes basically everything east of the Rocky Mountains, is where the plate is the thickest, 200 kilometers or more. The west coast of the United States, on the other hand, only has a thickness of less than 100 kilometers. That's because it is much younger.

It formed only during the last couple of hundred million years. While this part of the North American plate formed 1.5, 2, and to a large degree, even more than 2.5 billion years ago. As a result, the crust under the central and eastern United States is much colder, and most importantly, much denser than the crust below the western United States.

Think of it that way. The western crust is like a piece of wood. not very dense with lots of air pockets that can absorb shock waves, while the eastern crust is more like a piece of metal that can transfer vibrations really, really well and over large distances.

Let's now compare what happens when you have two equally strong earthquakes in these very different parts of the continent. This is a Did You Feel It map compiled by the USGS for a 5.7 magnitude earthquake in Greenville, California in 2013. It is essentially data collected from the population about where and how strong the earthquake was felt, which is then compiled into this color-coded map. Yellow and orange mark strong and very strong shaking that cause minor to moderate damage. This was mostly felt in the immediate epicenter region. Blues and greens mark zones with weak to light shaking that didn't cause any damage but can nevertheless be felt.

This zone extended, for this particular earthquake, maybe 200 to 300 kilometers in every direction. Beyond that, the seismic waves became too weak to even be felt. Now, let's compare this with an equally strong earthquake in the eastern United States. This is the 2011 Virginia earthquake. The zoom factor is exactly the same.

The difference is pretty obvious right off the bat. We have severe and violent shaking near the epicenter that can and has caused significant damage to houses in that region. But more importantly, for this comparison, we have a very large area of strong and very strong shaking extending all the way to Washington and Baltimore.

Landslides were still triggered 250 kilometers away from the epicenter, 200 kilometers further than during any similar-sized earthquake on the west coast, and indeed elsewhere in the world. This increases the area where landslides are expected to occur in this region by 20 times. This increase is also mirrored in how far the earthquake was felt. If we zoom out, you can see that this area too is about 20 times larger. Here's the data for another earthquake of that magnitude in Oklahoma in 2016, which is a bit closer to the New Madrid seismic zone.

These three earthquakes released the same amount of energy, and yet they were felt much, much further in the east, which shows that you can't really compare them, even if, or precisely because, they had the same magnitude. By now you should understand why, from a geological standpoint alone, this is a very bad spot for strong earthquakes. First, you have an old dense crust that can transfer seismic waves really, really well over very large distances, and then, on top of that, in particular in the epicentral region of the Mississippi Valley, you have a huge area of loose waterlogged sediments, which will shake like crazy. And if all of this wasn't already bad enough, the damage potential is of course even further amplified by the fact that this part of the country isn't even remotely as prepared for strong earthquakes as, say, California is, both from an engineering but also from a societal standpoint. We will talk about that in more detail in part 2. Now, one thing we also haven't touched on thus far is, of course, why earthquakes happen here.

For that, we have to dive a bit deeper into the geology of the New Madrid seismic zone, which, because this video is already very long, we will also have to do next time. Hopefully, you will join us for this video too.

Transcript for:Exploring the New Madrid Earthquakes

Transcript for:
Exploring the New Madrid Earthquakes