The Earth Story

Irish Sand Volcano This is a cross section of a “sand volcano” from Clare County, Ireland. These features are found in the Carboniferous aged Gull Island formation – a deposit of marine shales and sandstones. About 300 million years ago, this area was shaken by large enough earthquakes to produce liquefaction of water-rich sediment. The mixture of water and sand, pressurized by the weight of the layers above, broke through to the surface and erupted, forming a feature called a sand boil/sand blow or a sand volcano. These ancient sand volcanoes have been buried and lithified over time; similar features have been observed associated with large modern earthquakes (https://tmblr.co/Zyv2Js2NjegtT). -JBB Image credit: http://bit.ly/2z1A14T Reference: http://bit.ly/2z2sti8 https://www.geolsoc.org.uk/GeositesKilkee

A hidden hotspot in the Eastern US

This image shows seemingly plain fields and houses from the interior of the United States. They seem to be just average residential plots, but look closely at the fields. They’re dotted with patches of lighter material.

Those patches of lighter material are sand that shot upwards out of the ground. Their remnants on the surface today testify to the events of 1812, when the largest earthquake in recorded history struck the interior of the United States at a place called New Madrid, Missouri. They were produced after the earthquake when sand layers liquefied and were shot upwards into the air due to the shaking; they’re known as sand blows. The New Madrid system has long been difficult to explain. Unlike places along the Pacific Coast where earthquakes strike, New Madrid is in the center of a plate, far away from major tectonic boundaries, yet it is capable of producing earthquakes large enough to devastate major cities.

New research just published in the journal Nature Geoscience puts out a provocative new hypothesis for New Madrid; that the entire seismic system is being driven by a plume in the mantle, hidden by the crust in the Eastern U.S.

Hotspot tracks created by mantle plumes are something people might be familiar with in the ocean. The classic example is Hawaii, where volcanism has produced a chain of islands thousands of kilometers long, with new islands created as the Pacific Plate moves across the hotspot.

Hotspots and plumes of various types have been proposed to happen at many places on the Earth’s surface, and they can hit continents as well. The Yellowstone supervolcano and the Snake River Plain are probably a plume/hotspot track, a mark left by the interaction of that hotspot with the continental crust.

A team of researchers at the California Institute of Technology, led by a scientist who is now at the Chinese Academy of Sciences in Wuhan, China, took advantage of another recent earthquake in the Eastern U.S., the Virginia earthquake of 2011. That Earthquake sent seismic waves through rocks across the entire Eastern U.S., and those waves were detected by dozens of seismic stations positioned throughout the U.S.

When the scientists looked at the data from this earthquake, they found that the seismic waves in an area of the continent stretching from Missouri through Kentucky and Virginia moved more slowly than the same seismic waves outside this area. The difference was only a few seconds, but that is more than enough to be measured by modern seismometers. There is a long path of seismically slow material underneath the Eastern U.S, stretching across this entire region, cutting across the Appalachian Mountains and then turning to the north along the coastline.

Seismically slow material can have several causes, but the most common assumption is that seismically slow material is hotter than the mantle material around it. If that assumption is right, then sometime, perhaps 100 million years ago, a plume of hot mantle rose up beneath the United States and hit the bottom of the continental crust. The crust was so thick and cold that it didn’t produce large volcanoes at the surface but it left a remnant path of hot material at the top of the mantle.

That plume kept supplying hot material as the continent drifted over it, leaving a trail of hot mantle stuck to the continent from Missouri to Virginia and then turning north along the coastline heading towards Maine.

Without direct sampling of the rocks it’s hard to know for sure what the seismic data is telling us but it is clear there is something very weird about the rocks in this region and the area where the track starts: Missouri, Kentucky, Arkansas, and Southern Illinois, is right at the heart of the New Madrid seismic zone.

If there is a hot plume of material sitting beneath the continent, pushing up against the crust and trying to break through, one consequence of that plume could be earthquakes as the crust strains and cracks.

This is a very interesting story but it will need a lot of effort to confirm it. The authors did note that there are samples of kimberlite rocks in Kentucky, produced in small volcanic eruptions which sample the mantle and could possibly be consistent with their hypothesis. If their story is correct, then one certainly wonders what other impacts this possible plume might have had on the geology of the Eastern United States.

-JBB

Image credit: http://web.mst.edu/~rogersda/nmsz/

Original paper: www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1949.html

Press report: http://phys.org/news/2013-09-evidence-hot-eastern-underside-tectonic.html

The New Madrid Seismic Zone

About once a year, residents of the counties at the border between Kentucky, Tennessee, Missouri, and Arkansas will feel the ground roll beneath their feet. This image maps out the location of earthquakes in this area over a 30-year period and clearly illustrates a major feature: the New Madrid Seismic Zone. This zone produces about 1 quake that can be felt per year in addition to many small earthquakes…and has historically produced really big ones.

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.

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

This topic was a requested post from our blog, http://the-earth-story.com/, hope you don’t mind the length but I wanted to do it justice.

Read more (tons of references): http://earthquake.usgs.gov/earthquakes/states/10_largest_us.php 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://esp.cr.usgs.gov/pdf/BulletinoftheSeismological2002Tuttle.pdf http://bit.ly/1OvtNsu http://www.shakeout.org/centralus/