A dangerous earthquake zone?
By Larry Larason
Last spring my wife and I took a trip to some places we had wanted to visit for quite a while. After stopping to see family in Oklahoma, we continued to Cahokia State Park in Illinois, the archaeological site of what was once the largest Native American city north of Mexico. Then we continued to New Madrid, Missouri, site of the strongest earthquakes in the center of North America. You may recall that last spring [2011] the Mississippi River was in flood. The New Madrid town museum is situated at the base of the river levee. We took time to walk up some steps to the top and look at the river. It was like an inland sea. A few locals, probably worried about flooding, climbed up beside us for a look and left without saying a word. Our last stop on the trip was the Crater of Diamonds State Park at Murfreesboro, Arkansas. [No, I didn’t find a diamond.]
Some time later I mentioned the New Madrid earthquake to a friend. He had never heard of it. Maybe most of you haven’t heard of it either, and since this is almost the 200th anniversary of the event, here’s the story.
Thousands of micro-quakes still occur in the New Madrid seismic zone each year.
Today New Madrid [pronounced MAD-rid] is a small town of less than 3500 people but with a long history. European settlement began in the 1780s around a trading post while the region was still under Spanish control. The population was never large due to natural disasters such as flooding. [The town has moved three times.] Citizens expected floods since the town was built in a swampy area next to a powerful river, but no one expected earthquakes. The first record of an earthquake in the area was mentioned in his journal by a missionary travelling with French explorers near the future Memphis in 1699. The residents of New Madrid were almost certainly unaware of this.
We talk about the New Madrid earthquake, but it was actually a series of quakes occurring over a period of eight weeks beginning on December 16, 1811 at 2:30 am. Slippage of about 30 feet along 90 miles of the Blytheville Fault threw people from their beds and toppled chimneys. Most estimates of the magnitude of the tremor place it at 7.5 to 8.0 on the Richter scale. More than 50 aftershocks followed of magnitude 5.0 or more. Magnitude 5.0 is considered moderate; things may rattle on shelves, but little damage occurs. Still, shaking by the aftershocks caused wet sand and river sediments to liquefy; two nearby towns sank into the muck and one person went missing.
Aftershocks continued almost daily until January 23, 1812, when another large tremor struck. This one was felt from Canada to Mexico. The North Carolina legislature adjourned, thinking the quake had been local. People living in the region reported that the ground seemed to shudder constantly until February 7. Early that morning the largest of the quakes pushed a 20 square mile portion of the flood plain upwards about 30 feet, and related subsidence created Reelfoot Lake in Tennessee. The Mississippi River was blocked by the uplift and had to cut through the sediment in a series of waterfalls. What was left of New Madrid was destroyed by this last quake and the subsequent flooding by the river. It would be rebuilt later.
The Great Earthquake at New Madrid, a nineteenth-century woodcut from Our First Century (Devens 1877).
The region that felt the ground shaking of the New Madrid quakes was about 2.5 million square kilometers; that’s ten times the size of the area affected by the San Francisco earthquake of 1906.
One phenomenon of the quakes in this area is what is called sand boils, or sand volcanoes. These happen when the shaking liquefies sand saturated with water beneath a layer of clay under the surface. Pressure causes the slurry to be ejected upwards through the clay to form a ring of sand around a crater on the surface. There are thousands of these in the region. Farmers can recognize them because they hold water poorly, making bare spots in tilled fields. One such bare spot now hosts a cell tower built atop a concrete pad two feet thick. It has been dubbed the “Richter dipstick” because sand boils are unstable and, in a big quake, this 700-feet high tower is likely to sink or fall over.
Why did earthquakes of such magnitude occur in the middle of the continent? Several explanations have been offered, but the most convincing one, to me, relates to deeply buried scars in the continent’s basement rocks. The Mississippi River flows for much of its distance through an ancient rift zone. The rift dates back to when the supercontinent of Rodinia, containing most, if not all, of Earth’s landmass broke up about 750 million years ago. As it split apart, cracks formed in what would become North America. Though the continent did not separate, the deep rent in the “foundation” remained. Covered by sediment it was probably quiet until the formation of another supercontinent, Pangaea. Then, faults defining the rift were reactivated, especially as Pangaea began breaking up about 175 million years ago. Some volcanism occurred during both of these episodes. One result of that volcanism is the diatreme that is now the Crater of Diamonds State Park in Arkansas. The existing rift was then reburied by sediment and lay mostly quiet again until recently, although geologists have found evidence of other large earthquakes in the New Madrid seismic zone at 300, 900, and 1400 C.E.
Death and destruction during the New Madrid quakes of 1811-12 were minimal because there were few people living in the region at that time. If a swarm of similar earthquakes occurred today, the result would be catastrophic. Will it happen again? Probably, but not soon. USGS monitoring equipment detects little strain on the faults in the region. On the other hand, small aftershocks, the type felt only by seismographs, have continued since the 1800s. The thousands of micro-quakes each year may be relieving pressure that otherwise might lead to a severe temblor.
Most earthquakes occur near tectonic plate boundaries. Those in the middle of a continent are poorly understood. Not all the earthquakes in the Midwest occur in the New Madrid seismic zone. There are many buried faults in the center of the country. A similar area in Illinois/Indiana is called the Wabash Valley seismic zone. Earthquakes have taken place all across southern Illinois and adjoining Missouri and Arkansas, including Saint Louis and Memphis. Clearly, the region along the central Mississippi River is unstable.
New Mexico has a rift of its own. Do we face comparable dangers? Our rift is not as old as the one in Missouri. The Rio Grande Rift formed during the Miocene, 23 to 5 million years ago, as a part of the basin and range development. The majority of the quakes in our state are associated with the rift, but none have shown the power of the ones at New Madrid. Socorro is the most active spot, possibly because magma is slowly rising in the rift about 12 miles below the surface. Overall, New Mexico is considered a moderate seismic area.
If you’re interested in earthquakes you can find a great deal of information on the WWW. For example, the USGS has an earthquake tracking site.
