Your Fault, My Fault, and the New Madrid Fault
IN the earth, a fault is a line of fracture in the rocks where the two sides move by each other. The movement can be up, down or sideways, and it is caused by pressure and tension in the rock. When a sudden movement happens along one of these fault lines, an earthquake happens.
A fault can be very small, it can be seen in a single quartz crystal, or it can be very long. The Great Rift Valley in east Africa is over 6000 miles long!
California has a famous fault called the San Andreas fault, where the Pacific plate slips past the North American plate. Photos of the ground where faults occur show how rock beds have shifted over the years. Even the river channels get crinked when movement happens along a fault. Although California has more earthquakes, we've had bigger ones in the middle of the continent.
Measuring earthquakes
A recording device called a seismometer is used to measure earthquakes. The Richter scale is what we hear about mostly, and scientists also use a scale called the Moment-Magnitude.
The Richter Scale. A very important fact in this scale is that as each number goes up, the earthquake increases 10 times in power
1: Felt by instruments only
2: Felt by sensitive people and sensitive animals (10 times #1)
3: Felt by many people--feels like a passing truck (100 times #1)
4: Felt by everyone; pictures fall off the wall (1,000 times #1)
5: Damage--may cause weak walls to crack and fall (10,000 times #1)
6: A destuctive earthquake in populated areas; falling smokestacks, etc. (100,000 times #1) Marked Tree, AR, on the southern end of the New Madrid fault, has had quakes this large.
7: A major earthquake causing serious damage (1,000,000 times #1)
8: A disaster--a great earthquake that produces total destruction to nearby communities {1906 in San Francisco--8.3} (10,000,000 times #1)
9: Sendai, Japan had a devastating earthquake in 2011. (100,000,000 times #1)
The New Madrid fault
Located in the "Boot Heel" of Southeast Missouri, the town of New Madrid sits on a major fault that extends into Arkansas. In the early 1800's, a series of magnitude 8 earthquakes occured. Written accounts from eyewitnesses to this earthquake are quite remarkable, see more details below.
The size of this event leads geologists to suspect the the New Madrid fault is a failed rift valley: a place where the North American continent almost split into two pieces. What we know from history is that if something geologic has happened in the past, it can happen in the future. Earthquakes are not a matter of *if* they will happen, but *when* they will happen.
Earthquake Risk in the New Madrid Seismic Zone
by Ann G. Metzger and Jill Stevens Johnston
Center for Earthquake Research and Information
University of Memphis
Memphis, TN 38152
Plate Tectonics
Most earthquakes occur along the boundaries of the large, rigid crustal plates that make up the outermost shell of the earth. These plates, which range in thickness from about 5km to 70km are in motion relative to each other and to the earth's interior. In some areas, such as along the Mid-Atlantic Ridge, the plates are moving apart and new oceanic crust is forming as molten rock rises from within the mantle. The west coast of South America is an example of a boundary where two plates are coming together, and the heavier oceanic crust is being forced downward under lighter continental crust. A third type of plate boundary is illustrated by the San Andreas Fault, where the North American Plate and the Pacific Plate are moving past each other with a stick-slip motion that is responsible for the earthquakes which occur so frequently in California. A fourth boundary exists in the Himalayas, where mountains are slowly being uplifted due to the collision of two plates.
Scientists call this concept of moving plates plate tectonics. These plates are part of a dynamic system, a gigantic recycling system; with new crust created at spreading centers and older crust thrust down and melted in the mantle at subduction zones. Plate boundaries, therefore, are where the most dynamic processes of plate tectonics take place, and where 95% of all the earthquakes in the world occur each year. For those of us who live in the central U.S., however, it's the other 5%, those that occur far from plate boundaries, that are so crucial to our understanding of earthquake risk and the necessity for preparedness and mitigation.
Mid-Plate Earthquakes and the New Madrid Seismic Zone (NMSZ)
The central Mississippi Valley has the dubious distinction of being a mid-plate or intraplate area with an active seismic zone--the most active seismic zone east of the Rocky Mountains. Named for a small town on a bend of the Mississippi River near the junction of Missouri, Kentucky and Tennessee, the New Madrid seismic zone forms a zig-zag pattern from Cairo, IL, southwest to New Madrid, MO, then southeast to Ridgely, TN, and from Caruthersville, MO southwest to Marked Tree, AR. About 150 earthquakes occur each year in this 200 mile long, 15 mile wide band. Only about eight of these quakes are large enough to be noticed by the inhabitants of the area, but they are all recorded by sensitive instruments operated by St. Louis University and the Center for Earthquake Research and Information (CERI). Even though this area is not on a plate boundary, the geologic structure with which the activity is associated and the present seismicity are a result of plate tectonics.
More than 600 million years ago, in the Proterozoic Era the area now known as the Mississippi Embayment was pushed upward by molten rock from the underlying mantle. Faults formed, and over many millions of years, a rift structure now known as the Reelfoot Rift developed. Dense mantle material was injected into the lower crust, creating a pillow-shaped structure which was heavier than the surrounding rocks. As the upwelling ceased, the entire rift subsided, and filled with sediments eroded from its flanks. Then seas covered the area, laying down thick sequences of sediments which eventually hardened into limestones, sandstones and shales. During the Mesozoic Era, about 200 million years ago, rifting took place along the east coast of North America as the Atlantic Ocean began to open, resulting in the continent being stretched or extended, and in the Reelfoot Rift being pulled apart in a new episode of rifting.
Plutons (deep reservoirs of magma) formed along the flanks and axis of the rift, as molten rock moved upward along the ancient faults and then cooled before reaching the surface. Once again the rifting ceased and again, the ocean advanced over the area and receded; this time the sands, clays and gravels it deposited were not buried deeply enough, or long enough to become rock. At Memphis, this prism of unconsolidated material is approximately 3200 feet thick and covers the terrain from Little Rock, AR to the Tennessee River.
It is the reactivation of these ancient buried faults under the stress of continuous intracontinental pressure from the east and west (called a compressive stress regime) that is responsible for the earthquakes occurring in the NMSZ at present. The continuous pressure results in strain energy accumulating in the buried faults, a very small portion of which is released in the numerous low-magnitude earthquakes recorded in the NMSZ each year.
If so few of the earthquakes occurring within the zone today are large enough to be felt by the inhabitants, should we be concerned? Yes, because even while small earthquakes are occurring, more and more strain energy is quietly and continuously accumulating in the fault system, energy that will be released one day as a damaging earthquake, anywhere along the 200 mile length of the NMSZ. And presently no methodology exists to reliably predict the time, magnitude or location of an earthquake.
The Great Earthquakes
In December, January and February of 1811-12, three great earthquakes, all having magnitudes estimated to be greater than eight on the Richter Scale, shook this region, altering the landscape dramatically. The earth was observed to roll in waves a few feet in height. When these swells burst, large fissures were formed. One family that lived on a short bend of the Pemiscot River in the Missouri bootheel found that the entire river had been diverted through one of these fissures, resulting in the point of land on which their well and smokehouse were located moving to the opposite bank. Caving banks, along with ground motion, created tsunami-like effects on the Mississippi; many boats were swamped, while others were "...cast high and dry upon the shores". Two waterfalls were formed; one upstream from New Madrid, the other downstream, causing the river to reverse its flow for several hours. An island in the river completely disappeared, taking with it a band of river pirates.
Large areas of land were uplifted while much land sank, draining existing lakes and creating others, such as Reelfoot Lake in northwest Tennessee. Sand blows erupted like geysers, spreading sand over large areas where it is still visible today, after many years of cultivation. These eruptions of sand and water were called sand volcanoes by observers.
The damage area for the 16 December 1811 New Madrid earthquake was 15 times as large as the area of similar damage for the magnitude 8.3 San Francisco earthquake of 1906. And the third of these great earthquakes, in February of 1812, is the largest known earthquake in the continental U.S. Only Alaska has had a larger one, the Great Alaska earthquake of 1964.
Earthquake Risk and the Importance of Mitigation
What are the chances that we may experience a great earthquake early in the next century? Work done in 1985 by Dr. Arch Johnston and Sue Nava of the Center for Earthquake Research and Information (CERI) indicates that the average recurrence interval for great earthquakes in the New Madrid seismic zone is roughly 550 to 1200 years, and recent geophysical studies support the statistical estimations that great earthquakes are long term events. The most recent occurred in 1812 and the probability that one will occur within 15 years is just 0.3% - 1.0%. Even extending the time frame to 50 years, the probability only increases to 2.7% - 4.0%.
These time dependent probabilities were based on one 1985 study and were updated in 2004 to reflect new research and understanding by seismologists. The new time independent probabilities are:
Probability of a repeat of the 1811 - 1812 earthquakes (magnitude 7.5 - 8.0) = 7 - 10% in a 50-year time window.
Probability of a magnitude 6.0 or larger = 25 - 40% in a 50-year time window.
This is no reason for complacency, however, for it's not just great earthquakes that cause damage. In 1933, a magnitude 6.3 earthquake reduced most of the schools in Long Beach, CA to rubble. A magnitude 6.2 earthquake caused extensive damage to freeways and hospitals in San Fernando, CA, in 1971, while the 1983 Coalinga, earthquake, magnitude 6.5, damaged 68% of the town's residential units, making 35% uninhabitable. A magnitude 5.9 earthquake in Whittier, CA in 1985 caused 350 million dollars worth of damage in just 15 seconds, while the magnitude 6.8 Northridge earthquake in January, 1994 resulted in over 20 billion dollars worth of damage in 40 seconds. California is far better prepared for a damaging earthquake than the central U.S.,with a long history of seismic building codes and preparedness activities.
How often do these moderate to strong earthquake events occur in our area? The average repeat time for a magnitude 6.3 New Madrid earthquake is 70 years, plus or minus 15 years. In 1843, such a quake occurred near Marked Tree, AR causing damage in the then sparsely settled Memphis area. Not since the 1895 Charleston, MO, earthquake has the central Mississippi Valley been shaken by an earthquake in the magnitude 6 to 7 range. And with the increase in population in the region since then the number of buildings, lifelines and inhabitants that can be affected by an earthquake has increased dramatically. The probability that a significant damaging earthquake will occur within 15 years is 40% - 63%. Extending the time frame to fifty years, the probability increases to 86% - 97%. This is not a cause for panic. It is instead a powerful reason for people who live in this area to plan and implement the many steps that individuals and communities can take to enhance their ability to survive an earthquake, lessen property damage and shorten the recovery period. There is every indication that major earthquakes will occur in the New Madrid zone in the future, and experience has shown that individual action is effective. Earthquake hazard mitigation is cost effective, easy to implement, and significantly reduces the impact an of an earthquake on an individual, a family, and a community. The ball is in our court.
Some info on seismology and seismologists
A seismologist is someone who studies earthquakes and the internal structure of the earth, by analyzing both natural and artificially generated seismic waves.To be a seismologist you need an undergraduate degree in geology, geophysics,or mathematics with post graduate work in geophysics and seismology. For example; the University of Memphis Ph.D. Program in Geophysics requires the following courses:
1. Methods of Mathematical Physics
2. Advanced Geophysics
3. Tectonics
4. Earthquake Seismology
5. Seismotectonics
6. Advanced Seismology
A person with this type of training would be a qualified seismologist but class requirements may vary from school to school. I would further suggest that you search the web using the key phrase "Graduate programs in geophysics and seismology".
A Selection of Earthquake Web Sites
Arkansas Earthquakes Updates
http://www.geology.ar.gov/geohazards/earthquakes.htm
Earthquake Information Central U.S. Center for Earthquake Research and Information, The University of Memphis
http://www.ceri.memphis.edu
The Virtual Times Great New Madrid Earthquake Site
http://www.hsv.com/genlintr.newmadrd/
Earthquake Preparedness Information
http://www.fema.gov/home/fema.quakef.html
http://www.seismo.berkeley.edu/seismo/resource/preparedness.html
Boy Scouts:
I am a geologist and would be glad to answer any questions from your troop.
Regards,
Gary Patterson
Center for Earthquake Research and Information
3890 Central Avenue Phone: 901-678-2007
Memphis TN 38152
email: patterson@ceri.memphis.edu