MISSOURI'S EARTHQUAKE ZONES
Since 1990, when Iben Browning drew national media attention to Missouri by predicting a large earthquake might occur on the New Madrid fault, many Missourians have been aware that Missouri has some potential for being shaken by an earthquake. Geoscientists have long known about this potential and have, for many years, been investigating the earthquake threat in and around Missouri to the extent that funding has been available. Partly due to the Browning prediction, which caused increased public awareness of the earthquake risk in the central United States, but also due to media coverage of devastating earthquakes elsewhere in the United States and around the world, increased earthquake studies have been undertaken in Missouri and the central United States. These investigations have shed new light on the earthquake risk in Missouri and have shown that the New Madrid seismic zone is not the only threat.
New Madrid Update
Seismic activity in the New Madrid seismic zone continues to be monitored by Saint Louis University and the University of Memphis. An updated monitoring network is in the process of being installed with funding support from the U. S. Geological Survey. Access to recent and historical earthquake data is now readily available to the public as well as scientist via Internet World Wide Web sites at Saint Louis University:
http://www.eas.slu.edu/Earthquake_Center/
the University of Memphis
and the U. S. Geological Survey
http://earthquake.usgs.gov/neis/states/missouri/missouri.html
Small earthquake activity detected in the New Madrid seismic zone continues to average more than one earthquake every other day. Scattered earthquakes outside of the New Madrid seismic zone also continue throughout Missouri with highest concentrations in the eastern half of southern Missouri and adjacent southern Illinois.
A study by the University of Memphis using a temporary array of portable seismographs to more precisely locate small earthquakes has led to a better understanding of the three-dimensional geometry of some of the major faults in the New Madrid seismic zone. The northeast-southwest oriented, northern and southern fault segments are nearly vertical faults moving primarily with a sideways or strike-slip motion (when facing the fault the near side moves to the left and the far side moves to the right). The northwest-southeast oriented central fault segment slopes at a relatively shallow angle into the ground toward the southwest and is in compression. This segment is a reverse or thrust fault with the southwest side moving up and over the northeast side. How such a complex system evolved and is likely to react in the future is not well understood.
Paleoseismology studies, the study of prehistoric earthquakes as preserved in geologic materials, have shed additional light on the frequency and magnitude of earthquakes in the central United States. In the New Madrid area earthquakes cause the extensive deposits of water-saturated sandy soils to liquefy when shaken and thereby create sand blows (similar to small muddy geysers) and other features that have been preserved in the ground. Paleoliquefaction investigations have located and dated, using radiocarbon dating and archaeological materials, a significant number of these features over a large area. By correlating the age and distribution of these features an estimate of the timing and magnitude of prehistoric earthquakes can be made. Recent studies by the University of Maryland, the U. S. Geological Survey and others indicate large magnitude 7- to 8-size earthquakes, similar to those of 1811 and 1812, occurred at about 900 AD and 1530 AD. This suggests the recurrence interval for an earthquake of this size is about 300 to 500 years.
Selected earthquakes and potential source areas affecting Missouri
Ground movement or deformation studies by several groups, including Northwestern University, University of Missouri, University of Memphis, and Stanford University, have recently been initiated using the Global Positioning Satellite (GPS) system to precisely measure deformation in the New Madrid area. Preliminary published results have varied, but have tended to show a low level of strain accumulation. Controversy has been generated over how to interpret these measurements. Some people have interpreted these results to indicate that either smaller or less frequent large earthquakes have happened near New Madrid than has previously been accepted. This would lower the earthquake risk near New Madrid, but not elsewhere in the region. Others think this conclusion is premature considering the other conflicting data such as the previously mentioned paleoseismology data. Only with additional data collection and studies in the future will this matter be resolved. In the meantime, there is still a need for a defensible public policy for how the government and citizens are to deal with the earthquake threat. Regardless of size and frequency, the risk exists, and major damage and economic loss is possible.
Commerce Geophysical Lineament
Recent geophysical studies by the U. S. Geological Survey have identified a deeply buried (a mile deep or greater) feature in the ancient rocks underlying the central United States. This feature, named the Commerce Geophysical Lineament (named after the Commerce fault, which in turn is named after the town of Commerce, Missouri), was identified based on the magnetic and gravity signatures of these old rocks and is thought to represent a deep-seated weakness in the Earth's crust similar to the Reelfoot Rift that hosts the New Madrid seismic zone. The northeast-southwest trending Commerce Geophysical Lineament passes through southeast Missouri about 30 miles to the northwest of the New Madrid seismic zone. It extends southwestward to near Little Rock, Arkansas and northeastward to central Indiana. Several studies in Missouri have associated near surface features showing geologically recent earthquake activity with the lineament.
On Crowleys Ridge in the Benton Hills of Scott County, Missouri, trenching and seismic reflection studies by the Missouri Department of Natural Resources' Geological Survey Program, the U. S. Geological Survey and the University of Missouri - Rolla have documented several sites where geologically recent fault movements have occurred. At the intensively studied English Hill site, trenching investigations have been interpreted to show two or three periods of faulting with associated earthquakes in about the last 10,000 years. Several periods of even older fault movement have also been interpreted. Reliable data do not yet exist to estimate the magnitude of these earthquakes. However, large fault offsets and evidence of surface rupture suggest a minimum of magnitude 5 to 6, but probably higher. Seismic reflection geophysical surveys, using reflected waves of vibrations sent into the ground to produce an image of the underground layers and faults, have confirmed that surface features found in the trenches are connected to faults deep in the bedrock, and have been repeatedly activated throughout vast amounts of geologic time. Numerous seismic reflection surveys along the straight southeast flank of the Benton Hills have shown intense faulting above the trace of the Commerce Geophysical Lineament, suggesting a relationship between the lineament, the intermediate depth faulting and the surface faulting. Many of the imaged faults show displacement of the youngest geologic materials observable. The town of Commerce, located on the Mississippi River at the south edge of the Benton Hills, is the site of the Commerce fault and is close to three other sites where young faulting has been documented. The largest earthquake in Missouri in the last 10 years was located in the Benton Hills near the Commerce Geophysical Lineament. This was the September 26, 1990, magnitude 4.6 earthquake that happened during the Iben Browning incident.
About 40 miles to the southwest along the Commerce Geophysical Lineament is another paleoseismology site referred to as Holly Ridge. This site is located in the Bloomfield Hills, also on the southeast flank of Crowleys Ridge and directly above the lineament. At this site, which has only had a preliminary investigation by the Missouri Department of Natural Resources' Geological Survey Program, cut bank exposures show complex multistrand faulting in very young deposits. Large fault offsets postdate the youngest geologic unit, which is less than about 10,000 to 12,000 years old. Because of the site location on a hillside and the preliminary nature of the initial investigation, the tectonic (earthquake) origin of these faults has not yet been verified. A landslide origin is also a possibility. However, a seismic reflection survey about 1.5 miles to the northeast along the lineament trend shows tectonic faults in the youngest imaged deposit, which lends support to the argument for a tectonic origin.
Ranging from about 20 to 30 miles farther to the southwest along the Commerce Geophysical Lineament are several paleoliquefaction sites that can not be attributed to New Madrid seismic zone earthquakes. At the southern end of this range, near the town of Qulin and the Missouri-Arkansas border, another seismic reflection survey has shown faulting above the lineament in the youngest deposits imaged.
In Arkansas along the Black River, additional young paleoliquefaction sites have been found above the lineament. In 1982, a swarm of hundreds of small earthquakes occurred near Enola, Arkansas, which is also near the lineament. The largest earthquake in the central United States this century, a magnitude 5.5 quake, occurred on November 9, 1968. It was located in southeastern Illinois near the Commerce Geophysical Lineament. Along the Wabash River valley separating Illinois and Indiana, and along its tributaries in west central Indiana and southeastern Illinois, a large number of paleoliquefaction sites have been located and studied. These studies indicate that several large earthquakes have occurred in the last 10,000 years in that area, which is also located near the lineament.
All of this suggests the Commerce Geophysical Lineament is another earthquake source zone that needs to be considered when evaluating earthquake risk in the central United States. This has been done by the U. S. Geological Survey in preparing their latest seismic risk maps for the United States. The lineament is closer to most Missouri cities than the New Madrid seismic zone. It is about 75% closer to Cape Girardeau and Poplar Bluff and about 25% closer to St. Louis.
A new paleoliquefaction study report by the University of Maryland that was funded and published by the U. S. Nuclear Regulatory Commission has just been released. The study area was along the Mississippi River and its tributaries from Cairo, Illinois, to St. Louis, Missouri. Paleoliquefaction features were found in several areas including along the lower Meramec River just south of St. Louis and along the Kaskaskia River in Illinois east of St. Louis. Preliminary age dating of the features has had only limited success resulting in poorly constrained and overlapping age ranges. Also, the liquefaction susceptibly of the sediments has not yet been evaluated in detail. This results in several possible earthquake scenarios, which could explain the current data. These include a magnitude 7 earthquake about 40 miles east of St. Louis about 6,500 years ago or two separate earthquakes of about the same age with one being a magnitude 6 about 40 miles east of St. Louis and the other being a magnitude 5 in the south part of metropolitan St. Louis.
Earthquakes of these sizes located close to St. Louis could do much more damage in St. Louis than a much larger quake located in the more distant New Madrid seismic zone. For comparison, the November 9, 1968, magnitude 5.5 earthquake in southeastern Illinois was 100 miles from St. Louis, but even it caused some minor structural damage in St. Louis. As a reminder that St. Louis in not immune from earthquakes, a magnitude 2.4 quake struck near Fenton on the Jefferson - St. Louis County border on January 15,1998. This earthquake was felt over a large area of south St. Louis County and northeast Jefferson County.
Northwest Missouri
Western and northwestern Missouri are not immune from earthquakes just because they are located so far from the New Madrid seismic zone. Another buried feature named the Nemaha uplift and its associated Humbolt fault stretch from central Oklahoma to southeast Iowa across eastern Kansas and southeastern Nebraska. An elevated level of small to moderate earthquakes occur along this trend. Moderate earthquakes near Topeka, Kansas, in 1867 and 1906, caused Modified Mercalli Intensities VIII and VII, respectively, in the epicentral area and Intensities VI and IV, respectively, in both Kansas City and St. Joseph, Missouri, with values of VII and V, respectively, at points in Missouri between those cities. Intensity VII can cause considerable damage in poorly built buildings and slight to moderate damage in well-built buildings. Three small earthquakes have been felt in northwestern Missouri is the last five years. On May 13, 1999, a magnitude 3.0 earthquake located in Kansas City, Kansas, caused damage to two medical buildings there, and it was felt in Kansas City, Missouri. On February 11, 1995, a magnitude 3.1 quake shook Nodaway County and Maryville, Missouri. On March 31, 1993, a magnitude 2.9 earthquake struck near Nemaha, Nebraska, along the Missouri - Nebraska border, very close to the Cooper nuclear power plant on the Missouri River. This quake was felt in St. Joseph, Missouri.
Central to Southeast Missouri
Central Missouri, near Camdenton and Lake of the Ozarks, was shaken by a magnitude 3.1 earthquake on January 21, 1992. Earthquakes in this part of the state are rare but they do happen. As one proceeds southeast from central Missouri, earthquake frequency increases significantly to a moderate level, but is still well below the frequency of the New Madrid seismic zone. Over the years numerous earthquakes in the range of magnitude 3 to 5 have been located here. Small to moderate future earthquakes should be expected.
Education and Preparedness Overcomes Worry
Being aware of the earthquake threat, understanding the consequences and being willing to undertake preventive measures can greatly reduce the damage and economic impacts caused by an earthquake, even a large earthquake. Poorly constructed man-made structures are the main source of earthquake damage, casualties and economic losses. These are preventable with proper knowledge and actions and frequently have only a modest cost, especially where new construction is being done.
The good news is that most homes have wood frame construction that is relatively earthquake resistant. Your home therefore is usually the safest place to be in an earthquake. Larger buildings, especially reinforced concrete frame and masonry structures, where people work and congregate are usually more susceptible to earthquake damage and therefore require greater engineering care in design and quality control in construction. These structures can be made safe by paying close attention to the details and at a relatively modest cost when included at the time of construction. Strict adherence to building codes and their seismic provisions can make these structures more resistant to damage. They can also be designed to remain functional after an earthquake, which can prevent major economic impacts from business interruption and consequent loss of jobs.
Investigation and discussion of earthquake problems should not be considered a threat to attracting and keeping business and industry or to development and growth. Education and understanding is the key to success. Most responsible businesses would prefer to know about potential problems and have a plan for resolving them rather than being caught by surprise and unprepared, which could put them out of business. California and Japan have a lot more earthquakes than Missouri but businesses go there and stay there, they just take adequate precautions to survive an earthquake. A damaging earthquake can happen almost anywhere so moving to a different area will not ensure escape and may put you at risk to other perils. Not all faults that cause earthquakes are known. This has been demonstrated by recent devastating earthquakes in California and Japan on unknown faults. Awareness and preparation are the best defense.
David Hoffman
Geologist
Missouri Department of Natural Resources
Geological Survey Program
Excavating Old Quarry trench at English Hill. Note two faults visible in the right trench wall just to the right of the crawler-loader. Faults are less than about 10,000 years old.
Close view of trench wall showing one of many faults at English Hill. Fault extends from top center-right to bottom left corner. Note approximate 2-foot vertical offset of soil layers, with left side down relative to right side (e.g. dark soil layer at top right should align with dark soil layer at left center). Faulting offsets, and therefore is younger than, the Peoria silt. Deposition of Peoria silt ended about 10,000 to 12,500 years ago. The largest direction of fault movement is thought to be horizontal (into or out of the picture) not vertical.
Liquefaction caused sand dike cutting through clay and silt deposits exposed in the west bank of Dudley Main Ditch. Note that the sand dike has the characteristic taper of liquefaction dikes, larger near the bottom source area to smaller at the top where it was trying to erupt onto the ground surface. Age of dike is poorly constrained between 22,700 and 3,750 years old.
One of many faults at Holly Ridge site. Fault extends from upper left to lower right of the approximately 15-foot high cutbank. Wilcox sand is on the left and Peoria silt is on the right. Vertical offset component of faulting is greater than 30 feet. Age of faulting is younger than about 10,000 years.