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The New Madrid Seismic Zone
Although the West Coast and Alaska generally has the greatest earthquake activity, the potential for moderate-to-large earthquakes exist in the Midwest where three of the largest historical earthquakes occurred in the winter of 1811-1812 along the New Madrid Seismic Zone (NMSZ) in southeastern Missouri. The three main earthquakes of moment magnitude 7.0-8.0 occurred on December 16, 1811; January 23, 1812; and February 7, 1812.
The NMSZ stretches from just the west of Memphis in Tennessee into southern Illinois, and consists of three fault segments: the northeast and the southwest segments which are strike-slip faults, and the Reelfoot or central segment which is a thrust fault. The probability of a New Madrid earthquake of magnitude 6.0 or greater occurring in the next 50 years is 25-40%, while a repeat of the 1811-1812 earthquakes is 7-10% according to the United States Geological Survey (USGS) and the Center for Earthquake Research and Information (CERI) of the University of Memphis.
The Region of Potential Impact
Earthquakes in the Central and Eastern United States (CEUS) are less frequent than in the Western United States (WUS). However, they affect much larger areas because of the regional geology being different east and west of the Rocky Mountains, where the CEUS is a more stable region in which the energy released from a large earthquake is transmitted into the surrounding regions more efficiently, while the energy is relatively quickly absorbed in the WUS. For comparison, the 1906 San Francisco earthquake of magnitude 7.8 was felt 350 miles away in the middle of Nevada, while church bells rang in Boston, Massachusetts, from the New Madrid earthquake of December 1811 of magnitude about 7.5, which is 1,000 miles away. Therefore, a moderate-to-large earthquake activity in the NMSZ is capable of causing widespread damage over a large region in the Midwest. The region of potential impact is comprised of eight states: Alabama, Arkansas, Illinois, Indiana, Kentucky, Mississippi, Missouri, and Tennessee.
Ground Shaking Intensity
The intensity of ground shaking that a building will experience during an earthquake is a function of three main factors:
The earthquake moment magnitude where the larger the earthquake, the stronger is the shaking.
The closest distance from the segment of the fault that ruptured to the building, where the nearer the building is to the fault, the greater is the shaking.
The type of soil materials beneath the building, where soft soils amplify and prolong the shaking even at great distances from the ruptured fault, while hard bedrocks do not amplify the shaking. Deep soils in the Mississippi River valley would shake more than bedrock in the hills as a result of the persistent shaking where the seismic waves are trapped and reverberate. Unfortunately most urban development is in valleys.
Hazards Associated with Ground Shaking
Hazards associated with ground shaking in the Midwest includes:
Liquefaction susceptibility where partially saturated soil deposits may completely lose cohesion during prolonged shaking, behave like a liquid, and lose their ability to support structures. The highest hazard is in low-lying areas where there are loose, sandy soils or poorly compacted artificial fill. A liquefied sand layer can shoot to the surface through cracks, forming a sand-blow, and depositing sand on the ground. This results in permanent ground deformations such as lateral spreading and settlement, both of which increase the likelihood of damage to buildings and infrastructures.
Landslide susceptibility where steep sloping areas underlain by loose or soft rock are most susceptible to earthquake-induced landslides, damaging buildings and other structures.
Fires where earthquakes in urban areas are often followed by destructive fires because of gas lines break and electrical shorts that may quickly spread due to clogged roads and collapsed bridges that prevent firefighter access, or damaged water tanks that limit water for firefighting.
Expected Regional Economic Losses
The region surrounding the NMSZ is home to millions of people, including those in the metropolitan areas of St. Louis and Memphis. Most structures were not built to withstand earthquake shaking. Moreover, earthquake awareness and preparedness have lagged far behind as compared to the West Coast. A worst-case scenario study was conducted by the Mid-America Earthquake (MAE) Center at the University of Illinois at Urbana-Champaign for each state affected by a hypothetical earthquake of magnitude 7.7 predicted by the USGS.
The study concluded that Tennessee, Kentucky, Missouri and Illinois would incur significant direct economic losses that reach $56, $45, $40 and $35 billion, respectively, while Arkansas and Mississippi would incur nearly $19 and $9.5 billion in direct economic loss, respectively. On the other hand, Indiana and Alabama would experience limited direct economic losses of approximately $1.5 and $1.0 billion, respectively. It shall be noted that the regional direct economic losses can not be determined by adding the above mentioned values. These values would be significantly reduced for a moderate earthquake of magnitude 6.0-6.5. Indirect economic losses due to business interpretation and loss of market share may be much higher than the direct economic losses. For comparison, the insured damages from Hurricane Katrina in 2005, the largest single natural disaster in the United States in recent years, were $45 billion. Additional federal aid and reconstruction expenditures were $127 billion.
Expected Buildings Damage
The MAE study demonstrates that residential buildings would incur more damage than any other building usage where wood frame buildings are the most prevalent building type in the Midwest and comprise a large portion of all building damage, while unreinforced masonry (URM) buildings comprise a much smaller portion of the regional building inventory. However, estimates show a much higher percentage of URM buildings would be damaged even from moderate shaking. In Tennessee; over 250,000 buildings are expected to be moderately or severely damaged from which 50,000 at least moderately damaged URM buildings from rupture of the southwestern segment of the NMSZ. Over 260,000 people would be displaced with over 60,000 casualties (injuries and fatalities). In Missouri; over 80,000 buildings are expected to be damaged leaving more than 120,000 people displaced and causing over 15,000 casualties.
How to Survive a New Madrid Earthquake?
Seismic Retrofitting
If your home is constructed before the 1980's, you probably need to retrofit it, especially if you live within the NMSZ (refer to the red, orange and yellow contours of the USGS seismic hazard map in the earthquake hazards section at http://earthquake-preparedness.net ). Seismic retrofitting measures ensure the structural integrity of your home to withstand earthquakes with little structural damage. Retrofitting is your own decision at your own risk according to your financial situation as it may be costly. However, it may be addressed one step at a time. The best return on the investment of retrofitting efforts is to initially address the foundations issues followed by bracing the cripple walls, then bracing the soft story, and ending with bracing or anchoring the masonry chimney.
Contents Mitigation
You should mitigate the contents of your home whether you retrofit it or not. Earthquake mitigation measures are simple techniques taken to secure non-structural elements and contents of your home to the studs of the interior walls, in order to reduce the risk to lives and investments, using inexpensive hardware tools and materials found in hardware stores and home centers. Items in the garage should also be secured to reduce damage to vehicles. Self-mitigation techniques can be undertaken by homeowners as they are simple to explain and require no special expertise, materials or tools to implement.
Earthquake Insurance
Homeowners insurance does not provide coverage for earthquake damage to your home. If you live within the NMSZ, you should consider earthquake insurance as an effective way to manage potential costs of future inevitable earthquakes.
Earthquake Preparedness Plan
You should review, update, or develop your own earthquake preparedness plan that includes maintaining emergency food and water for 1-2 weeks at all times along with personal survival kits and a household emergency kit. The magnitude 7.0 Haiti earthquake demonstrates the necessity of earthquake preparedness for anyone living in an active seismic region.
Concluding Remarks
Haiti earthquake is a wake up call for earthquake preparedness in the Midwestern States that has lagged far behind as compared to the West Coast. If you live within the region of the potential impact of New Madrid earthquakes, you should re-consider retrofitting your home or at least mitigating its contents to protect yourself and your family. The benefits of seismic retrofitting and mitigation go well beyond being simply reducing financial losses. It will make your home safer and help in returning your family much more quickly back to their normal life style. In addition, earthquake insurance would help in managing potential costs of future earthquakes. Finally, develop an earthquake preparedness plan that includes maintaining emergency food and water for 1-2 weeks along with personal survival kits and a household emergency kit.
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