Missouri has already experienced earthquakes much greater than either the Northridge or Kobe events. During the winter of 1811-1812 three earthquakes estimated to have been magnitude 7.5 or greater were centered in southeast Missouri. Thousands of aftershocks continued for years. While estimates of the recurrence intervals of the large 1811-1812 earthquakes are about 500 - 1000 years, smaller, but still destructive, earthquakes are even more likely. The recurrence interval for a magnitude 6 earthquake is about 100 years. The last such earthquake was in 1895 near Charleston, Missouri.

A sequence of powerful earthquakes struck the mid-Mississippi Valley in the winter of 1811-1812. No fewer than 18 of these events were felt on the Atlantic seaboard, which implies that their magnitudes were greater than 6 - 6.5. The main shocks on December 16, 181 1, January 23, 1812 and February 7, 1812 had magnitudes greater than 7.5, making them some of the largest earthquakes known in the continental United States. Since then, large earthquakes have continued to occur: significant earthquakes, each about magnitude 6, occurred in 1843 near Marked Tree, Arkansas, and on October 31, 1895 near Charleston, Missouri. .PP Earthquakes affecting the region are not restricted to the Bootheel. The magnitude 5.5 earthquake, November 9, 1968 in southeastern Illinois caused minor damage in St. Louis. Other earthquakes have occurred throughout southeastern parts of Missouri. The figure on the cover of this report shows the distribution of historic known earthquakes in the region.

Active research projects on the nature of the earthquake problem are being sponsored through the National Earthquake Hazards Reduction Program (NEHRP). These investigations have documented large earthquakes prior to 1811-1812, thus reinforcing concern for the future. Other efforts generated updated probabilistic hazard maps that show the degree of the earthquake threat in southeastern Missouri.

The most important direct earthquake hazard is ground shaking. Ground shaking affects structures close to the earthquake epicenter but can also affect those at great distances. Certain types of buildings may be damaged by earthquakes at a significant distance from the epicenter. This places most of southeastern Missouri, as well as the St. Louis metropolitan area, at risk. Unreinforced masonry structures, tall structures without adequate lateral resistance, and poorly-maintained structures are specifically susceptible to large earthquakes. Owners of these structures should be aware of their potential for seismic damage.

Indirect hazards may also occur at great distances from large earthquakes. Liquefaction, landslides and life-line disruptions will most affect areas closest to the epicenter, but may occur at significant distances. The impact on the general public, small- to medium-size businesses, life-line services, and the infrastructure may be radically lessened if precautions are undertaken at multiple levels.

The flat-lying, southeastern (Bootheel) section of Missouri is most susceptible to earthquakes because it overlies the New Madrid fault zone. It is the epicentral area of the 1811-1812 earthquakes and seismic activity continues there. It also has the highest risk because its subsurface conditions -- loose sediments and a high water table -- tend to amplify earthquake ground shaking. The immediate vicinity of the Ozarks is also at risk from is also at risk from earthquakes in the New Madrid fault zone. Like the Bootheel, subsurface conditions of the Mississippi and Missouri River valleys tend to amplify earthquake ground shaking. As a result, these areas, including much of metropolitan St. Louis, are also at high risk from earthquakes. Earthquake hazards in the western part of the state also exist because of the historical earthquakes in eastern Kansas and Nebraska. No area of Missouri is immune from the danger of earthquakes. Minor, but potentially damaging, earthquakes can occur anywhere in the state.

Increased education, concern and subsequent action can reduce the potential effects of earthquakes, and this can be done in conjunction with preparations for other natural hazards. A program that recognizes the risk of flooding, landslides and other dangers and which incorporates earthquake issues, will be the most beneficial to our citizens.

Individuals and all levels of government have roles in reducing earthquake hazards. Individuals can reduce their own vulnerability by taking some simple and inexpensive actions within their own households. Local government can take action to lower the threat, through the proper use of poor sites, assuring that vital or important structures (police, fire and school buildings) resist hazards, and developing infrastructure in a way that decreases risk. State agencies and the legislature can assist the other levels of action and provide incentives for minimizing hazards.

Last Changed 04/29/97