Fault Hazards Increase Seismic Threat for St. Louis Area

New Madrid, Missouri, is well-known as the "bulls eye" for seismic hazard in the midwestern U.S. -- huge earthquakes were centered here in 1811 and 1812 and low-level seismic activity has continued ever since. But a closer study of faults to the north of the New Madrid seismic zone reveals that surprisingly recent and large-magnitude earthquakes have occurred 20 miles closer to St. Louis than previously believed. The discovery raises the earthquake risk for this metropolitan area by an amount that remains to be determined.

The English Hills faults are located on the southeast flank of the Benton Hills in Scott County, Missouri. Originally described by geologists in the 1930s, the faults had been ignored for decades: their very existence had even been challenged. The faults have been located in a series of trenches dug in the spring of 1995 by geologists Dave Hoffman, Jim Palmer, and Jim Vaughn from the Missouri Department of Natural Resources, Division of Geology and Land Survey and by Richard Harrison and others from the U.S. Geological Survey. The trenches are located about 15 miles south of Cape Girardeau, Missouri and about 115 miles south-southeast of St. Louis. This "rediscovery" of the English Hills faults is especially significant for several reasons.

First the material displaced by faulting in the trenches is quite young geologically. It appears that, while 8 to 12 feet of vertical displacement has occurred at the trench site, most of the displacement is horizontal---how much horizontal displacement is not yet known. Modern soil profiles, which are usually less than 10,000 years old, have apparently been displaced by large earthquakes along the English Hills faults. Faulting within the past 10,000 years is considered "active." It must be assumed that active faulting will continue in the future.

Second, the evidence shown in the trenches suggests that more than one type of faulting occurred. This is important because it shows that the forces deep within the earth's crust, which are released through earthquakes, have changed over time. The English Hills faults are a distinct weakness in the earth's crust. The earth has "used" that weakness as a way to release energy during more than one mechanical adjustment to its physical arrangement. Earthquakes have occurred along the English Hills faults for a long time--- probably for more than 100,000 years.

Finally, and perhaps most importantly, the faults are outside the area generally considered the "New Madrid fault zone" (NMFZ). The NMFZ is an area that more-or-less extends from near Charleston, Missouri to near Marked Tree, Arkansas---with doglegs near New Madrid, Missouri and Tiptonville, Tennessee. The northernmost edge of the NMFZ is about 20 miles south of the English Hills faults. Because essentially all risk-estimation studies for the St. Louis metropolitan area (including the 1993 BOCA building code) have been based on earthquakes occurring on the NMFZ, this newly identified, highly capable fault, which is 20 miles closer to St. Louis, will increase the risk to the entire St. Louis metropolitan area. The degree of that increased risk has not yet been estimated.

The English Hills faults affect the earthquake risk for the St. Louis metropolitan area, much of Missouri, and southern Illinois: the risk posed has been there for many thousands of years. However, our recognition of that risk is new---and our reaction to it is quite warranted. Much more work is needed.

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