NEW FINDINGS REGARDING THE SEISMIC THREAT POSED BY THE NEW MADRID, SOUTH CENTRAL ILLINOIS AND WABASH VALLEY FAULT ZONES ON STRUCTURES IN THE ST. LOUIS METRO AREA

ROGERS, J. D., Department of Geological Sciences & Engineering, 129 McNutt Hall, 1870 Miner Circle, University of Missouri-Rolla, Rolla, MO 65409-0230, KARADENIZ, D., Department of Geological Sciences & Engineering, 129 McNutt Hall, 1870 Miner Circle, University of Missouri-Rolla, Rolla, MO 65409-0230, rogersda@umr.edu, dkrz9@umr.edu.

Recent paleoseismic work in the New Madrid Zone has identified at least six major quake sequences (M 7.3 to 7.8), with an average recurrence frequency of about 530 years and M 6 events every 70 to 90 years. The last significant event was a M 6.6 quake centered around Charleston, Missouri in October 1895. Other historic earthquakes have emanated from south central Illinois, which spawned M 5.0 events in 1838, 1857 and 1917. Little is known about what tectonic mechanisms causes these. In just the past several years a dearth of paleoliquefaction features have been discovered along the Wabash and Kaskaskia Rivers in southeastern and east central Illinois, along the Illinois-Indiana border. This area appears to have spawned M. 5.0 (1891), M. 5.4 (1968) and M 5.0 (1987) quakes in the last 110 years. This appears to be a separate intraplate seismic zone, similar to the New Madrid, but slightly less active in terms of recorded microseismicity.

The most likely earthquake facing the American Midwest is a M 6.0 to 6.8 event, emanating from any one of the three fault zones. Site response characteristics for long span bridges across the Missouri River near St. Louis were determined using synthetic ground motions for M 6.0 to 6.8 events at distances between 115 and 232 km. Response spectra generated for these sites suggest that lower magnitude quakes generate increased magnification of spectral accelerations, as would be expected. Between 8X and 12X magnification of spectral accelerations occurs in weaker ground motions at great distance, similar to what occurred at Mexico City in 1985. Weaker base rock motions resulting greater site amplification because those sites don't suffer from nonlinear behavior, which tends to damp out seismic energy trapped in the sediment pile. The resulting site response spectra exhibit a classic bimodal distribution resulting from the resonant frequency of the sediment pile laying on top the Paleozoic bedrock. The trailing peaks coalesce between periods of 0.67 and 1.5 seconds, which are bad for simply supported multiple span bridges. These findings suggest that M 6.4+ quakes at ranges of 200 to 260 km could be expected to trigger foundation liquefaction and resonant frequency problems for multiple span bridges and tall buildings (10 to 25 stories) in old channel corridors containing 100 to 150 feet of unconsolidated sediment.