The probabilistic calculations by Chris Cramer, a University of Memphis seismologist at the Center for Earthquake Research and Information, and 15 colleagues including the Illinois State Geological Survey’s Robert Bauer, was recently published in Seismological Research Letters. The calculations in the manuscript were based on a number of different earthquake scenarios and on high-resolution 3D maps of near-surface unconsolidated deposits (glacial and postglacial) down to bedrock, developed over the past 20 years by ISGS and the Missouri Department of Natural Resources.
The thickness and material properties of individual glacial layers play a role in how earthquake motions coming up from the bedrock will be amplified by the column of the glacial materials, and thickness of land and depth to water table provide information on liquefaction susceptibility.
The Illinois side of the urban area has been a priority for ISGS map makers for decades, partly because of its seismic hazards. Maps used in this study were primarily produced by David Grimley and Andrew Phillips, ISGS geologists who have published more than 20 maps covering parts of Jersey, Macoupin, Madison, Monroe, and St. Clair Counties, as well as full county surficial geologic maps of Madison and St. Clair Counties.
This interest arises partly from the fact that Illinois has experienced historic (12 produced damage in the past 205 years) and prehistoric earthquakes.
There is ample evidence of the Midwest’s seismic history – from signs of liquefaction in Ice Age human settlements, to the great New Madrid quakes of 1811 and 1812, to more recent measurements of ground shaking with modern instruments. The famous New Madrid seismic zone is less than 150 miles to the south, which produced major quakes up to 7.8 magnitude.
The seismic activity in the middle of a continental plate is still an enigma in geology, according to Tim Larson, an ISGS senior geophysicist. The dominant theory is that a supercontinent began to break apart 750 million years ago along what today is eastern Missouri through Illinois, Kentucky and into Indiana. But the split stalled, leaving the Reelfoot Rift zone among thicker, stable rock layers. Another idea is that the North American plate slowly drifts, causing old faults to reactivate and causing earthquakes.
The geology of the region tells the tale of why even infrequent temblors pose a risk of severe damage in the highly populated area, according to Cramer’s calculations. Four major rivers formed extensive alluvial plains with sediments up to 50 meters deep. Shaking in the lowlands on such sandy ground with high water tables leads to liquefaction, which can cause structures to sink into the ground.
ISGS’s work makes the St. Louis metro region of Illinois among the most thoroughly studied in the country. The work uses sound migration-reflection from thousands of active and abandoned wells, and bore holes drilled to complete the maps contribute to a high-resolution of about 500 meters.
Such high-resolution data means subsequent synthesis (models and algorithmic) frameworks like Cramer’s start on solid scientific ground, Bauer explained. Most of the highest-risk areas identified by the study are on the Illinois side of the Mississippi River, but the Missouri side also includes much territory with high-risk of damage. Substantial areas are built on artificial fill and thus have unknown risk of damage.
Such maps and their use for predictive syntheses are valuable for regional planners and first responders for disaster planning, said Bauer.
In estimating damage from future earthquakes, Bauer explained that ground shaking is distinguished by frequency (vibrations per second) and as a percentage of the Earth's gravity (1.0 g).
Earthquakes in the Midwest pose an added threat because the seismic energy travels farther and weakens less than in the geology of the western U.S.
In fact, measuring distant earthquakes was an important part of deep mapping of the crust under Illinois, Missouri, Kentucky, and Indiana completed by ISGS last year as part of the EarthScope project. Similar to the way magnetic resonance imaging reveals the inner structure of the body, the seismometers listened for earthquakes, revealing the geologic structures underground. They did this by measuring dozens of small local earthquakes, and by recording how hundreds of faraway earthquakes passed through underground layers.
ISGS geologists placed two large arrays of 60 seismometers at intervals across that area for 18 months. Illinois averages a magnitude three temblor every year, a magnitude four every five years, and a magnitude five every 20 years. The National Science Foundation project deployed thousands of seismometers and other sensors throughout North America to develop a deeper understanding of the underground structure and how they contribute to earthquakes.
ISGS also recently published 12-layer maps for the EarthScope project.
ISGS is also part of the Central U.S. Earthquake Consortium, which works with other state geological surveys in the Central U.S. to provide information on geologic factors contributing to earthquake hazards (i.e. soil liquefaction, slope stability, and soil amplification).