Measuring Soil and Rock Properties in the St. Louis Area to Help Map Potential Earthquake Ground Motions
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Researchers at the University of Missouri, Rolla, Saint Louis University, Missouri Department of Natural Resources, Illinois Geological Survey, and the USGS are collaborating to compile subsurface geologic data and to conduct high-resolution seismic imaging investigations at over 100 locations in the St. Louis metropolitan area. These data will help to better characterize earthquake hazards and ground motion in this region. For the seismic studies we use small hammers to gently shake the ground either vertically (called P-wave) or horizontally (called S-wave) and then record the "echoes" from soil and rock layers at a given site. From these echoes, we can create a subsurface image and estimate how fast seismic waves travel beneath a site. Both the image and the wave speeds are important to more accurately determine how the soils and rock will behave during an earthquake. Shallow S-wave velocities have a large effect on the ground motion experienced at a site during an earthquake. The goals of the seismic imaging experiment are:
The St. Louis area is underlain by hard Paleozoic limestones which can sometimes be observed along highway road cuts. Knowing the depth of this limestone bedrock surface will be critical to accurately forecasting earthquake ground motions. Overlying the limestones are soils, weathered bedrock, and alluvium deposited in the river valleys. Measurements to date indicate that the thickest sections of material overlying the limestone bedrock surface can be up to about 45 meters in the river valleys. Bedrock depths in the upland areas are generally in the 0 to 15 meter range. All data from these studies will be incorporated into a subsurface model of the St. Louis area, and when combined with estimates of earthquake magnitudes and locations, earthquake ground motions can be estimated at vibration levels and frequencies relevant to the St. Louis built environment.
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