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Original published in D. W. Steeples, ed., 1989, Geophysics in Kansas: Kansas Geological Survey, Bulletin 226, pp. 129-164 |
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Kansas Geological Survey
The article is also available as an Acrobat PDF file.
Historically, refraction surveys have been conducted in hopes of mapping distinct layers within the earth. Refraction is a useful tool provided its limitations and the assumption that layers increase in seismic velocity with increasing depth are kept in mind. A traditional reversed-refraction profile was conducted along a 500-km (300-mi)-long east-west line extending from Concordia, Kansas, to Agate, Colorado. Analysis of the data showed an average crustal velocity of 6.1 km/sec (3.7 mi/sec) and an average upper-mantle Pn phase velocity of 8.29 km/sec (4.97 mi/sec) with a Moho depth calculated to be 36 km (23 mi) on the eastern end and 46 km (29 mi) on the western end. Some evidence suggests velocities as high as 7.2 km/sec (4.3 mi/sec) in the crust at various locations along the survey line. The strong east-west regional gravity gradient of -0.275 mgal/km supports the seismically drawn conclusion of a thinning of crust in north-central Kansas. In order to supplement the data from this refraction survey, we took advantage of the Kansas earthquake seismograph network. A crustal study using earthquakes as energy sources and a regional earthquake network as seismometer locations resulted in a crustal-velocity model that will improve determination of local earthquake locations. A large anomalous body in the upper mantle/lower crust, assumed to be related to the Precambrian-aged Midcontinent Geophysical Anomaly (MGA), resulted in early P-wave arrivals from refracted energy from the Moho recorded at Concordia, Salina, Tuttle Creek, and Milford. An omnidirectional positive Pn residual zone near El Dorado may be related to the Wichita geomagnetic low. Some evidence suggests the presence of a lower velocity material on the western and eastern flanks of the MGA, possibly representing the Rice Formation. A Pn velocity of 8.25 km/sec±0.15 km/sec (4.95 mi/sec±0.09 mi/sec) with the crust thinning from west to east and an apparent thinning from the north and from the south was determined from the 16 regional earthquakes studied. Crustal thickness from central Kansas through western Missouri seems to be relatively consistent.
Kansas Geological Survey
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