AN INVESTIGATION OF SEISMIC WAVE PROPAGATION IN THE EASTERN UNITED STATES
MICHIGAN UNIV ANN ARBOR INST OF SCIENCE AND TECHNOLOGY
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The paper describes the travel-time anomalies and attenuation losses of seismic compressional waves generated by a series of underwater explosions in the eastern United States. The efficient tamping of the shots fired in water provided a seismic source that could be detected at much larger ranges than could be accomplished by equivalent sized shots fired underground. A number of mobile field recording stations equipped with three-component matched short period seismometers and magnetic tape recorders were used to record 273 individual shots. A total of 1,295 recordings were obtained along a reversed profile extending from International Falls, Minnesota to the Atlantic Coast. The underwater shots were fired in Lake Superior and the Atlantic Ocean. Precise travel times were obtained by recording radio time signals at all of the recording stations. An analysis of the travel-times of the seismic waves disclosed that the earths crust varies in thickness from 28.1 km near the Atlantic Coast in North Carolina to 50.4 km near the Keweenaw Peninsula in upper Michigan. The crust in North Carolina was found to be comprised of a single layer with a compressional wave velocity of 6.0 kmsec. A two-layer crust with compressional wave velocities of 6.2 and 7.7 kmsec was disclosed in upper Michigan. Travel time residuals across the Appalachian Mts. would indicate a mountain root system similar to that found under the Rocky Mts. The Lake Superior area was found to be more efficient in the coupling of energy into seismic waves by the underwater shots than in the Atlantic Ocean.