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Discontinuities in the earth's upper mantle as indicated by reflected seismic energy

DEPARTMENT OF GEOPHYSICS, UNIVERSITY OF UTAH,, SALT LAKE CITY, UTAH

Abstract

Thirty-three seismograms from nine large quarry blasts ranging in size from 50,000 to 2,138,000 lb of explosives were analyzed for possible reflections from inhomogeneities in the earth's upper mantle. Of the 33 seismograms, four were obtained at temporary seismograph stations positioned between 90 and 243 km from the explosions and an array of three to four seismometers was used at each of the stations. The remaining twenty-nine seismograms were obtained from ten permanent seismograph stations located between 76 and 1,009 km from the explosions. Seven of these latter seismograms were obtained from the seismograph station at Salt Lake City, Utah, and six were obtained from the seismograph station at Eureka, Nevada. Each arrival on these 13 seismograms was noted and then correlated to determine which arrivals were common to all seismograms having nearly constant epicentral distances.

Of the nine quarry blasts recorded, seven were detonated at Promontory, Utah, and two were detonated at Lakeside, Utah, which lies about 33 km west of Promontory. This multiplicity of blasts resulted in two groups of seismograms for both the Salt Lake City and Eureka stations with one group at each station having a different epicentral distance from the other group at the same station. A comparison was made between the seismograms of each station based on the apparent velocity of the arrivals across this difference in epicentral distance. Seismic arrivals having apparent velocities that would be representative of deep reflections were selected from the aforementioned arrivals common to most records.

The remaining 16 seismograms, which were from eight permanent seismograph stations located at epicentral distances in excess of 500 km, were used to check the results from the analysis of the Salt Lake City, Eureka, and temporary stations.

Times of possible reflected events are presented which could result from energy reflected at discontinuities in the upper mantle at depths of about 190, 520, and 910 km. The depths were computed using average velocities based on velocity-depth curves given by Jeffreys and Gutenberg (Jacobs 1953, p. 187) for the deeper portions of the upper mantle and assuming that linear ray paths pertained.

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