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Seismic investigation of crustal structure in the eastern part of the basin and range province

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

Abstract

Results are presented of seismic studies of nine quarry blasts, ranging in size from 50,000 to 2,138,000 lbs of explosives, at Promontory and Lakeside, Utah, during 1956 to 1959, and also of the Rainier (1.7 kilotons) and Blanca (23 kilotons) nuclear explosions near Mercury, Nevada, on September 19, 1957, and October 30, 1958, respectively. Seismic waves from the blasts were recorded at 17 temporary stations to a distance of about 280 km and 15 permanent seismograph stations to a distance of about 1,009 km. Time-distance graphs were plotted using first and later arrivals at all of the stations. Depths to refracting horizons were computed using refraction equations for horizontal layers having constant velocities.

The time-distance plot of first seismic arrivals from Promontory, Utah, to Eureka, Nevada, (nearly 355 km) shows the following velocities: 5.73 km/sec to a horizontal distance of approximately 76 km, 6.33 km/sec from 76 to 132 km, and 7.59 km/sec from 132 to 355 km. The data are interpreted to indicate two velocity discontinuities, one at a depth of about 9 km, and the other at a depth of approximately 25 km.

A composite time-distance plot of seismic arrivals from all blasts at Promontory and Lakeside, Utah, shows that the velocity of recognizable first arrivals changes from 7.59 km/sec to 7.97 km/sec at a horizontal distance of about 650 km from the blast sites. Assuming flatlying layers, the depth to this third velocity discontinuity is computed to be 72 km.

The time-distance plot of first arrivals for the Blanca nuclear explosion, which was obtained from four stations in northern Utah at horizontal distances between 380 and 630 km from the shotpoint, shows a velocity of 7.44 km/sec for the recognizable first arrivals of energy between 379 and 464 km and 7.46 km/sec for arrivals correlated from 380 to 630 km.

For seismic energy that traveled beneath mountains in the medium characterized by a velocity of approximately 7.5 km/sec, the measurements of arrival times of first seismic energy showed that any warping of the top of the 7.5 km/sec layer beneath the mountains was not more than a few kilometers.

Footnotes

The four authors are, respectively, Associate Professor of Geophysics, Professor of Geophysics and Head of the Department, former graduate student, and former graduate student, at the University of Utah.

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