Traveltimes and amplitudes from nuclear explosions; Nevada Test Site to Ordway, Colorado

Abstract

This paper treats the results of a study of seismic waves generated by eight nuclear explosions and recorded at 31 locations between the Nevada Test Site (NTS) and Ordway, Colorado. The line of recording stations crosses the eastern part of the Basin and Range Province, the Colorado Plateau, the southern Rocky Mountains, and extends into the Great Plains. In the eastern Basin and Range Province and the western margin of the Colorado Plateau (0 ≤ Δ ≤ 385 km ), the time-distance curves for P g and P n can be expressed, respectively, as T 1 = 0.8 + Δ/6.0. T 3 = 5.8 + Δ/7.6. A third phase, tentatively identified as P*, is represented by the equation T2 = 3.8 + Δ/6.5. Using the crustal structure and P n velocity (7.9 km/ sec) found for the NTS region by other authors, these relations indicate that the thickness of the crust increases from about 25 km at NTS to about 42 km in the western part of the Colorado Plateau Province. East of this boundary the velocity of P in the upper mantle increases to 8.0 km/sec; depth to the Mohorovicic discontinuity is approximately constant over the range 435 ≤ Δ ≤ 645 km. Beyond 850 km, first arrivals indicate an apparent velocity of about 8.4 km/sec. Amplitudes of P n attenuate according to the equation A = A o Δ -1/2 (Δ -d) -3/2 e -0.0022Δ over the distance range 150 ≤ Δ ≤ 850 km. This relation yields a value of Q, for P n of about 520. The amplitudes of P g attenuates extremely rapidly, and beyond about 130 km this phase cannot be identified with certainty. An extension of the P g traveltime branch at large distances could be associated with waves reflected beyond the critical angle, from the base of the crust. This phase, called ?P after Mohorovicic, appears to attenuate as A = Ao e -0.076Δ Δ -1/2 . The value of Q indicated by this equation is about 200.