Summary report of strong-motion measurements, underground nuclear detonations

Abstract

Subsurface and surface motion measurements were made on six underground nuclear detonations in the Oak Springs tuff of Nevada Test Site in Operation Hardtack II: Shots Mars (∼13 tons), Tamalpais (∼77 tons), Neptune (∼115 tons), Logan (∼5 kt), Evans (∼30 tons), and Blanca (∼19 kt). Free‐field peak radial acceleration decreased as the inverse third or fourth power of slant range, as for Rainier. Particle velocities attenuated at a rate between the inverse square and inverse cube. Maximum radial and tangential subsurface stress varied as the inverse cube of radial range. Observed peak strain suggested attenuation at a rate between inverse cube and inverse square of range. Maximum upheaval at Blanca surface zero was about 25.5 feet; ∼2.5 feet at 750 feet radial range; and 1.5 feet at 910 feet. Reed gage spectra indicated a shift of maximum energy to lower frequencies with increasing ground range. All components of surface acceleration followed an empirical equation of the form All components of surface displacement did not follow a comparable relationship. Displacement is more precisely predicted than acceleration. The velocity of the tuff was determined to be 6200 ft/sec, with the velocity of the underlying dolomite 11,700 ft/sec. If a two‐layer model for the crust beneath the Nevada Test Site is assumed, appropriate values for the thicknesses and velocities are 24 km and 5.69 km/sec for the top layer, and 36 km and 7.65 km/sec for the intermediate layer. The top of the mantle beginning at a depth of 60 km has a velocity of 8.12 km/sec and dips eastward.

Seismological evidence pertaining to the structure of the Earth's upper mantle

“…The authors proposed a crustal model consisting of an upper layer 28.5 km thick, an intermediate layer 24.5 km thick, and a Pn velocity of 8.11 km/sec. Adams et al [1961] reported the travel times of P waves from Blanca and Logan to distances as great as 3850 km. They found that their data, out to 900 km, could be fitted by three straight-line travel-time curves.…”

Partitioning of Seismoacoustic Energy and Estimation of Yield and Height-of-Burst/Depth-of-Burial for Near-Surface Explosions

“…Adams et al (1961) and Murphy and Lahoud (1969) found that of three types of seismic observables, displacement, velocity, and acceleration, only displacement legitimately follows cube-root scaling. They reason that outside the hydrodynamic region, in the seismic far field, the elastic medium introduces other characteristic lengths and a low-frequency observation such as displacement is less susceptible to these effects.…”

“…A few particular studies are relevant to this study. Adams et al (1961) and Murphy and Lahoud (1969) reported empirical scaling relations for far-field seismic motions (displacement, velocity, and acceleration) versus range. Many studies found different behavior for explosions from different emplacement material strength (e.g., Perret and Bass, 1975) or recording site lithology (e.g., Murphy and Lahoud, 1969).…”

A geophysical shock and air blast simulator at the National Ignition Facility

“…Seismometers, key diagnostic tools of seismic signatures, measure only displacement. Previous researchers 31,32 have noted that only displacement legitimately follows cube-root scaling analogous to the types presented for air blast effects in Sec. V B. Koper et al 4 have concluded that the peak displacement of the first seismic-signal arrival is the most robust estimator of the yield of the source of all the observables they studied.…”