Amplitudes of seismic arrivals from the M discontinuity

Theoretical wave forms for the first cycle and a half are calculated for Romney's experimental recordings of underground nuclear explosions Blanca, Logan, and Tamalpais at distances of 96 to 714 km. Models of the crust are constructed from travel times. Zvolmskii s near-front approximation is used to form the basis of amplitude calculations which include head coefficients, geometrical spreading, and corrections for superposed layers. The source function is scaled from measurements made of the Rainier shot. The effects of attenuation and instrument response are included. By convolving these factors, theoretical displacement amplitudes are calculated in millimicrons for the first half-cycle which agree with the experimental measurements of Logan and Bianca from 300 to 600 km within +4 to −16 per cent. A single-layer crustal model with a Q of about 400 is indicated by the amplitude calculations. The amplitudes of later half-cycles are influenced by the reflection or interaction at the surface of the Rainier mesa. Additional data and calculations indicate that the surface reflection or interaction is nonlinear and has an amplitude about three times that expected on an elastic basis.

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