Seismic and Underwater Responses to Sonic Boom

Sonic booms produced by aircraft moving at supersonic speeds apply moving loads to the earth's surface. In deep water, a moving underwater pressure field is observed to accompany the hyperbolic boom trace sweeping over the surface. The pressure waveform underwater near the surface is almost identical to that of the N wave in air, but it is rapidly smoothed and attenuated with depth, typically becoming one-tenth as large at a depth less than 0.6 of the N wavelength. Overpressures may exceed background noise pressures by factors of up to 100 at moderate depths for frequencies between 2 Hz and 100 Hz, but are less than 0.16% of pressures known to harm marine life in single exposures. Adequate quantitative theories for the underwater effect have been developed, and have been verified by scale-model experiments. On land, which is generally stratified, there are two major effects: the “static” deformation field traveling with the surface load, and air-coupled Rayleigh wavetrains following each N-wave transient. The latter have frequencies and amplitudes determined by the geology and the aircraft speed. The former has always been the largest effect in over 1003 seismograms recorded in field tests. Its amplitude is proportional to the sonic-boom overpressure. The maximum ground motion recorded was about 100 times the largest natural, steady seismic noise background, but was still less than 1% of the accepted seismic damage threshold for residential structures. Movement is greater in soft ground than on hard rock, and decreases rapidly with depth. Present quantitative theories for the major seismic effects agree reasonably well with the experiments. Seismic forerunner waves, which begin at least 7 sec before arrival of the sonic boom, might be exploited for automatic warnings to lessen the startle effect. Sonic booms probably cannot trigger earthquakes, but might possibly precipitate incipient avalanches or landslides in exceptional areas which are already stressed to within a few percent of instability.