Quasi‐static loading of the Earth by propagating air waves

Acoustic gravity waves from an event on October 14, 1970, presumed to be a Chinese atmospheric explosion, were observed on high-gain vertical- and horizontal-component seismograms written at three different locations in the world. Reliable group velocity data for the atmosphere over propagation paths as long as 97° (10,800 km) were obtained for the period range 30–375 sec. These data are in agreement with theoretical dispersion curves based on the Cospar model atmosphere with the effects of winds included. Seismic body and surface waves from this atmospheric event were also recorded at the three stations and were used to determine an epicenter. On April 14, 1971, slow-speed gravity waves from another pressure disturbance of meteorological origin were observed with periods between 240 and 360 sec on seismograms at Ogdensburg, New Jersey. The seismic recordings of the acoustic and slow-speed gravity waves are attributed to ground motion produced by surface loading and not to direct pressure effects on the instruments. The observed displacements and tilts from both events are in agreement with those predicted by static-loading theory. At the Ogdensburg station the rock layer above the instruments (543 meters) acts as a wavelength filter to suppress wind noise of short wavelength and to enhance signals from long-wavelength (coherent) disturbances in the atmosphere.