abstract

Simultaneous recordings of acoustic pressure waves and seismic body waves were obtained for three earthquakes of magnitude 2.0, 2.6, and 2.8 in the Imperial Valley, California, at hypocentral distances of 7 to 8 km. The recordings show that an acoustic signal with frequencies between 50 and 70 Hz begins within 0.02 sec of the P-wave arrival and gradually dies out by the time of the S-wave arrival about 2 sec later. These recordings are consistent with reports that a low rumbling sound frequently accompanied earthquakes in the January 1975 swarm. They also explain reports that the sound was often heard several seconds before the earthquakes were felt in that perceptible shaking apparently begins with the S wave for local earthquakes of magnitude less than about 3. Calculations of the transmission coefficients for seismic P and SV waves converted to acoustic waves at the Earth's surface confirm that body-wave ground motion from local earthquakes is sufficient to generate audible acoustic pressure waves.

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