Background of gravitational-wave antennas of possible terrestrial origin. I.

The data collected in May 1980 with two gravitational-wave antennas operated simultaneously, one (M equals 20 kg) in Rome, the other (M equals 389 kg) in Frascati, show a few high-energy events recorded by the two stations at times which differ by a few seconds. The probability for these 'coincidences' to be accidental is of the order of 0.0010. Various considerations tend to exclude that these events are due to gravitational waves of extraterrestrial origin. In addition, a few types of trivial local disturbances have been excluded. A study of the occurrence times of the events recorded at Frascati shows the presence of two periods (53.1 min and 54.7 min) equal to those of the free oscillations of the earth.