GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 2.1 GNGTS 2024 gravimeter B on the bottom of Fig. 2. It should be highlighted that these are not the errors of the instruments as such standard deviations contain the tide contributions. The two graphs show on the abscissa the 378 and 417 measurements in the times of execution of the two gravimeters and on the ordinate the 24 values of the hourly averages joined by broken lines. Polynomial fits of the averaged periods are also reported to retrieve maximum and minimum periods corresponding to minimum and maximum gravity values, respectively. These graphs involve simultaneity and repetitiveness in the measurements of tidal forces validated by water levels in Venice on the same day and have surprised leading experts in the field of tidal study and tidal force recording. Gravity data referred to a geographical point on Earth surface, which is usually a stable value, in turn is integrated with the addition of many very small perturbations. Generally of a transitory nature, small perturbations are observed to be induced by gravitational influences of the other celestial bodies of the solar system with greater mass (Campion, 2022), such as Jupiter and Saturn and/or from greater proximity such as Venus. These terrestrial gravity variations are difficult to determine with current gravimeters in a continuous way as they present variations in the sixth and seventh decimal of the period. Moreover, whenever detected in a sufficiently rigorous manner, gravity variations can provide valuable astronomical information. A further case concerns the ability of the damped pendulum gravimeter to highlight seismic events. It was used to record the gravity of the ground before, during, and after an earthquake highlighting some peculiar variations which were more intense during strong quakes (Campion, 2022). Polesine had always been considered an area with low seismic risk, but the strong earthquake in Modena, Italy in 2012, which affected the upper part on the border with Emilia and Lombardy, suggested that this was not the case. The signal reported in Fig. 3 top coincided with the seismic shock of 17 July 2011 in Ceneselli, 33 km away from Rovigo, towards Lombardy, having a magnitude ML = 4.5 without causing any particular damage. The double recording of the event in Rovigo, with both the seismograph and the gravimeter, provided the possibility to observe the event in depth by integrating the information obtained with the two measurement systems. The two graphs were drawn vertically on the same Fig. 3 top, the seismogram upward and the gravity diagram downward, making the times of the two signals coincide, so as to carry out a comparison. The recording made by the seismograph produced the main wave detected by the gravimeter, while the second part of the same recording was attributable to the return to equilibrium of the ground. The first part of the gravimeter recording from 19:41:57 to 20:27:35, for a total of 45 minutes, shows three regular oscillations of the ground which were followed by a flat trend in gravity. The gravity flattening that preceded the seismic event could be interpreted as a sign of soil stretching and the critical phase before the shock. The shock itself, which seismic recording showed to last approximately 7 minutes, corresponds to a direct wave of the ground lasting 5 minutes and a reverse wave lasting 2 minutes, as measured by the gravimeter. Direct and reverse waves are due to the integration of seismic oscillations with the damped pendulum oscillations, these waves seem to indicate a first phase of gravity reduction followed by a rebound phase of gravity increases. A series of larger fluctuations followed the end of the event. An earthquake of magnitude ML = 4.2 struck the area 1 km E in Ceneselli, on October 28, 2023, at 17:29:23 Italian time with geographical coordinates 45.015N, 11.388E at a depth of 8 km. The area affected by this earthquake was characterised by medium seismic danger and by strong earthquakes that had occurred in the past (INGV, 2015). The epicentre appears to fall in an area with few known earthquakes: the 2011 event discussed above, the sequence in the Po Valley in May-June 2012, and another event of magnitude ML = 4.2 occurred on October 25, 2023. The