Ulteriori precisazioni sull'origine dei microsismi

Caloi, Pietro ; Migani, Marcello

Atti della Accademia nazionale dei Lincei. Rendiconti della Classe di scienze fisiche, matematiche e naturali, Série 8, Tome 60 (1976) no. 6, pp. 824-833

Résumé

A series of experiments was performed in the dams of La Maina (Sauris) and Pieve di Cadore, able to explain some aspects of microseisms of local character. It is confirmed that, in small closed basins, the microseisms originate by rapid variations of the atmospheric pressure passing over the lake, owing to wavy systems with small wave-length, only affecting the superficial layer of the lake. That is, three seismographic stations are arranged into the dam, at three different heights between top and bottom: the first only records well developed microseisms, which are totally absent on the bottom and appear solely in the form of traces in the intermediate station. Another experiment, performed in the Pieve di Cadore dam, with seismographic station arranged at the same height, but in dam ashlars 120 meters distant between themselves, demostrated that local microseismes change from point to point, in form and amplitude. The absence of vertical component—recorded only as a trace—excludes the formation of Rayleigh waves. On changing the action line of the perturbing cause, microseisms increase in amplitude near a station, and decrease near the other one, separated from the first by an angular distance of about 55°. The recorded waves have character of longitudinal waves and they can be regarded as Rayleigh waves “fringes”, occurring in the Lamb theory. The extension of these conclusions to seas and oceans consents to affirm that sporadic, temporary, accidental microseisms, able to animate only short extents of coast, conditioned by the thickness of the sea layer, liable to the action of small wave-length microbarographic perturbations (and that take rise contemporary to fundamental microseisms—associated to systems which touch the sea bottom—destined to large distance), are essentially formed by longitudinal waves.