More on the Somigliana waves (Ci.j and PL)

Previously Caloi has shown how a few corrections and rectifications of a theory on surface waves published by Carlo Somigliana in 1917, permit a quite new interpretation of the classic Rayleigh equation. In the light of this new interpretation, contrary to the Rayleigh theory, the real roots greater than 1 of the velocity equation have a correct physical meaning. More than a period of thirty years of useless attempts, one of use f1-2-7), finally, have reached a strict theoretical justification about conspicuous systems of typical surface waves, which start at the bottom of the Earth' crust layers, when the angles of incidence of the longitudinal waves or transverse waves are effective. These are the so-called PL¡,¡ and Ci,¡ waves. These waves are very interesting because they evince a new mechanism of elastic waves propagation, and they permit to study the fundamental characteristics of the crust stratifications. Owing to the considerable wave-lenghth, Somigliana waves present appreciable advantages as compared to ordinary longitudinal waves or transverse waves, both they are not subjected to phenomena of anomalous dispersion (see the case of the ordinary body waves of high frequency), as well as for they indicate the existence of discontinuities in the likeness of transition-zones, which generally escape the investigations when the same are performed by ordinary longitudinal waves and transverse waves. Besides the new very clear examples of these particular types of waves — very often of remarkable amplitude — the Authors study the limits in which the Somigliana waves rise: then waves may start also at the greatest distances (like the Co,2 waves), and sometimes they form the most large phase of a seismogram.