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E' assolutamente così ma l'amplificazione del segnale e la sua trasmutazione in frequenza parrebbe essere solo dovuta a fenomeni di risonanza tra il bedrock e i passaggi di velocità netti superiori che generano forti contrasti di impedenza.

L'affermazione di WILLIAM B. JOYNER, RICHARD E. WARRICK and THOMAS E. FUMAL in Bulletin of the Seismological Society of America; August 1981; v. 71; no. 4; p. 1333-1349
"The data illustrate that site amplification may be less a matter of resonance involving reinforcing multiple reflections, and more the simple effect of the low near-surface velocity. Application of traditional seismological theory leads to the conclusion that the site amplification for peak horizontal velocity is approximately proportional to the reciprocal of the square root of the product of density and shear-wave velocity."
Non l'ho più ritrovata, ma tieni conto che sono un autodidatta in materia.
L'articolo esponeva The effect of quaternary alluvium on strong ground motion in the Coyote Lake, California, earthquake of 1979.
The effect of alluvium on strong ground motion can be seen by comparing two strong-motion records of the Coyote Lake, California, earthquake of 6 August 1979 (ML = 5.9). One record at a site on Franciscan bedrock had a peak horizontal acceleration of 0.13 g and a peak horizontal velocity of 10 cm/sec. The other, at a site 2 km distant on 180 meters of Quaternary alluvium overlying Franciscan, had values of 0.26 g and 32 cm/sec, amplifications by factors of 2 and 3.
Che dovrebbe essere il campo di lavoro di molti.

Chi sa se qualcuno ci vuol dire qualcosa in proposito su una questione così importante.
Alcuni Autori (Jonathan P. Stewart UCLA http://cee.ea.ucla.edu/faculty/Papers/draft_report.PDF) citano questo articolo come base teorica per la applicazione del metodo della Vs30 da parte di Borcherdt (1994)
"Wave propagation theory suggests that ground motion amplitude should depend on the density and shear wave velocity of near-surface materials (e.g., Bullen, 1965; Aki and Richards, 1980).
Density has relatively little variation with depth, and so shear wave velocity is the logical choice for representing site conditions. Two methods have been proposed for representing depthdependent velocity profiles with a single representative value. The first takes the velocity over
the depth range corresponding to one-quarter wavelength of the period of interest (Joyner et al.,1981), which produces frequency-dependent values. Fumal and Tinsley (1985) developed 1-Hz Vs maps for the Los Angeles region by relating quarter-wavelength velocities inferred from 33 boreholes to geologic units."

Vi prego di tener conto che sono soltanto un collega che vuole capire ciò che sta facendo e non un esperto della materia, non vorrei influenzare con ragionamenti sbagliati i colleghi.