The 1783 Calabrian earthquake - 2

The seismic sequence that in 1783 hit Calabria and north-east Sicily marked the beginning of systematic studies aimed at a deeper understanding of seismic phenomena and their causes. Many scientists focused on peculiar effects induced by the earthquake. Among them there was the opposite rotation of two obelisks at the Charterhouse of Serra San Bruno, documented in the report of the seismological commission chaired by Michele Sarconi and reproduced in a picture contained in the enclosed atlas. This effect was mainly ascribed to vortical motions eventually associated with the earthquake. Interestingly, in his Principles of Geology Lyell wrote: “It appears that the wave-like motions, and those which are called vorticose or whirling in a vortex, often produced effects of the most capricious kind. […] Two obelisks placed at the extremities of a magnificent facade in the convent of S. Bruno, in a small town called Stefano del Bosco, were observed to have undergone a movement of a singular kind. The shock which agitated the building is described as having been horizontal and vorticose” (C. Lyell, 1830, p. 418). Later Robert Mallet questioned Lyell’s interpretation and wondered how the earthquake shaking the ground back and forth could twist a stone. He considered a lower stone (for instance the base of an obelisk) moving along a horizontal direction and identified the source of the opposite direction of motion of an overlying stone in its inertia. Among all the possible undulatory motions, the only one able to displace bodies by their inertia was then recognized to be “the transit of a wave of elastic compression, or of a succession of these, in parallel or intersecting lines, through the solid substance and surface of the disturbed country” (R. Mallet, 1848, p. 58). Mallet carried out an extensive study of earthquake waves, focusing on their velocity, amplitude and wavelength, as well as on the effects of reflection and refraction when impinging on a boundary between rocks of different density and elasticity properties,

___Salvatore Esposito & Adele Naddeo