We may thus conclude, with Professor Mercalli, that the earthquake
resulted from the almost immediate succession of two distinct shocks,
in each of which the nearly vertical vibrations were more marked at
the beginning, while the slower undulations predominated towards the
close, those of the second phase generally becoming vorticose through
the superposition of movements coming from different directions. The
second part of the shock in all of the more carefully written accounts
is described as the stronger, especially as regards the subsultory
vibrations in the meizoseismal area; except in the immediate
neighbourhood of Nice, where the second phase was generally regarded
as the weaker, or at any rate as not stronger than the first.
_Origin of the Double Shock._--These observations show, not only that
the principal earthquake consisted of two distinct shocks, but also
that the shocks originated in different foci. For, if the vibrations
of both had started from one focus, the second shock would have been
everywhere the stronger; instead of which there was a small area near
Nice where the intensity of the first was the greater. This points
clearly to the existence of another focus situated not far from Nice;
and it is evident that the greater intensity of the first part in that
district was due solely to the proximity of this focus, for, still
farther to the west, at Antibes, the second part was again the
stronger.
There is thus a striking agreement in the inferences drawn from
observations on the direction, time of occurrence, and nature of the
shock. In the face of such concurring testimony, little doubt can
remain as to the existence of two foci, one to the south of Oneglia
and the other to the south of Nice, the initial impulse at the latter
being decidedly the weaker, and preceding that at the eastern focus by
an interval of some seconds, long enough at any rate for the resulting
vibrations to reach the Oneglia focus and to spread beyond it before
the vibrations from that focus started on their outward journey.
_Seismographic Records._--In 1887, the Riviera and the districts
adjoining it were unprovided with accurately constructed seismographs.
The observatories at Alessandria, Milan, Monza, Parma, Florence, and
other places in Italy contained seismoscopes and other pendulums, and
these all registered the fact that an earthquake had occurred, and in
many cases traced a series of elliptical or elongated curves. A record
of the shock was also given by a Cecchi seismograph at Perpignan in
France, but the distance from the epicentre was too great to allow
details to be shown. The most valuable record was that obtained from a
Cecchi seismograph at the observatory of Moncalieri, near Turin, about
ninety miles north of the principal epicentre.
Public-domain text, read in full here on John Shaqi.
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