Relazioni fra sismicità e moti lenti nella crosta terrestre



At Somplago near Tolmezzo, two horizontal pendulums,
reaching periods up to 20 minutes, are operanting (experimentally) in a
130 m high, excavated mountain pit; the pit is connected with the subterranean
power plant house where, in a room, is installed an electromagnetic
seismic station. The sensibility of the pendulums to the variations of the
apparent vertical is extreme. At present, their period is on the order of
5 minutes.
In the first half of January 1969, in the region of the Ambiesta (a
right-side confluent of the Tagliamento river, with which it unites near
Tolmezzo) there occurred a small seismic period, characterized by two small
shocks, almost unnoticed by the local population; the first on the 7th and
the second, stronger one, on the 14th of January. After about 10 years
of near immobility, at the end of November and along all of December,
the clinographs functioning in the rocks, on the right of the Ambiesta at
the level of the dam barring te river, showed a perceptible, slow ondulation
of the ground. This variation of the vertical can be put in relation with
the minor shock of January 7th 1969, that was perceived by the personnel of the Ambiesta-dam and recorded by the local seismic station as well as
by the seismic stations of Tolmezzo and of Somplago. This first shock is
to be considered definitely superficial.
The second shock occurred on January 14th, in the region between
the Ambiesta-river and Somplago. Its depth has been estimated 011 the
order of 13 km. This shock was preceded by a very small displacement
of pendulum " A " at Somplago on January 12th, and then followed —
several hours after the earthquake — by a perceivable deviation of the
vertical, recorded by both pendulums " A " and " B ". The displacement
(slightly above one second of arc) occurred more or less in direction of
the source of the earthquake, with trend S 46° from Somplago. This is
in accordance with the fact that the Somplago seisndc station recorded the
earthquake as starting in the form of a compression (upward thrust). The
maximum displacement was reached in only a few hours, while it took about
20 days to bring the pendulums back to their primitive position. If —
as everything seems to indicate — the displacement showed by the longperiod
pendulums is to be related to the consequences provoked by the shock
in his ipocentral location, it can be deduced that the propagation in direction
Somplago of the recorded ondulating movement has been very slow (with
a velocity inferior to 1 km/h). In other words, the displacement of matter
subsequent to the breack in the elastic field occurred in the liypocenter,
lias taken on the form of an extremely slow ondulation, evidently associated
with the gradual proceeding of the deformed stratification to a new system
of equilibrium. There have been other occasions, in the Tolmezzo area,
permitting to detect a clear relation between slow variations of the vertical
and local earthquakes. In the contemplated case at Somplago, the ondulations
following the earthquake of 14. January have been definitely greater
than the ones preceding the quake.
In my opinion, it is with this course of action — geodynamic, geodetic,
geomagnetic and geoelectric observations — that one can hope to gather
the information apt to confer a more realistic character to the forecasting
of seismic phenomena. To insist — as it is yet done in Italy — with the
statistical methods, is plainly illusory work. Max Planck spoke ironically
of the pretense of some of his colleagues to resolve fundamental problems
of physics basing on the probability table. " Only the strictly dynamic
laws — the famous physicist pointed out — satisfy wholly our need of
knowledge, while no statistical law can actually satisfy, because it has no
absolute value but admits of exceptions in the single cases and thus leaves
us confronting the problem to know which are the cases where such exceptions
may occur " . " Therefore it appears convenient — it is still Planck
speaking — to claim among the postulates of physics not only the existence
of laws, but also their strictly causal character and to never consider closed
the investigation, until each and every statistic law is resolved into one
or more dynamic laws ".

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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X