Relazioni fra sismicità e moti lenti nella crosta terrestre

Abstract

At Somplago near Tolmezzo, two horizontal pendulums,reaching periods up to 20 minutes, are operanting (experimentally) in a130 m high, excavated mountain pit; the pit is connected with the subterraneanpower plant house where, in a room, is installed an electromagneticseismic station. The sensibility of the pendulums to the variations of theapparent vertical is extreme. At present, their period is on the order of5 minutes.In the first half of January 1969, in the region of the Ambiesta (aright-side confluent of the Tagliamento river, with which it unites nearTolmezzo) there occurred a small seismic period, characterized by two smallshocks, almost unnoticed by the local population; the first on the 7th andthe second, stronger one, on the 14th of January. After about 10 yearsof near immobility, at the end of November and along all of December,the clinographs functioning in the rocks, on the right of the Ambiesta atthe level of the dam barring te river, showed a perceptible, slow ondulationof the ground. This variation of the vertical can be put in relation withthe 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 asby the seismic stations of Tolmezzo and of Somplago. This first shock isto be considered definitely superficial.The second shock occurred on January 14th, in the region betweenthe Ambiesta-river and Somplago. Its depth has been estimated 011 theorder of 13 km. This shock was preceded by a very small displacementof pendulum A at Somplago on January 12th, and then followed —several hours after the earthquake — by a perceivable deviation of thevertical, recorded by both pendulums A and B The displacement(slightly above one second of arc) occurred more or less in direction ofthe source of the earthquake, with trend S 46° from Somplago. This isin accordance with the fact that the Somplago seisndc station recorded theearthquake as starting in the form of a compression (upward thrust). Themaximum displacement was reached in only a few hours, while it took about20 days to bring the pendulums back to their primitive position. If —as everything seems to indicate — the displacement showed by the longperiodpendulums is to be related to the consequences provoked by the shockin his ipocentral location, it can be deduced that the propagation in directionSomplago of the recorded ondulating movement has been very slow (witha velocity inferior to 1 km/h). In other words, the displacement of mattersubsequent to the breack in the elastic field occurred in the liypocenter,lias taken on the form of an extremely slow ondulation, evidently associatedwith the gradual proceeding of the deformed stratification to a new systemof equilibrium. There have been other occasions, in the Tolmezzo area,permitting to detect a clear relation between slow variations of the verticaland local earthquakes. In the contemplated case at Somplago, the ondulationsfollowing the earthquake of 14. January have been definitely greaterthan 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 gatherthe information apt to confer a more realistic character to the forecastingof seismic phenomena. To insist — as it is yet done in Italy — with thestatistical methods, is plainly illusory work. Max Planck spoke ironicallyof the pretense of some of his colleagues to resolve fundamental problemsof physics basing on the probability table. Only the strictly dynamiclaws — the famous physicist pointed out — satisfy wholly our need ofknowledge, while no statistical law can actually satisfy, because it has noabsolute value but admits of exceptions in the single cases and thus leavesus confronting the problem to know which are the cases where such exceptionsmay occur . Therefore it appears convenient — it is still Planckspeaking — to claim among the postulates of physics not only the existenceof laws, but also their strictly causal character and to never consider closedthe investigation, until each and every statistic law is resolved into oneor more dynamic laws