M o v e m e n t s o f t h e f a u l t o f t h e L a k e o f Cavazzo i n c o n n e c t i o n w i t h t h e l o c a l r a i n f a l l s P . C A L O I * - M . M I G A N I * * R e c e i v e d on O c t o b e r 15, 1971 SUMMARY. — U n d e n i a b l e w i t n e s s e s a r e g i v e n f o r t h e e x i s t e n c e of m i c r o - m o v e m e n t s of t h e f a u l t of t h e L a k e of C a v a z z o ( T o l m e z z o ) i n c o n n e c t i o n w i t h local r a i n f a l l s . T h e y a r e r e c o r d e d a s v e r t i c a l r o t a t i o n s b y t h e c l i n o - g r a p h i c s t a t i o n s i n c o r p o r a t e d i n t o E N E L ' s p o w e r s t a t i o n of S o m p l a g o w h i c h lias b e e n b l a s t o u t of t h e r o c k o n t h e W e s t e r n e d g e of t h e l a r g e f a u l t of t h e l a k e . T h e f a u l t in i t s e l f , w h i c h e x t e n d s p r o b a b l y d o w n t o t h e c o n f l u e n c e of t h e J l e l o t o r r e n t w i t h t h e T a g l i a m e n t o R i v e r a t a b o u t 10 k i n s S o u t h of t h e S t a t i o n , lies in t h e N W q u a d r a n t of t h e v a s t a l l u v i u m a r e a of t h e T a g l i a m e n t o e x t e n d i n g b e t w e e n V e n z o n e , G e m o n a a n d O s o p p o . T h e r o t a t i o n of t h e v e r t i c a l t o w a r d S S E m a y b e a t t r i b u t e d t o t h e w e i g h t i n c r e a s e of t h e a l l u v i u m d u e t o r a i n f a l l s . I n f a c t , t h e d e v i a t i o n p r o v e s t o b e p r o p o r t i o - n a l t o r a i n f a l l s : t h e h e a v i e r a r e t h e l a t t e r , t h e l a r g e r is t h e f o r m e r . T h i s s u g g e s t s t h e i d e a t h a t b e l o w t h e b e d of t h e T a g l i a m e n t o R i v e r t h e r e m u s t b e a l a r g e u n d e r g r o u n d flood w h i c h is a l i m e n t e d b y l e a k a g e of r a i n w a t e r . I t s h o u l d b e c o n s i d e r e d in t h i s r e s p e c t t h a t d u r i n g t h e S u m m e r w h e n r a i n f a l l s a r e s c a r c e , t h e v e r t i c a l is s u b j e c t e d t o a slow o p p o s i t e m o v e m e n t a t S o m p l a g o , a s if i t w e r e u n d e r t h e i n f l u e n c e of a c o r r e s p o n d i n g slow l i g h t e n i n g of t h e S S E a r e a . T h e m i c r o m o v e m e n t s of t h e t w o l e d g e s of t h e f a u l t m a y e x e r t a " r e l e a s e " e f f e c t on t h e f a u l t a r e a s u c h a s t o p r e v e n t s i z e a b l e a c c u m u l a t i o n s of e l a s t i c t e n s i o n s i n t h e s u r r o u n d i n g m e d i u m . A s a m a t t e r of f a c t , t h e S o m p l a g o a r e a h a s n e v e r b e e n in t h e e p i c e n t r e of d e s t r u c t i v e e a r t h q u a k e s a s f a r a s r e c o r d e d h i s t o r y g o e s b a c k , n o t w i t h s t a n d i n g t h e h i g h s e i s m i c c h a r a c t e r of C a r n i a . * D i r e t t o r e d ' O s s e r v a t o r i o , I s t i t u t o N a z i o n a l e di Geofísica, R o m a . ** E . N . E L . C o m p a r t i m e n t o di V e n e z i a . 32 P . C A L O I - M . M I G A N I 1. - I t was given in account of working — in experimental stage — of two horizontal p e n d u l u m s of exceptional power (x) located in a well of 136 metres hight which had been sank in t h e rock a n d contains t h e e q u i p m e n t of E N E L ' s power station of Somplago (Tolmezzo). Under normal conditions t h e recordings of t h e p e n d u l u m s are those pertaining to t h e E a r t h ' s tides. Local p e r t u r b a t i o n s depending on t h e m o v e m e n t s of water in t h e piezometric well of t h e S t a t i o n are n o t missing, b u t t h e y can be readily distinguished. There are often abnormal p e r t u r b a t i o n s of tectonic origin or attri- b u t a b l e to o u t w a r d reasons. The l a t t e r are t h e subject of this s t u d y . 2. - The recording of variations of t h e vertical as a consequence of passages of more or less a c c e n t u a t e d variations of t h e atmospheric pressure is certainly nothing new. Such variations h a v e been recorded m a n y times during t h e last decades. The e x t r e m e sensitivity of t h e clinographie s t a t i o n of Somplago to such p h e n o m e n a and to others connected with t h e m appears to be q u i t e new, however. E v e r since the station of horizontal p e n d u l u m s is operating a t t h e Somplago power station, the a m o u n t of cases recorded was such as to remove a n y d o u b t : t h e r e is no passage of atmospheric p e r t u r b a t i o n of some intensity which did n o t bring a b o u t t h e more or less rapid shift- ing of t h e image of one of t h e two components (the S33°E) out of t h e recording-paper, causing s h a r p deviations also of t h e other component a t 90°. We are limiting ourselves to report some of t h e most recent ex- amples: 20-25 F e b r u a r y 1969 (fig. 1), 21-26 April 1969 (fig. 2), 16-20 May 1969 (fig. 3), 3-9 J u n e 1969 (fig. 4), 7-13 J u l y 1969 (fig. 5) and m a n y others. Making a first comparison between t h e clinographic curves and t h e atmospheric pressure it m a y lead to t h i n k of an obvious correspondence between t h e deviation (more or less sharp) of the N33°YV-S33°E clinographic component and t h e decrease of atmospheric pressure observed in t h e area, b u t when looking closer one observes two f a c t s which cannot be explained: t h e time gap between t h e two events (variation of the vertical and decrease of atmospheric pressure) and t h e a p p a r e n t independence between t h e amplitudes of the two pheno- mena, since small pressure variations correspond sometimes to strong vertical deviations, a n d viceversa. M O V E M E N T S O F T H E F A U L T O F T H E L A K E O F C A V A Z Z O 1 7 Here one question had to be p u t : is t h e sudden and ample devia- tion of « A » t o w a r d S33°E, followed b y a rapid shifting of t h e image out of the recorder, really b o u n d — and exclusively so — to t h e passage of a minimum of pressure above Somplago! One f a c t is assured: gener- ally, when t h e atmospheric pressure gives a strong, negative leap, t h e vertical is deviated with t h e image being shifted out of t h e recor- der. The contrary, however, is n o t always true, since there are f r e q u e n t and ample shiftings of t h e light-image toward S33°E w i t h o u t t h e at- mospheric pressure having suffered variations beyond normal. W e won- dered if this could be due to exceptional high tides of t h e Venetian lagoons (Grado), b u t we got a negative answer. I t could be excluded as well t h a t phenomena of this kind are brought a b o u t by variations in t h e level of t h e near L a k e of Cavazzo, of too limited a surface t h a t even considerable variations of its level (daily variations of some decimeters) according to t h e requests of t h e near power s t a t i o n are a p t to cause such large effects, a n d it could be considered v e r y un- likely t h a t such a regular a n d f r e q u e n t p h e n o m e n a should h a v e some tectonic origin, so t h a t another weather f a c t o r h a d to be suspected: the rainfalls. Which was actually t h e case. To give one example only, let us consider t h e event of November 19, 1970. As will be seen f r o m fig. 9, t h e variation of atmospheric pressure on t h a t d a y was negli- gible. This notwithstanding, however, t h e image of component «A» of the horizontal p e n d u l u m to overstep quickly a n d a b u n d a n t l y its limits. The rainfall — a t Somplago solely p a s t 90 m m s in 24 hours — on t h a t d a y justifies t h e strong a n d sudden v a r i a t i o n of t h e vertical which happened in the area. And by referring to t h e rainfall, all t h e ob- served vertical variations, besides those reported by figures 1 and 16, are justified as well. 3. Which is t h e reason for such an outspoken sensitivity to phe- nomena which elsewhere remains almost unobserved? I t should n o t be forgotten t h a t t h e upper suspension of t h e p e n d u l u m extends vertically lip to 130 ms inside t h e mountain. W a s there a differential action be- tween t h e upper and t h e lower connection point a t t h e basis of t h e power station which m a y have enhanced t h e effect of rotation? Or did the entire m o u n t a i n rotate, as a rigid means? The a l t e r n a t i v e could be settled only by an horizontal pendulum which was conditioned ex- clusively by its base support, which was done. Dr. Migani had two horizontal p e n d u l u m s built and fixed on t h e ground floor of t h e power station, near t h e seismic station and t h e other clinographic station. 1 8 P . C A L O I - M . M I G A N I T h e new pendulums, whose sensitivity was much less t h a n t h a t of t h e original couple, offered an a d v a n t a g e over t h e l a t t e r by allowing generally the entire recording of the phenomenon, with t h e exception of particularly a c c e n t u a t e d variations of t h e vertical. T h e recordings of t h e new couple of p e n d u l u m s allowed to prove t h a t as a m a t t e r of f a c t t h e entire m o u n t a i n is uniformly subjected to t h e inclination, even in t h e case of superficial p e r t u r b a t i o n s such as t h e one here examined. Figures 14-16 show example recordings of the p h e n o m e n o n b y t h e new couple of horizontal pendulums. 4. - The rainfalls in t h e area a r o u n d Somplago are, therefore, t h e main reason of t h e vertical deviation recorded b y t h e clinographic stations installed in the power station which had been blast out of t h e rock in t h e extreme N o r t h of the L a k e of Cavazzo. How can this high sensitivity of t h e clinographic stations to rainfalls be explained? Spontaneously one is led to t h i n k of a p a r t i c u l a r tectonic f e a t u r e of t h e area where t h e above said stations are located. Geologists observe t h a t t h e p r e d o m i n a n t tectonic f e a t u r e of t h e area consists of a series of tectonic parallel chips which overslid t o w a r d s S and SYV, b y overlapping surfaces falling off t o w a r d N. There are, besides, transverse faults. One of t h e most i m p o r t a n t of t h e m is t h e f a u l t which can be distinguished along t h e W e s t side of t h e L a k e of Cavazzo and which was discovered and studied first by E . Feruglio (4) (figs. 18,19), a f r a c t u r e caused by a n horizontal shifting crosswise to t h e folds and m a r k e d by their sudden deviation t o w a r d S in correspondence to t h e Melò Valley. The f r a c t u r i n g plane is accompanied b y a vertical ejection a r o u n d which t h e rock was thinly smashed. The f r a c t u r e m a y be followed only for a limited length as b o t h in the N o r t h and in the South it disappears under t h e lake and t h e al- luvium. I t is impossible to check w h e t h e r it proceeds — as E . Selli (5) believes — u n t i l t h e confluence of t h e Melò in t h e Tagliamento River near which, on the W e s t e r n side of t h e Melò, a n o t h e r transverse frac- t u r e comes in sight (fig. 18). This f a u l t belongs to more recent orogenetic periods (post-lielvetian). Between the H e l v e t i a n and t h e Q u a t e r n a r y occurred the most intense diastrophic period with one-sided t h r u s t s f r o m S o u t h which caused an heavy crush followed by upheaval of t h e m o u n t a i n folds and by M O V E M E N T S O F T H E F A U L T O F T H E L A K E O F C A V A Z Z O 1 9 their upset or sliding, while a vigorous compression f r o m E a s t twiste d them and caused transverse f r a c t u r e s (*). 5. - The clinographic movements recorded a t Somplago on the Western edge of t h e Lake of Cavazzo a f t e r heavy rainfalls m a y be in some way connected with the f a u l t ' s activity. A more or less deep f r a c t u r e in the E a r t h ' s crust causes t h e bonds of t h e edges to slacken and permits relative micromovements with a certain degree of freedom. This is w h a t m a y h a p p e n in t h e area of t h e Lake of Cavazzo during rainfalls. The area West of t h e f a u l t is altogether mountainous. I n t h e E a s t , between Gemona, Osoppo, and Venzone, f u r t h e r there is a v a s t extension of alluvium which is p a r t l y cultivated and a p t to absorb a m o u n t s of water when it rains, with a considerable increase of the " g r o u n d wreight" so as to a t t r a c t t h e W e s t e r n edge of the fault. I t m u s t be emphasized t h a t t h e r e s u l t a n t of t h e observed bendings is always directed toward SSE, t h a t is toward the g r a v i t y centre of t h e area considered. I n t h a t case it m u s t be a d m i t t e d , how- ever, t h a t according to Selli's hypothesis, t h e f a u l t is n o t limited to t h e Western side of the L a k e of Cavazzo, b u t extends f u r t h e r towrard South where it links up with the f r a c t u r e which comes to sight between Ava- sinis and t h e confluence point of t h e Melò with t h e Tagliamento Kiver (fig. 18). One f u n d a m e n t a l f e a t u r e of the m o v e m e n t is its taking place with sudden and swift deviations, while a t the end of the rainfalls t h e r e t u r n to the original position of equilibrium is very slow (figs. 15, 16), as if an underground flood, suddenly alimented, loses gradually its flow when the rain has subsided. A p a r t from the way in which the micromovements of the f a u l t of the Lake of Cavazzo t a k e place in connection with rainfalls, and which could be looked into closer in following studies, there remains t h e out- standing f a c t of a large f r a c t u r e which is a n i m a t e d by an unsuspected outward cause. If the cause is t h a t which has been viewed in the pre- vious section, there is no d o u b t t h a t the two contact surfaces of t h e f a u l t (*) I n t h i s c o n n e c t i o n , it is e m p h a s i z e d t h a t , p r e s c i n d i n g f r o m t h e p h e - n o m e n o n o b j e c t of o u r s t u d y , i n t h e r e c o r d i n g s of t h e c l i n o g r a p h i c s t a t i o n of S o m p l a g o , a s l o w d e v i a t i o n N o r t h w a r d is a l w a y s p r e s e n t . T h e p e r m a n e n c e of a m i c r o r o t a t i o n N o r t h w a r d ( g e n e r a l l y N o r t h - W e s t w a r d , s o m e t i m e s N o r t h - E a s t w a r d ) , m a y b e of t e c t o n i c o r i g i n a s a p e r s i s t e n t p u s h s o u t h e r l y i n t o t h e a r e a . 2 0 P . C A L O I - M . M I G A N I carry out relative m o v e m e n t s when t h e cause is in action. T h e frequency of rainfalls in this area is r a t h e r high so t h a t t h e said micromovements recur q u i t e o f t e n and more or less a c c e n t u a t e d according to t h e inten- sity of t h e p e r t u r b i n g cause. I t m a y therefore well be affirmed t h a t a t least in the outer, surface p a r t of t h e f a u l t hardly great elastic tensions will a c c u m u l a t e d as t h e y are continuously " r e l e a s e d " by t h e above said relative micromovements. I n other words, t h a n k s t o those micromovements no surface e a r t h q u a k e s of high intensity should t a k e place in this area. As a m a t t e r of f a c t , t h e L a k e of Oavazzo has never been in t h e epicentre of large e a r t h q u a k e s as f a r as recorded his- tory can tell. As can be seen f r o m t h e s t u d y of Marcelli — Feliziani (2>3) (fig. 20), t h e area of t h e L a k e of Cavazzo appears surrounded b y t h e epicentres of e a r t h q u a k e s , a m o n g which very h e a v y ones, from the year 0 up to 1963. This does n o t give t h e impression of mere coin- cidence. R E F E R E N C E S (*) C'AI.OI P . , 1909. - Relazioni fra sismicità e moti lenti nella crosta terrestre. (Relations between seismicity and slow movements in the Earth's crust). « A n n a l i di G e o f i s i c a », X X I I , 3, p p . 2 0 9 - 2 2 8 . (2) FELIZIANI P . , MARCELLI L . , 1965. - Il terremoto di Tolmezzo del 26 Aprile 1959. I P a r t e : Venti secoli di storia sismica della Regione Cantica e dell'Italia Nord-Occidentale. « A n n a l i di G e o f i s i c a » , X V I I I , 3, p p . 165-207. (3) FELIZIANI P . , MARCELLI L . , 1966. II P a r t e : Studio macrosismico-geolo- gico: determinazione analitica delle grandezze ipocentrati e natura della scossa. « A n n a l i di G e o f i s i c a » , X I X , 2, p p . 191-214. (4) FERUGLIO E . , 1925. - Le Prealpi fra l'Isonzo e VArzino. « E s t r . B o l l . A s s . A g r . F r i u l a n a ». (S) SELLI R . , 1963. - Schema geologico delle Alpi Gamiche e Giulie occidentali. « G i o r n a l e di G e o l o g i a », X X X .