Land s u b s i d e n c e i n V e n i c e a n d P o r t o C o r s i n i (*) M . C A P U T O - L. P I E R I - P. Rossi T E S I ( * * ) R e c e i v e d o n J a n u a r y 22, 1972 SUMMARY. — T h i s p a p e r is a c o n t r i b u t i o n t o t h e s t u d y of t h e p r o b l e m of s u b s i d e n c e in t h e P o d e l t a a n d in t h e V e n e t i a n l a g o o n . W e h a v e c o m p a r e d t h e d a t a s u p p l i e d b y t i d e g a u g e s i n T r i e s t e , V e n i c e a n d P o r t o Corsini ( F i g . 1) d u r i n g t h e p e r i o d 1896-1966, a n d i n G e n o a i n t h e p e r i o d 1894-1966. A l t h o u g h d a t a w e r e r e c o r d e d in s l i g h t l y d i f f e r e n t p e r i o d s , t h e s e p e r i o d s a r e s u f f i c i e n t l y o v e r l a p p i n g t o a l l o w f o r a s i g n i f i c a n t s t a t i s t i c a l c o m p a r i s o n . W e n o t e t h a t t h e m o v e m e n t of t h e a n n u a l m e a n s e a l e v e l in G e n o a a n d T r i e s t e is g r e a t l y d i f f e r e n t f r o m t h a t of P o r t o C o r s i n i a n d V e n i c e a n d , i n o r d e r t o s e p a r a t e t h e c o m p o - n e n t d u e t o e u s t a t i c e f f e c t f r o m a n y p o s s i b l e m o v e m e n t of t h e c o a s t , w e cal- c u l a t e t h e polynomial c o e f f i c i e n t s , t h a t b e s t f i t t h e r e a l m o v e m e n t s . A p p r o x i - m a t i n g p o l i n o m i a l s a r e of f i r s t d e g r e e f o r G e n o a a n d T r i e s t e a n d of s e c o n d d e g r e e f o r P o r t o C o r s i n i a n d V e n i c e . W e h a v e e v a l u a t e t h e c o e f f i c i e n t s of c r o s s c o r r e l a t i o n firstly o n o r i g i n a l d a t a a n d t h e n s u b t r a c t i n g f r o m o r i g i n a l d a t a t h e b e s t fit p o l i n o m i a l s . T h e c o r r e l a t i o n is v e r y s i g n i f i c a n t a n d t h e t r e n d is s i m i l a r in T r i e s t e a n d G e n o a . T h e p o w e r s p e c t r u m a n a l y s i s w i t h t h e d i r e c t F o u r i e r ' s m e t h o d s h o w l o n g p e r i o d s w i c h a r e s h a r e d b y t i d e g a u g e s . W e c o n c l u d e e s t i m a t i n g 1,35 m m / y e a r t h e i n c r e a s e of t h e m e a n s e a l e v e l f o r t h e e u s t a t i c e f f e c t in t h e M e d i t e r r a n e a n 3,9 m m / y e a r t h e s u b s i d e n c e in P o r t o Corsini a n d 1,4 m m / y e a r t h e s u b s i d e n c e i n V e n i c e . RIASSUNTO. — Allo s c o p o di c o n t r i b u i r e allo s t u d i o d e l l a s u b s i d e n z a nel d e l t a p a d a n o e n e l l a l a g u n a v e n e t a a b b i a m o c o n f r o n t a t o i d a t i m a r e o g r a - fici di T r i e s t e , V e n e z i a e P o r t o Corsini nel p e r i o d o ( 1 8 9 6 - 1 9 6 6 ) e quelli di G e n o v a nei p e r i o d i (1884-1910) e (1928-1966). L ' a n a l i s i di t a l i d a t i m o s t r a u n a n d a m e n t o s e c o l a r e cui si s o v r a p p o n g o n o v a r i a z i o n i p e r i o d i c h e ; r i s u l t a i n o l t r e c h e i m o v i m e n t i del livello m e d i o del m a r e a P o r t o Corsini e a V e n e z i a d i f f e r i s c o n o n e t t a m e n t e d a quelli di G e n o v a e T r i e s t e c h e i n v e c e s o n o in o t t i m o a c c o r d o t r a di loro. A b b i a m o poi c a l c o l a t o le linee di r e g r e s s i o n e c h e si a d a t - t a n o meglio ai v a l o r i o s s e r v a t i : t a l i l i n e e r i s u l t a n o delle r e t t e p e r G e n o v a e T r i e s t e e delle c u r v e di I I g r a d o p e r P o r t o Corsini e V e n e z i a . I c o e f f i c i e n t i (*) T h i s w o r k h a s b e e n c a r r y o u t b y t h e c o n t r i b u t i o n of C . N . R . ( L a v o r o e s e g u i t o c o n il c o n t r i b u t o del C . N . R . ) . (**) I n s t i t u t e of G e o d e s y a n d T o p o g r a p h y U n i v e r s i t y of B o l o g n a ( I t a l y ) . 5 0 M . C A P U T O - L . P I E R I - F . R O S S I T E S I (li c o r r e l a z i o n e t r a i v a l o r i o r i g i n a l i e t r a q u e l l i c o r r e t t i p e r l ' a n d a m e n t o s e c o l a r e d i m o s t r a n o l ' o t t i m a c o r r e l a z i o n e t r a i d a t i r e g i s t r a t i i n t u t t e le l o c a l i t à . L ' a n a l i s i s p e t t r a l e , e f f e t t u a t a c o n il m e t o d o d i r e t t o d i F o u r i e r , m o s t r a c h i a r a m e n t e i s e g u e n t i p e r i o d i a n n u i p e r t u t t i i m a r e o g r a f i : 2 0 . 0 ; 1 2 . 5 ; 8 . 3 ; 0 . 7 ; 4 . 0 ; 3.4. Si c o n c l u d e o s s e r v a n d o c h e l ' i n n a l z a m e n t o a n n u o e u s t a t i c o d e l l i v e l l o m e d i o m a r i n o n e l M e d i t e r r a n e o r i s u l t a d i m m 1,4 n e l l ' u l - t i m o s e c o l o , m e n t r e l a s u b s i d e n z a a P o r t o C o r s i n i è di m m 3 , 9 e, a V e n e z i a , d i m m 1,4 a n n u i . 1 . - S T A T E M E N T O F T H E P R O B L E M An analysis of long time records of tide gauges located in differents stable areas should give t h e secular sea level variation. U n f o r t u n a t l y it is not easy to prove t h e stability of stations where mareographs are located. W e h a v e collected d a t a on four italian stations located in geolo- gically different areas covering a b o u t 70 years a n d a t t e m p t e d t o esti- m a t e t h e secular variation of t h e sea level a n d also t h e subsidence of some of the stations. To h a v e more accurate results t h e analyses of a larger n u m b e r of stations should be made; however we feel t h a t t h e d a t a t h a t we analyse give consistent and accurate results. 2 . - A V A I L A B L E D A T A D a t a are supplied by t i d e gauges in Trieste, Venice, P o r t o Corsini and Genoa, t h e y are yearly averages of observations carried o u t in t h e periods listed in Table 1. F i g 1 L A N D S U B S I D E N C E I N V E N I C E A N D P O R T O C O R S I N I (il Iii t h e period 1922-1034 in P o r t o Corsini occasional recordings were m a d e which allowed for a n interpolation t h a t covers the whole lapse of time 1896-1966. I t was impossible to perform the same oper- ation in Genoa, because t h e local tide gauge h a d been damaged in t h e course of various war events and remained inactive between 1911 and 1927. The tide gauges of Genoa and Trieste are located on solid rocks those of Venice and P o r t o Corsini are on thick sediments which are known to subside. Table 1 summarizes t h e periods for which we have information. T a b l e 1 P l a c e P e r i o d T r i e s t e 1896-196(5 V e n i c e 1 8 9 6 - 1 9 6 6 P o r t o C o r s i n i 1 8 9 6 - 1 9 6 6 G e n o a 1 8 8 4 - 1 9 1 0 1 9 2 8 - 1 9 6 6 3 . - DATA ANALYSIS. Pigs. 2, 3, 4, 5, concerning t h e recorded tide gauge d a t a , clearly show t h a t the m o v e m e n t of the m e a n a n n u a l sea levels in P o r t o Cor- sini a n d Venice greatly differs f r o m t h a t observed in Genoa and Trie- ste. A survey of t h e d a t a suggest t h a t in each s t a t i o n there is a secular t r e n d on which periodic oscillations are placed on. I n order to separate the periodic m o v e m e n t of the sea level and the component due to glacial eustatic effect f r o m t h e secular trend, we computed best fit polynomials of 1st degree for Genoa a n d Trieste and 2nd degree for P o r t o Corsini and Venice. The correlation coefficients between t h e raw d a t a and t h e d a t a af- ter subtracting t h e secular t r e n d are given in Tables 2 and 3. I t is 50 M . C A P U T O - L . P I E R I - F . R O S S I T E S I F i g . 3 - M e a n a n n u a l s e a l e v e l i n P o r t o C o r s i n i . L A N D S U B S I D E N C E I N V E N I C E A N D P O R T O C O R S I N I (il F i g . 4 - Mean a n n u a l sea level i n V e n i c e . 350 s-J1 1 1 , -g- J 1 ' ' 1 í 1 1 1 s 1 1 s 1 1 s 1 1 A 1 ' l — i F i g . 5 - M e a n a n n u a l sea level in G e n o v a . obvious t h a t the correlation is very significant a f t e r removal of t h e secular effect which proves the physical significant of the d a t a for t h e long periodic variations of t h e sea level. T a b l e 2 - C O R R E L A T I O N C O E F F I C I E N T S F O R T H E R A W D A T A P C V E G E T S P C 1 V E 0,92 1 — — G E 0,78 0,89 1 — T S 0,75 0,90 0,91 1 50 M. C A P U T O - L . P I E R I - F . R O S S I T E S I T a b l e 3 - C O E F F I C I E N T S O F C O R R E L A T I O N A F T E R S U B T R A C T I O N O F L I N E A R E F F E C T F O R T R I E S T E A N D G E N O A A N D O F 2 N D D E G R E E E F F E C T F O R V E N I C E A N D P O R T O C O R S I N I . P C V E G E T S P C 1 — — V E 0 , 8 3 1 — — G E 0 , 8 1 0 , 8 4 1 — T S 0 , 8 3 0 , 9 1 0 , 8 9 1 4 . - S P E C T R A L A N A L Y S I S . The power spectrum was also obtained, by F o u r i e r ' s direct m e t h o d . The plot of t h e power s p e c t r u m clearly show the following periods (in years) which are shared by all tide gauges: 2 0 . 0 1 2 . 5 8 . 3 6 . 7 5 . 6 4 . 5 4 . 0 3 . 4 T h e statistical t e s t of W a l k e r on t h e power spectrum show t h a t t h e peaks r e l a t e to period of 20.0 a n d 12.5 years are highly signifi- cance (97,5% confidence level); t h e others h a v e a confidence level lower t h a n 6 0 % . I t should be noted t h a t some periods related to t h e Genoa record- ings are slightly different f r o m those recorded by t h e other tide gauges, f r o m this it is p o i n t out also a period a t 25 years which does n o t appear in t h e other records. This difference is m o s t probably due to t h e different situation in t h e sea basins. Also t h e power spectrum concerning Genoa d a t a is based on a less complete time series. 5 . - S E C U L A R T R E N D A N A L Y S I S . As we mentioned t h e tide gauges in Venice a n d P o r t o Corsini are located on thick sediments in areas wich are known to subside. The linear secular t r e n d in Trieste a n d Genoa which are located in stable rock of geological different areas are: 1.40 m m / y e a r a n d 1.36 m m / y e a r in perfect agreement. This linear t r e n d is very p r o b a b l y due to eu- static sea rise. L A N D S U B S I D E N C E I N V E N I C E A N D P O R T O C O R S I N I ( i l S u b t r a c t i n g this trend f r o m the q u a d r a t i c secular t r e n d of P o r t o Corsini and Venice we should o b t a i n the subsidence in these stations; rrvqutncy ciclM/Vt*r Fi<. 6 we see t h a t t h e lowering of the land level during t h e last 70 years has had the average rate of 3.9 m m / y e a r in P o r t o Corsini and 1.4 m m / y e a r a t the tide ganges in Venice.