T e c t o n i c Stress Field in Central E u r o p e G. B A N A L I ^ - A . E . S C I I E I D E G G E R ( * ) K i c e v u t o il .6 M a r z o 1 9 6 7 SUMMARY. — S c l i e i d e g g e r ' s s t a t i s t i c a l m e t h o d f o r t h e t e c t o n i c i n t e r - p r e t a t i o n of e a r t h q u a k e f a u l t p l a n e s o l u t i o n s is a p p l i e d t o r e c e n t l y p u b l i s h e d s o l u t i o n s of e a r t h q u a k e s w h i c h o c c u r r e d i n t w o a r e a s of C e n t r a l E u r o p e . T h e r e s u l t s of t h e c a l c u l a t i o n s s h o w t h a t i n S o u t h w e s t e r n G e r m a n y , e a s t of t h e e a s t e r n b o r d e r of t h e R h i n o T r e n c h , t h e t e c t o n i c s t r e s s s t a t e is a t e n - s i o n a l o n e , w h i l e i n B e l g i u m i t s e e m s t o b o c o m p r e s s i o n a l . T h e P a n d T d i r e c t i o n s , h o w e v e r , a r e v e r y s i m i l a r i n b o t h r e g i o n s . RIASSUNTO. — I l m e t o d o s t a t i s t i c o d i S c h e i d e g g e r p o r l ' i n t e r p r e t a z i o n e t e t t o n i c a d e l m e c c a n i s m o d e i t e r r e m o t i a l l ' i p o c e n t r o ( " e a r t h q u a k e f a u l t p l a n e s o l u t i o n s " ) è q u i a p p l i c a t o a r e c e n t i i n d a g i n i di s i s m i o c c o r s i i n d u e d i f f e r e n t i r e g i o n i d e l l ' E u r o p a c e n t r a l e . I r i s u l t a t i d e i c a l c o l i m o s t r a n o c h e n e l l a G e r m a n i a s u d - o c c i d e n t a l e , a d E s t d e l b o r d o o r i e n t a l e d e l l a f o s s a r e n a - n a , l o s t a t o t e t t o n i c o p r e d o m i n a n t e è a c a r a t t e r e t e n s i o n a l e , m e n t r e i n B e l g i o s e m b r a e s s e r e a c a r a t t e r e c o m p r e s s i o n a l e . T u t t a v i a le d i r e z i o n i d e l l e c o m - p r e s s i o n i e d e l l e t e n s i o n i s o n o m o l t o s i m i l i i n a m b e d u e l e r e g i o n i c o n s i d e r a t e . I N T R O D U C T I O N . I t is a well known f a c t t h a t t h e crust of t h e E a r t h is subject to a " tectonic " stress s t a t e which m a y v a r y in its c h a r a c t e r f r o m region t o region. I t r e m a i n s one of t h e more i m p o r t a n t problems in geophy- sics t o ascertain t h e precise n a t u r e of this stress s t a t e inasmuch as it is n o t easily possible to measure u n d e r g r o u n d stresses b y direct means. Thus, m o s t of our knowledge of tectonic stresses has come f r o m indi- rect evidence, such as analyses of surface f a u l t p a t t e r n s , etc. (*) U n i v e r s i t y of I l l i n o i s , U r b a n a 1 9 4 G. R A N A L L I - A . E . S C I I E I D E G G E R One of the most promising alleys for investigating t h e regional tectonic stresses is based on seismic evidence. A statistical analysis of f a u l t plane solutions of e a r t h q u a k e s yields directly t h e direction of greatest compression (P-axis), t h e direction of least compression (T- axis) a n d t h e axis of relative motion (normal t o ' t h e i n t e r m e d i a t e principal stress |\B] axis) in a n y one a r e a (see Sclieidegger, °). This statistical t r e a t m e n t of e a r t h q u a k e f a u l t p l a n e solutions has already given i n f o r m a t i o n a b o u t t h e tectonic stress s t a t e in various areas of t h e world [Scheidegger (9), (10), ( n ) , (12), (13)]. Recently, new sets of f a u l t plane solutions h a v e been calculated. These news solutions available m a k e now possible a detailed s t u d y of some region previously n o t v e r y A v e l l k n o w n f r o m t h e p o i n t of view of seismotecto- nics. I n this instance, t h e present p a p e r deals with t w o areas of c e n t r a l E u r o p e n o r t h of the Alps, n a m e l y S o u t h w e s t e r n G e r m a n y a n d Belgium. As to Southwestern G e r m a n y , eleven e a r t h q u a k e s which h a v e oc- curred in t h e Southern Black F o r e s t , in t h e southwestern p a r t of t h e Swabian J u r a , a n d in t h e western p a r t of t h e Molasse Basin h a v e been studied. The region in question is roughly couflned between 47°-49° if and 7°-10° E . The references in t h e l i t e r a t u r e h a v e been given b y H o r d e j u k (8), Hiller (5), (6), a n d Schneider, Schick a n d Berck- hemer ("). The directions of t h e P , T, a n d B axes h a v e been cal- culated for each one of these e a r t h q u a k e s . The f a u l t p l a n e solutions are listed in Table I ¡in t h e s t a n d a r d f o r m 'employed b y F a r a (4). The k e y to t h e l i t e r a t u r e references is as follows: 1: H o r d e j u k (8); 2,3: Hiller (5), (6); 4,5,6,7,8,9,10,11: Schneider, Schick a n d Berckhemer ( u ) . 'For t h e first t h r e e e a r t h q u a k e s , planes I a n d I I h a v e also been calcu- l a t e d . I n seven o u t of t h e eight e a r t h q u a k e s studied b y Schneider, Schick a n d Berckhemer (14), t h e B axis has b e e n a s s u m e d to be ver- tical, a n d accordingly, t h e P a n d T axes h a v e been assumed to be ho- rizontal, since t h e A u t h o r s give no i n f o r m a t i o n a b o u t t h e dip of t h e B axis, b u t only t h e projections of t h e t w o nodal lines on t h e surface of t h e E a r t h a n d t h e horizontal directions of t h e P and T axes. Some- times t h e nodal lines are n o t perpendicular t o each o t h e r ; this can be explained b y a slight t i l t of t h e force system. A n o t h e r case in which t h e n o d a l lines were n o t perpendicular has been studied b y Caloi (2). This e a r t h q u a k e occurred on October 18, 1936, a n d its epicenter was located n e a r t h e t o w n of Belluno, in n o r t h - e a s t e r n I t a l y ; its epicentral T a b l e I - F A U L T P L A N E S O L U T I O N S O F E A R T H Q U A K E S I N S O U T H E R N G E R M A N Y D a t e E p i c e n t e r P T B P l a n e I P l a n e I I N o . 1 6 . 1 1 . 1 1 4 8 . 3 N 0 0 9 . I E 00 N 7 5 W 3 8 S 6 0 E 5 1 N 2 1 E 0 7 N 6 8 W 8 4 N 6 3 E 10 *D 1 1 1 0 8 . 0 2 . 3 3 4 8 . 8 N 0 0 8 . 2 E 00 N 2 5 W 0 0 N 6 5 E 0 0 N 0 0 E 9 0 S 7 0 E 9 0 N 2 0 E 90 S 2 2 7 . 0 6 . 3 5 4 8 . I N 0 0 9 . 4 E 00 S 0 5 E 0 0 N 8 5 E 0 0 N 0 0 E 9 0 N 5 0 W 9 0 N 6 1 E 90 S 0 1 3 0 2 . 0 5 . 4 3 4 8 . 3 N 0 0 9 . 0 E 00 N 4 0 W 0 0 N 4 6 E 0 0 N 0 0 E 9 0 4 2 8 . 0 3 . 6 0 4 8 . 3 N 0 0 9 . 0 E 00 N 4 3 W 0 0 N 4 3 E 0 0 N 0 0 E 9 0 5 19.04.61 4 8 . I N 0 0 9 . 0 E 00 N 2 8 W 0 0 N 6 3 E 0 0 N 0 0 E 9 0 6 2 8 . 0 4 . 6 1 4 7 . 7 N 0 0 7 . 9 E 00 N 3 8 W 0 0 N 5 1 E 0 0 N 0 0 E 9 0 7 2 7 . 0 3 . 6 5 4 8 . I N 0 0 9 . 5 E 00 N 0 2 W 0 0 N 8 8 E 0 0 N 0 0 E 9 0 8 3 0 . 0 3 . 6 5 4 8 . I N 0 0 9 . 5 E 00 N 0 3 W 0 0 N 8 7 E 0 0 N 0 0 E 9 0 9 2 5 . 0 5 . 6 5 4 8 . I N 0 0 9 . 5 E 00 NO 1 WOO N 8 9 E 0 0 N 0 0 E 9 0 10 19.09.65 4 7 . 9 N 0 0 8 . 2 E 00 S 2 3 W 1 0 N 2 3 E 8 0 S 6 7 E 0 0 11 I!)f> G . K A N A L L I A. E . S C H E I D E G G E l l coordinates are a p p r o x i m a t e l y 46° N a n d 12° E . I t is interesting to n o t e t h a t t h e horizontal projections of t h e P a n d T axes are N 29° W and N 61° E respectively, i.e. v e r y similar to those of southwestern G e r m a n y a n d Belgium. I n e a r t h q u a k e (11) t h e B axis is horizontal, a n d t h e dips of t h e P a n d T axes h a v e been calculated on t h e basis of a m e t h o d proposed b y Holler (7). F i g . 1 - P - d i r e c t i o n s f o r e a r t h q u a k e s i n S o u t h w e s t e r n G e r m a n y . Fig. 1 shows t h e P directions for each of these e a r t h q u a k e s which h a v e occurred in S o u t h w e s t e r n G e r m a n y . T h e second region considered is Belgium. The a r e a u n d e r consi- deration is roughly confined between 50°-52° N a n d 4 ° - 6 ° E . The d a t a h a v e been calculated f r o m i n f o r m a t i o n given in p a p e r s b y Ahorner and V a n Gils (*), a n d V a n Gils (16). Three e a r t h q u a k e s h a v e been t a k e n into account, b u t , since in t h e e a r t h q u a k e of December 15, 1965, t h e foreshock and t h e m a i n shock showed a completely reversed ground motion direction, f o u r cases are listed in Table I I . The k e y to litera- t u r e references is: 12: Ahorner & V a n Gils (x); 13,14,15: V a n Gils (16). Cases listed as (13) a n d (.14) refer respectively t o t h e foreshock and t o t h e m a i n shock of t h e above m e n t i o n e d e a r t h q u a k e of Decem- ber 15, 1965. F o r e a r t h q u a k e (12), planes I a n d I I h a v e also been cal- culated. The B axis has been assumed t o be vertical in (13), (14), V T a b l e 11 - F A U L T P L A N E S O L U T I O N S O F E A R T H Q U A K E S I N B E L G I U M . D a t e E p i c e n t e r P T B P l a n e I P l a n e I | N o . 2 5 . 0 6 . 6 0 5 1 . 2 N 0 0 5 . 7 E 00 4.2 N 0 0 E 0 0 N 9 0 E 3 2 N 9 0 W 5 8 N 9 0 E 77 NOOE 00 * 12 1 5 . 1 2 . 6 5 5 0 . 2 N 0 0 4 . 0 E 00 4.4 N 57 WOO N 3 3 E 0 0 N 0 0 E 9 0 13 15.12.65 5 0 . 2 N 0 0 4 . 0 E 00 4.4 N 3 3 E 0 0 N 5 7 W 0 0 N 0 0 E 9 0 14 2 1 . 1 2 . 6 5 5 0 . 4 N 0 0 5 . 3 E 00 4.4 N 6 6 W 0 0 N 2 4 E 0 0 N 0 0 E 9 0 15 198 G. R A N A L L I - A . E . S C I I E I D E G G E R (15). Since these solutions are based upon recordings f r o m Belgian stations only, t h e y have no exaggerate claim of accuracy. Fig. 2 shows t h e P directions for each case. T R E A T M E N T OF DATA . The f a u l t plane solutions listed in Tables 1 a n d 2 have been t r e a t - ed b y a statistical m e t h o d developed b y Scheidegger (9); t h e best f i t t i n g P, T, a n d P - n o r m a l axes h a v e been calculated b y an eigen- value t e c h n i q u e w i t h t h e help of t h e University of Illinois I B M 7094- 1401 Computer System. The P - n o r m a l axis has the sense of direction of m o t i o n ; if it is more or less in t h e same direction of t h e best P axis, t h e tectonic area is a compressional one, in t h e other case, we are deal- ing with a tensional tectonic region. T a b l e 111 - R E S U L T S O F C A L C U L A T I O N S . A r e a N o . of Q u a k e s B e s t P B e s t T M o t i o n a x i s S o u t h w e s t e r n G e r m a n y B e l g i u m 11 4 N 1 9 W 0 1 N 3 3 W 0 0 N 7 0 E 0 4 N 5 1 E 10 S 6 0 W 0 1 N 0 0 E 0 0 T E C T O N I C S T R E S S F I E L D I N C E N T R A L E U R O P E 109 The results of t h e statistical c a l c u l a t i o n s are shown in Table I I I . The m e a n stress systems in Southerwestern G e r m a n y a n d Belgium are p l o t t e d in Fig. 3. B E S U L T S . W e shall consider separately t h e two geographical areas under con- sideration. I n Southwestern G e r m a n y , all e a r t h q u a k e s under consi- deration h a v e occurred in t h e region immediately east of t h e eastern border of t h e B h i n e Trench, namely in intensely f a u l t e d areas (as t h e Southern Black Forest, a n d t h e western p a r t of t h e Swabian J u r a near t h e Hohenzollern Trench). I n d i v i d u a l f a u l t plane solutions show t h a t b o t h strike-slip and dip-slip motions are present in this zone [Schneider, Schick a n d Berckhemer (")]. The result of the statistical calculations points out t h a t the stress system is a tensional one (the motion axis nearly coincides A v i t h t h e best fitting T axis). This is n o t unexpected in a border zone of a trench, a n d therefore this result is consistent with w h a t one m a y reasonably infer f r o m p u r e l y geological considerations. As regards t h e Belgian region, it is to be again pointed out t h a t t h e f a u l t plane solutions are based on d a t a f r o m few seismic stations, 2 0 0 G . R A N A L L I - A. E . S C I I E I D E G G E R a n d , moreover, t h a t t h e n u m b e r of e a r t h q u a k e s is v e r y small. A n y w a y , the result indicates t h a t t h e area is one of compressional tectonic stress (the m o t i o n axis is closer to t h e P axis t h a n t o t h e T axis). An interesting f e a t u r e is observed in t h e f a c t t h a t in b o t h , South- western G e r m a n y and Belgium, t h e P a n d T directions are almost iden- tical. This seems to indicate t h a t t h e tectonic stress field is more or less t h e same in all of central E u r o p e n o r t h of t h e Alps, a n d t h a t it is only t h e i n t e n s i t y (i.e. t h e predominance) of t h e P or T direction t h a t is different. Very recently, new light has been b r o u g h t , f r o m seismic evidence, into t h e tectonic f e a t u r e s of t h e M e d i t e r r a n e a n region [Constantinescu, R u p r e c h t o v a , and Enescu (3)], a n d into t h e present stress s t a t e of t h e Alpine r a n g e [Scheidegger (")]. The present s t u d y complements these other investigations with regard to t h e remaining p a r t of E u r o p e . A C K N O W L E D G M E N T S . T h e calculations r e p o r t e d here were performed on t h e University of Illinois I B M 7094-1401 c o m p u t i n g system operating u n d e r a g r a n t f r o m t h e N a t i o n a l Science F o u n d a t i o n . The writers are i n d e b t e d to Professor B e r c k h c m e r of F r a n k f u r t for l e t t i n g t h e m h a v e a copy of t h e p a p e r 011 f a u l t plane solutions of e a r t h q u a k e s in B a d e n - W ü r t t e m - berg prior to its publication. R E F E R E N C E S (1) AHOBNER L . , VAN GILS J . M . , Das Erdbeben vom 25 Juni I960 im Belgisch- nederländischen Grenzgebiet. A c a d . R o y a l e d e B e l g i q u e , " B u l l . Cl. S c i e n c e s " , S e r i e G e o p l i y s . , 6 7 , ( 1 9 6 4 ) . (2) CALOI P . , Bicerche su terremoti ad origine vicina. Scosse del Cansiglio del- l'Ottobre 1930. " L a R i c e r c a S c i e n t i f i c a " , 9 , 2, 1 - 4 1 , ( 1 9 3 8 ) . (:L) CONSTANTINESCU L . , R U P R E C H T O V A L . , E N E S C U U . , Mediterranean alpine earthquake mechanisms and their seismotectonic implications, " G e o p l i . J . R . A s t r . S o c . " , 1 0 , 3 4 7 - 3 6 8 , ( 1 9 6 6 ) . (') FARA H . D . , A new catalogue oj earthquake jault plane solutions, " B u l l . S e i s m . S o c . A m . " , 5 4 , 1 4 9 1 - 1 5 1 7 , ( 1 9 6 4 ) : (6) HILLER W . , Der Herd des Bastatter Bebens am S Februar 1933, " G e r ì . B e i t r . z u r G e o p l i . " , 4 1 , 2, 1 7 0 - 1 8 0 , ( 1 9 3 4 ) . T E C T O N I C S T R E S S F I E L D I N C E N T R A L E U R O P E 201 (6) HILLER W . , Das oberschwäbische Erdbeben am 27. Juni 1935. " W ü r t t . J b . f ü r S t a t i s t i k u . L a n d e s k d e " , J n g . 1 9 3 4 / 3 5 , 2 0 9 - 2 2 6 , ( 1 9 3 6 a ) . (7) IIILLER W . , Das Erdbeben im nördlichen Schicarzwald am 30. Dezember 1935, " S e i s m . B e r . W ü r t t b g . E r d b e b e n w " . J g . 1 9 3 5 , (1936ft). (8) HORDEJUK J . , Charakterystyka dynamiczna ogniska Trzesienia ziemi z dnia 1(1. 11. 1911, " A c t a g e o p l i . P o l o n i c a " , 5, 2, 103-109, (1957). (") SCHEIDEGGER A. E . , The tectonic stress and tectonic motion direction in Europe and Western Asia as calculated from earthquake fault plane solutions, " B u l l . S e i s m . S o c . A m . " , 5 4 , 1519-1528, (1964). (10) SCIIEIDEGGER A. E . , The tectonic stress and tectonic motion direction in the Pacific and adjacent areas as calculated from earthquake fault plane solu- tions, " B u l l . S e i s m . Soc. A m . " , 5 5 , 147-152, ( 1 9 6 5 a ) . (11) SCIIEIDEGGER A. E . , Grosstektonische Bedeutung von Erdbebenherdmechani- smen, " Z. G e o p l i . " , 3 1 , 300-312, (19656). (12) SCIIEIDEGGER A . E . , Tectonics of the artic seismic belt in the light of faultplane solutions of earthquakes, " B u l l . S e i s m . S o c . A m . " , 5 6 , 241- 2 4 5 , (1966). (13) SCIIEIDEGGER A. E . , The tectonic stress in the vicinity of the Alps, " Z. G e o p h y s i k " 3 3 , 167-181, (1967). (1 4) S C H N E I D E R G . , SCHICK R . , B E R C K I I E M E R I L , Faultplane solutions of earth- quakes in Baden-Württemberg, " Z. G e o p h y s i k " , 3 2 , 383-393, (1966). (15) VAN GILS J . M., Les tremblements de terre des 15 et 21 Décembre 1965 en Belgique. A c a d . R o y a l e d e B e l g i q u e . " B u l l . Cl. S c i e n c e s " , S é r i e G e o p h . 7 4 , 101-107, (1966).