T h e B a b o l - K e n a r ( 1 9 7 1 ) e a r t h q u a k e a n d t h e s e i s m o t e c t o n i c s o f t h e C e n t r a l A l b o r z ( I r a n ) J . S . T C H A L E N K O * - M . H . I R A N M A N E S H * * - A . M O H A J E R - A S H J A I * * * R e c e i v e d o n D e c e m b e r 23, 1971 SUMMARY. — T h e B a b o l - K e n a r e a r t h q u a k e (9tli A u g u s t , 1971) w a s a r e l a t i v e l y m i n o r e v e n t c o m p a r e d t o p r e v i o u s c a t a s t r o p h i c e a r t h q u a k e s in t h e C e n t r a l A l b o r z m o u n t a i n s of n o r t h e r n I r a n . I t ' s s t u d y h o w e v e r , is i m p o r t a n t f o r t h e u n d e r s t a n d i n g of t h e s e i s m i c i t y of a r e g i o n w h i c h h a s a l r e a d y e x p e r i e n c e d t w o l a r g e e a r t h q u a k e s d u r i n g t h i s c e n t u r y a n d w h i c h c o u l d b e e n t e r i n g a p h a s e of r e n e w e d s e i s m i c a c t i v i t y . T h e p a t t e r n d e d u c e d f r o m t h e s e q u e n c e of all r e c o r d e d e a r t h q u a k e s in t h e r e g i o n o u t l i n e s t h e s t r u c t u r a l u n i t s of t h e C e n t r a l A l b o r z a n d s u g g e s t s t h e e x i s t e n c e of s e i s m o - t e c t o n i c p r o v i n c e s w h i c h a r e a l t e r n a t i v e l y a c t i v e a n d q u i e s c e n t . RIASSUNTO. — Il t e r r e m o t o di B a b o l - K e n a r (9 A g o s t o 1971) è s t a l o u n e v e n t o r e l a t i v a m e n t e di s c a r s o r i l i e v o s p e c i e s e r a f f r o n t a t o a t e r r e m o t i c a t a s t r o f i c i p r e c e d e n t i a v v e n u t i nelle m o n t a g n e d e l l ' A l b o r z Central© ( I r a n s e t t e n t r i o n a l e ) . Il s u o s t u d i o , t u t t a v i a , è i m p o r t a n t e p e r c o n o s c e r e la s i s m i c i t à di u n a r e g i o n e c h e ili q u e s t o secolo h a g i à s u b i t o d u e gl'alidi t e r r e - m o t i e c h e p o t r e b b e q u i n d i e n t r a r e in u n a f a s e di r i n n o v a t a a t t i v i t à s i s m i c a . Il m o d e l l o d e d o t t o d a l l a s e q u e n z a di t u t t i i t e r r e m o t i r e g i s t r a t i n e l l a r e g i o n e , t r a c c i a i c o n t o r n i d e l l e u n i t à s t r u t t u r a l i d e l l ' A l b o r z C e n t r a l e e s u g - g e r i s c e l ' e s i s t e n z a di p r o v i n c e s i s m o t e t t o n i c h e c h e s o n o a l t e r n a t i v a m e n t e a t t i v e e t r a n q u i l l e . 1 . - I N T R O D U C T I O N The Babol-Kenar earthquake of magnitude Mb = 5.2 occurred on the 9th August 1971 a t 02h 54m 36.7s about 150 km northeast of Tehran in the Caspian Province of Mazanderan. The instrumental * E n g i n e e r i n g S e i s m o l o g y , I m p e r i a l College, L o n d o n . ** G e o l o g i c a l S u r v e y of I r a n , T e h r a n . *** G e o p h y s i c s S e c t i o n , I m p e r i a l College, L o n d o n . F i g . 1 - B A B O L - K E N A R E P I C E N T R A L R E G I O N ( A p p r o x i m a t e S k e t c h ) . D a s h e d l i n e i n d i c a t e s a u t h o r ' s r o u t e . INSERT: Full circle, t o w n s w i t h s l i g h t d a m a g e . Open circle, t o w n s w h e r e s h o c k w a s s t r o n g l y f e l t . Shaded rectangle, B a b o l - K e n a r e p i c e n t r a l r e g i o n . Asterisk, i n s t r u m e n t a l e p i c e n t r e . A = A b - A i i ; B = B a b o l ; B h = B e h s h a r h ; B s = B a b o l s a r ; D = D a m a v a n d ; F = F i r u z k u h ; G = G o r g a n ; N = N o w s h a h r ; P = P o l S e f i d ; S = S a r i ; S g = S h i r g a h ; S h = S h a h i ; S I = S a n g c l i a l ; T = T e h r a n ; Z = Z i r a b . 28 J . S. T C H A L E N K O - M. H . I R A N M A N E S H - A . M O H A J E R - A S H J A I epicentre was calculated by NOAA to be 30.2 X 52.7 E a n d t h e focal d e p t h 27 km. The Geological Survey of I r a n sent a t e a m to t h e area to define precisely the macroseismic epicentral region a n d to assess t h e d a m a g e a n d t h e seismotectonic implications of t h e event. T h e region of m a x i m u m d a m a g e was f o u n d to be a t t h e n o r t h e r n m o s t limit of t h e Alborz foothills, south of t h e town of Babol a n d a t a b o u t 15 k m f r o m t h e i n s t r u m e n t a l epicentre (Figure 1). A b o u t 900 houses i n 42 villages suffered d a m a g e of some kind (generally light) a n d one person died and 39 were i n j u r e d . A s u m m a r y of t h e t e a m ' s visit t o t h e region and conclusions reached is given below. 2 . - D A M A G E : 2.1. General considerations. The e a r t h q u a k e felt over m o s t of N o r t h - c e n t r a l I r a n a n d t h e shock was reported as being strong in T e h r a n , Ab-Ali, D a m a v a n d , F i r u z k u h , Gorgan, Behshahr, Nowshahr, as well as in t h e towns nearer to t h e epicentre: Babol, Amol, Shalii, Babolsar, Shirgah, Zirab a n d Pol Selid (Figure 1). Occasional damage occurred in a fewr places a t a considerable distance f r o m t h e epicentre: a t F i r u z k u h (65 km), t h e Amir K a b i r School built of kiln-bricks was slightly d a m a g e d a n d one person was i n j u r e d ; a t N o w s h a h r (120 km), some brick houses devel- oped cracks a n d glass panes were broken; a t Ab-Ali (100 km), cracks a p p e a r e d in one wall of t h e casino. I n this last instance, no other d a m a g e could be found cither in Ab-Ali or in a n y of t h e surrounding villages, a n d it m u s t be assumed t h a t i n m a n y of these anomalous cases d a m a g e was due to severe weaknesses or Haws in t h e building or in i t s foundations. I n localities nearer to t h e epicentral region, slight d a m a g e was more general. I n t h e suburbs of Babol, a m i n a r e t collapsed (in Bid- Abad) a n d some rows of houses were d a m a g e d and glass panes s h a t t e r e d (in Bid-Abad and Raste-Bazar). I n Babolsar, one old building was destroyed, the roofs of two houses caved in a n d several houses develop - ed cracks. I n Shirgah, m o s t houses were cracked, a n d in t h e Zivla district near t h e P o s t Office some houses p a r t l y collapsed. I n Sangchal, epicentre of the e a r t h q u a k e of 2nd J u l y 1957 (see n e x t section, seismo- tectonics), a few houses were cracked by t h e r e a c t i v a t i o n of old landslides. F i g . 1 - B A B O L - K E N A R E P I C E N T R A L R E G I O N ( A p p r o x i m a t e S k e t c h ) . D a s h e d l i n e i n d i c a t e s a u t h o r ' s r o u t e . INSERT: Full circle, t o w n s w i t h s l i g h t d a m a g e . Open circle, t o w n s w h e r e s h o c k w a s s t r o n g l y f e l t . Shaded rectangle, B a b o l - K e n a r e p i c e n t r a l r e g i o n . Asterisk, i n s t r u m e n t a l e p i c e n t r e . A = A b - A l i ; B = B a b o l ; B h = B e l i s l i a r h ; B s = B a b o l s a r ; D = D a m a v a n d ; F = F i r u z k u h ; G = G o r g a n ; N = N o w s h a h r ; P = P o l S e f i d ; S = S a r i ; S g = S h i r g a h ; S h = S h a h i ; S I = S a n g c l i a l ; T = T e h r a n ; Z = Z i r a b . 3 0 J . S . T C I I A L E N K O - M . I I . I R A N M A K E S I I - A . M O I I A J E R - A S H J A I The severest d a m a g e a n d all casualties occurred in the B a b o l - K e n a r region a b o u t 25 k m south of B a b o l (Figure 1). Mere, w i t h i n a radius of a b o u t 7 km, m o s t of t h e houses in all t h e villages were affected to some degree, albeit m a n y only lightly. Buildings were also d a m a g e d in B a n d P e v to t h e west of B a b o l - K e n a r and along t h e Shahi-Pol Sefid road (see Appendix). The d a m a g e in B a b o l - K e n a r is described in more detail below. 2.2 Epicentral region. B a b o l - K e n a r is located a t t h e limit between t h e Caspian plain a n d t h e n o r t h e r n m o s t Alborz foothills. The villages d a m a g e d by t h e e a r t h q u a k e are s i t u a t e d on either side of t h e R u d - e Babol river (also k n o w n as N a h r - e Mianrud). As in other p a r t s of M a z a n d e r a n , t h e villages here h a v e a loosely-knit s t r u c t u r e of disconnected hamlets in t h e midst of rice a n d c o t t o n fields and jungle forests. The t r a d i t i o n a l houses, either built entirely of t i m b e r or of a timber f r a m e w o r k filled with m u d a n d plaster, h a v e o f t e n two storeys a n d nearly always t h a t - ched roofs. The i t i n e r a r y followed in Babol-Kenar is shown in F i g u r e 1. In t h e peripheric villages such as E h d a s h t , D a r u n Kola Sharglii a n d Rcis Kola, d a m a g e was s c a t t e r e d a n d slight. F o r example in D a r u n Kola Sharghi, some walls showed cracks b u t only one house (owner: F s f a n - d y a r Mahdari) p a r t l y collapsed due to a f o u n d a t i o n weakness. In Govan Kola, where people r e p o r t e d to h a v e been t h r o w n down by t h e shock (?), t h e local t i m b e r houses resisted b e t t e r t h a n t h e kiln- brick a n d m o r t a r constructions (Plate 1). Deraz K o l a a n d K e b r i a Kola are typical of t h e more severely d a m a g e d villages. I n Deraz Kola, 40 o u t of t h e 320 houses would require extensive repairs before re-occupation (Plate 2). There were however neither complete collapses, nor serious i n j u r i e d due to collaps- ing elements. People were n o t t h r o w n down. A well built brick a n d m o r t a r residence a b o u t 50 years old (and k n o w n as R e z a S h a h ' s Palace) resisted with only v e r y minor d a m a g e . I t s owner confirmed i n f o r m a t i o n g a t h e r e d in the surrounding villages, t h a t t h e last strong e a r t h q u a k e in the region was felt in 1935, a n d t h a t t h e 1957 e a r t h q u a k e of was much weaker (see section on seismotectonics). The d u r a t i o n of severe shaking in 1971 was e s t i m a t e d between 6 a n d 8 seconds f r o m inter- views reconstructing t h e m o v e m e n t s of observers d u r i n g t h e e v e n t . T H E B A B O L - K E N A R ( 1 9 7 1 ) E A R T H Q U A K E , E T C . .3.3 P l a t e 2 - L o c a l t y p e h o u s e i n D e r a z K o l a . All i n n e r a n d o u t e r w a l l s will h a v e t o b e r e b u i l t , b u t t h e t i m b e r f r a m e w o r k a n d roof will o n l y n e e d m i n o r r e p a i r s . P l a t e 1 - S h o p s i n Grovan K o l a . T h e s e k i l n - b r i c k a n d m o r t a r c o n - s t r u c t i o n s g e n e r a l l y s u f f e r e d m o r e t h a n t h e local t y p e h o u s e s . T H E B A B O L - K E N A R ( 1 9 7 1 ) E A R T H Q U A K E , E T C . .3.3 Tt is of interest to note t h a t t h e two-storey talar (*) s t r u c t u r e showed no sign of damage or strain by this e a r t h q u a k e b u t were reported by locals as swaying violently during t h e e a r t h q u a k e of 1935. Tn conclusions, local t i m b e r - f r a m e d houses were severely cracked only if they were old or if they h a d weak foundations. No complete collapse was seen a n d partially collapsed houses were the exception. Well-built local houses resisted with much less d a m a g e t h a n t h e few small kiln-brick a n d m o r t a r houses. No evidence was f o u n d for g r o u n d f r a c t u r i n g of a n y type. 3 . - SEISMOTECTONICS 3.1 Previous catastrophic earthquakes Two e a r t h q u a k e s are known to have caused wide-spread destruc- tion during this century in t h e Central Alborz. The first one (n° 1 in Fig. 2) occurred on t h e 11th April, 1935 and was assigned a magni- t u d e of 0.75 (4). Little is known of the resulting damage, except t h a t 27 villages were destroyed and a b o u t 400 people were killed. The second e a r t h q u a k e , of m a g n i t u d e 7.4 (n° 3 in Fig. 2), occurred on t h e 2nd J u l y , 1957 a n d caused a b o u t 1000 deaths. M a x i m u m d a m a g e was in t h e Sangchal region (Fig. 1) even though extensive destruction accentuated by landslides and rockfalls occurred along t h e Ileraz a n d Talar valleys. Both e a r t h q u a k e s were followed by strong aftershocks: in 1935 these were confined to the eastern p a r t of the range, whereas in .1957 t h e y nearly all took place in the west. A smaller shock a t t h e eastern limit of t h e area studied here (n° 2 in Fig. 2) d a m a g e d Gorgan on t h e 5th April, 1944, killing 20 people in the town a n d surrounding villages. 3.2 Sequence of earthquakes W h e n t h e chronology of e a r t h q u a k e s in the Central Alborz of m a g n i t u d e greater t h a n 4 is considered, t h r e e distinct regions can be delineated. F r o m 1932 (May) to 1944 (November), all e a r t h q u a k e s (*) C a g e - l i k e r o o m s s e t o n w o o d e n p i l e s a n d r a i s e d b y a b o u t 4 m e t r e s f r o m t h e g r o u n d . T h e y a r e u s e d i n S u m m e r a s s l e e p i n g q u a r t e r s . 34 J . S . T C H A L E N K O - M. I I . I R A N M A N E S H - A. M O U A J E R - A S I I J A I occurred on t h e n o r t h e r n (Caspian) flank a n d east of t h e Talar and H a b l e h R u d valleys. F r o m 1957 (May) to 1962 (August) e a r t h q u a k e s were also located on the n o r t h e r n flank, b u t west of t h e Talar-IIableh R u d valleys (*). F r o m 1962 (September) onwards, these two regions were quiescent, almost all events being confined to t h e s o u t h e r n (Teh- ran-Semnan) flank of t h e range. The t h r e e seismicity regions, N o r t h e a s t - ern, N o r t h w e s t e r n a n d S o u t h e r n , can be deduced f r o m Figure 2 which shows all the i n s t r u m e n t a l l y determined epicentres in t h e Central Alborz. T h e B a b o l - K e n a r e a r t h q u a k e of 1971 is t h e first event to diverge appreciably f r o m this p a t t e r n , [t occurred in t h e N o r t h w e s t e r n seis- micity region, whereas t h e great m a j o r i t y of shocks f r o m September 1962 u p to August 1971 were s i t u a t e d in t h e S o u t h e r n region. I t s i n s t r u m e n t a l epicentre (n° 4 in Fig. 2) is slightly north of the 1957 epicentre, t h e macroseismic. d a t a in 1957 as well ,as in 1971 being in a g r e e m e n t with these positions. E v e n t h o u g h t h e intensity a t t h e epicentre was f a r g r e a t e r in 1957 t h a n in 1971, t h e areas of m i n i m u m d a m a g e were similar, all d i s t a n t locations h a v i n g suffered minor d a m a g e in 1971 being within t h e isoseismal VI (MM) of the e a r t h q u a k e of 1957. 3.3 Seismotectonic provinces. The differentiation of a n o r t h e r n a n d s o u t h e r n region is a s t r u c t u r a l characteristic of t h e Alborz t h r o u g h o u t its geological history. A n early r a n g e existed already in t h e Paleogene, later developing into a barrier s e p a r a t i n g t h e Caspian depression in the n o r t h f r o m t h e Neogene basins of Central I r a n in t h e south (7'8). Different regimes of sedi- m e n t a t i o n a n d tectonic m o v e m e n t s on either side of this barrier are known for various geological periods, a n d are p e r h a p s m o s t striking for t h e Eocene-Oligocene time. Eocene sedimentation a n d volcanism producing t h e " G r e e n B e d s " or " K a r a j F o r m a t i o n ' ' are widespread and up to 400m thick on t h e s o u t h e r n flank of t h e Alborz, b u t entirely missing in the n o r t h . The n o r t h e r n limit of these Eocene f o r m a t i o n s coincides a p p r o x i m a t e l y with t h e n o r t h e r n limit of t h e S o u t h e r n seismicity region seen in t h e previous section (Fig. 2). Several m a j o r f a u l t zones r u n parallel to t h e general t r e n d of t h e range, subdividing it into s t r u c t u r a l regions (1.2.3.«); There are some indi- (*) F r o m N o v e m b e r 1944 t o M a y 1957, w i t h o n l y 2 e x c e p t i o n s , n o e a r t h q u a k e s w e r e r e c o r d e d f o r t h e r e g i o n u n d e r s t u d y . T H E B A B O L - K E N A R ( 1 9 7 1 ) E A R T H Q U A K E , E T C . .3.3 cations t h a t t h e s o u t h e r n M u s h a - F a s h a n f a u l t zone a n d t h e zone a t t h e n o r t h e r n border of the range m a y be active. W h e t h e r t h e y are f u n d a m e n t a l to t h e seismicity of the Central Alborz it is not possible to determine with t h e limited d a t a available a t present. P r o b a b l y of greater significance to t h e seismotectonics of t h e region is t h e sudden change in the s t r u c t u r a l directions f r o m N W - S E in t h e west to N E - S W in t h e east. The m a j o r tectonic event of the Q u a t e r n a r y , the creation of the D a m a v a n d Volcano, is located near this hinge, and t h e delinea- tion of a N o r t h e a s t e r n and a N o r t h w e s t e r n region on t h e basis of earth- quake sequence indicates t h a t a t present seismicity is also controlled by this change in s t r u c t u r a l directions. The correspondence between t h e seismicity seen in t h e previous section and t h e regional tectonics suggests t h a t t h e Northwestern, N o r t h e a s t e r n a n d Southern regions are seismotectonic provinces which, to a certain e x t e n t , act i n d e p e n d a n t l y one f r o m another. 3.4 Conclusions The s t r u c t u r a l subdivision recognized by geologists for the Central Alborz, i.e. t h e n o r t h e r n and southern flanks, are also seismicity units on t h e basis of the chronological sequence of recorded e a r t h q u a k e s . F u r t h e r m o r e , t h e larger e a r t h q u a k e s are concentrated in t h e region where the s t r u c t u r a l t r e n d changes a b r u p t l y f r o m N W to N E . The three seismotectonic provinces which can t h u s be defined, t h e North- eastern the Northwestern a n d t h e Southern Seismotectonic Provinces, have been, since a b o u t 1900, alternatively active a n d quiescent. The Babol-Kenar e a r t h q u a k e of t h e 9th August 1971, occurring in a province which has been quiescent since t h e last aftershocks of t h e e a r t h q u a k e of 1957, m a y signify t h a t the n o r t h e r n flanc of t h e Alborz is entering a new phase of seismic activity. A C K N O W L E D G E M E N T S The field work reported here was u n d e r t a k e n as p a r t of a joint project between t h e Geological Survey of I r a n and t h e Engineering Seismology section, Imperial College, London, t h e l a t t e r being support- ed for this work by a g r a n t f r o m t h e National E n v i r o n m e n t a l Research Council, London. 3 6 J . S . T C 1 1 A L E N K O - M . H . I R A N M A N E S H - A. M OH A J E R - A S H J AI R E F E R E N C E S (') ALLENBACH 1'., 1966. - Geologie und Petrographie des Damavand und seiner Umgebung (Zentral-Elburz), Iran. " M i t t . C e o l . I n s t . E i d g e n . T e c h . H o c h s h u k U n i v . Z u r i c h " , N . F . 6 3 , p p . 144. (2) ASSERETO R . , 1966. - Geological map of upper Djajerud and Lar Valleys (Central Elburz, Iran). I n s t . Geol. U n i v . M i l a n o , S e r i e G . , P u h l . , N o 2 3 2 , p p . 86. (3) GANSSER A. a n d IIUBER, H . , 1962. - Geological observations in the Central Elburz, Iran. " S c h w . M i n . i ' e t r . M i t t . " , 4 2 (2), p p . 5 8 3 - 6 3 0 . ( 4 ) G U T E N B E R G B . a n d R I C H T E R C . F . , 1 9 5 4 . - Seismicity of the Earth, 2 n d E d . P r i n c e t o n , N e w J e r s e y : P r i n c e t o n U n i v e r s i t y P r e s s . ( 5 ) N A T I O N A L O C E A N I C A N D A T M O S P H E R I C A D M I N I S T R A T I O N ( N O A A ) : Pre- liminary determination of epicentres. (6) STEIGER R . , 1966. - Die Geologie der West-Firuzkuh Area (Zentral Elburz, Iran.) " M i t t . Geol. I n s t . E i d g e n . T e c h . H o c h s c h u l e U n i v . Z u r i c h " , N . F . 6 8 , p p . 145. (7) STOCKLIN J . , 1968. - Structural history and tectonics of Iran. " A M . A s s o c . P e t r o l . G e o l . , Bull. " 5 2 (7), 1229-1258. (8) STOCKLIN J . a n d NABAVI M., 1968. - Summary of the geology of the Alborz Mountains and Iranian Azarbaijan. " I n t . Session o n T e c t . M a p s of E u r o p e , N e a r a n d M i d d l e E a s t , B a k u . 1968 " .