D e l e r m i n a t i o n of earthquake m a g n i t u d e and intensity in the USSR E . E . S A V A R E N S K Y The determinatimi of earthquake magnitude in the Soviet Union is based on instrumentai observations using t h e international magni- t u d e scale. The estimations of M for distant earthquakes usually involve ob- servations of amplitudes and periods a t the m a x i m u m phase of surface waves. To simplify the work seismic stations use nomograms (fig. 1, N. V. Sliebalin and S. L. Solovyov f6'1')). In esthnating t h e seismic energy for near and weak earthquakes recorded b y higlily sensitive i n s t r u m e n t s t h e Galitzin method (') is used. Such estimates for locai earthquakes are made a t t h e I n s t i t u t e of Seismology of t h e Academy of Sciences of Tadjikskaya SSE (V. I. Bune (9'10)). Work is proceeding to improve methods of determining M from observations of body waves as well as to find regional corrections of M depending on t h e location of e a r t h q u a k e epicentres (S. L. Solovyov (MM'.IMI)). Devices recor- ding energy flux of earthquakes are now being designed which will m a k e M determinations more rehable. I n determining instrumentai magnitude M in the Soviet Union the scientists do not confine themselves to t h e instrumentai seismic investigations and to t h e connection of these with tectonic. processes (E. E. Savarensky ("•8' u- 15 •20)) b u t consider t h e practical problem of connecting t h e intensity near the epicentre with t h e magnitude M. This is i m p o r t a n t for earthquake-proof construction especiaUy in cases when forni erly non-inhabitant regions of seismic activity are considered. The first stage of this problem deals witli t h e determination of t h e connection between t h e intensity I0, t h e magnitude M and t h e focal depth H . This dependance was found by interpreting a few hundreds of earthquakes and comparing t h e intensity a t t h e epicentre with t h e magnitude M. Finally t h e following empirical formula was deduced: I„ = 1.5 M — 3.5 lgE + 3 . D E T E R M I N A T I C I OF E A R T H Q U A K E M A G N I T U D E , ETC. 3 7 1 The formula is t r u e for earthquakes with focal depths of no more t h a n 60 k m . A t greater depths the influence of a layer with a lowered velocity (asthenosphere) is being exerted. (N. V. Shebalin (6>l9'22)). The determination of I0 by M is seen from the above nomogram (fig. 1). The estimates assume t h a t average properties between t h e focus and surface of the E a r t h are obtained everywhere. I n conclusion mention is made on t h e systematization and catalo- guing of earthquakes by magnitude. W h e n compihng seismicity maps of respective zones of t h e USSR (E. E. Savarensky (20)) according to their magnitude ali earthquakes are divided into 5 groups: G r o u p M a n d Io - - i n t e n s i t y ( a t n o r m a l f o c a l d e p t h s ) D i s t a n c e of r e g i s t r a t i o n I 7 1/2 9 a n d m o r e r e c o r d e d b y t h e m o s t s t a t i o n s of t h e w o r l d I I 6.5-7.5 8-9 I I I 5 1/4-6.5 ca. 7 60° a n d m o r e I V 4 1/4-6 1/4 — 20-60° Y 4 1/4 — u p t o 20° Thus, the first three groups are based on practical considerations. To some e x t e n t t h e y include destructive earthquakes considered to be t h e most violent in the USSE. The lower groups include weak earth- quakes which are of interest for a general study of seismicity. I n some cases M is determined from the distance of registration. The catalogues a t t a c h e d to t h e seismicity maps are compded according to t h e f o r m : D a t e T i m e of E p i c e n t r e F o c a l M a g n i - M D i s t a n c e N o t e s e a r t h - c o o r d i n a - d e p t h t u d e of r e g i s t r a - ( I n f o r - q u a k e t e s d e p t h g r o u p t i o n a n d m a t i o n h . r n . s . n a m e of t h e on i n t e n - m o s t d i s t a n t s i t y s t a t i o n . A c c u r a c y c l a s s : A - e p i c e n t r a l e r r o r is less t h a n 25 k m . , B - le ss t h a n 50 k m . 3 7 2 E . F . S A V A R E N S K Y To m a k e a more accurate determination of I 0 b y M it is necessary to t a k e into account t h e influence of t h e locai geologie s t r u c t u r e in the epicentral region upon t h e a m p h t u d e of oscillations. Tliis influence is witnesses b y t h e results of numerous investigations of destruction caused b y earthquakes. The main principal problem is t o determine t h e interval of periods in which we m u s t t a k e into account t h e influence of t h e layer of comparatively sedimentary rocks. L e t us m a k e b u t a simple estimate of t h e effect of direct f r o m any direction SE waves and vertical P waves. Observations show t h a t such waves are most dangerous. L e t us consider piane waves of u n i t y a m p h t u d e . I t sliould be re- called t h a t a t the surface of a semi-space t h e amplitude of the oscillations considered is equal to 2. If a layer with thickness I I is situated on this surface, then, as Eayleigh showed, waves coming f r o m t h e semi-space a n d reflected m a n y times in the layer will interfere. F o r P waves t h e amplitude magniflcation takes place each time when t h e ratio of the layer thickness to t h e wavelengtli is multiple of the odd n u m b e r of quarters. A t these ratios the surface amplitude is m equal to 2 — » where m1 and m„ are the acoustic rigidity of t h e layer and of the semi-space. Witli t h e wavelength X considerably exceeding t h e layer thickness J J H , when fi = 2 n is small, t h e a m p h t u d e of oscillations at. t h e A layer surface will be near to 2 and to t h e influence of t h e layer can be neglected. This error will be determined by t h e difference: sin /3 — /S Thus, t h e m a x i m u m value of t h e period in the interval is determined. W e sha.ll f u r t h e r t a k e into account t h e absorption of t h e energy oscillations b y t h e layer assuming t h a t t h e absorption is inversly propor- tional to the wavelength or t o t h e period, which f a c t is conflrmed b y observations. I n this case t h e a m p h t u d e changes with distance Z b y t h e formula: —2jcx» —— e A where is t h e damping coefficient and t h e a m p h t u d e decrease with t h e increase of > or with t h e reduction of the wavelength. This damping A lowers resonance m a x i m a a t t h e layer surface. D E T E R M I N A T I C I OF E A R T H Q U A K E M A G N I T U D E , E T C . 3 7 3 Hence it follows t h a t a t sufficiently large values of t h e absorption will be so great t h a t the layer eft'ect will not be i m p o r t a n t . These d a t a help to find the minimum value of the period in t h e interval. F i g . 2 To calculate a m p h t u d e s on a layer surface (dry loess sandy soil, etc.) a 1 k m deep, lying on a granite semispace and where ratio of acoustic 771 rigidities — — = 10 and damping coefficient = 0.1 (E. Y. K a r u s (23,)) 7U1 t h e influence of t h e layer should be taken into consideration within t h e interval of periods from 0.5 to 7 seconds fig. 2). ABSTRACT The determination of earthquake magnitude in the Soviet Union is based on instrumentai onservations under the international magnitude scale. The dependence between intensity gradation B, magnitude M and focal depth H was found by interpreting a few hundreds earthquakes: B = 1.5 M — 3.5 Ig H + 3. 3 7 4 E . F . S A V A R E N S K Y The formula is true for earthquakes with focal depths of no more than 60 km (N. V. Shebalin). The estimates assume that medium properties between the focus and surface of the Earth are everywhere equal. To make a more accurate determination of B it is necessary to take into account the influence of the locai geological structure of the epicentral region upon the amplitude of oscillations. Compiling seismicity maps of respective zones of the USSR according to their magnitude ali earthquakes are divided into five groups: ^ 7 l / 2 ; 6 V.-7 7«i 5 1/4-6 I/2; 4 74-5 V«5 < 4 7«. In the published quarter bulletins of the seismic stations of the USSR M is given to within 1li and at M > 4. RIASSUNTO La determinazione della magnitudo dei terremoti nella Unione Sovie- tica è basala su osservazioni strumentali in base alla scala di magnitudo internazionale. La relazione tra gradazione B di intensità, magnitudo M e profondità focale E è stata determinata interpretando alcune centinaia di terremoti: B = 1.5 M — 3.5 Ig M + 3 . La formula è vera per terremoti con profondità focali non superiori a 60 km (N. V. Shebalin). Si presume che le caratteristiche del mezzo tra il fuoco e la superficie della terra siano ovunque eguali. Per effettuare una più precisa determinazione di B e necessario tener presente V'influenza della struttura geologica locale della regione epicentrica, rispetto all'ampiezza delle oscillazioni. Preparando le mappe della sismicità delle rispettive zone dell'U.R.S.S. secondo la loro magnitudo, tutti i terremoti sono stati suddivisi in cinque gruppi: > 7 V.; 6 V2-7 7.5 51/4-6 7 4 7 4 -5 y 4 ; < 4 7 4 . Nel bollettino trimestrale pubblicato dalle stazioni sismiche dell' U.S.S.R. M è dato entro l'intervallo compreso fra M — 1/i e M ^ 4. D E T E R M I N A T I C I O F E A R T H Q U A K E M A G N I T U D E , E T C . 3 7 5 R E F E R E N C E S ( ! ) ROJINQI:,IH B . E . , O 3CM.iemp/iceHuu 18 (fieepajm 1911 iod, H 3 B . H M N . A H , 9 9 1 - 9 9 8 , (1915)- ( 2 ) KOCEHKO C. H., K eonpocy o eumicnenuu snepeuu 3eMAempHcemiù. T p . reotJjHaH. 2 1 , 148, 3 - 1 5 ( 1 9 5 3 ) . ( 3 ) COJIOBBEB C . J I . , O K/iaccutfiuxatfuu seMAempnceuuù no ecAimuiie ux auepzmi. Tp. reoiJjHaH 30, 157, 3 - 2 1 , ( 1 9 5 5 ) . ( 4 ) CABAPEHCKHH E . O . , CeùcMUHHOcmb CCCP, limoni u nepcnenmuBU ee tayueiiun. Tp. reoiJ)HaH 25, 152, 5 - 2 4 , ( 1 9 5 4 ) . ( 5 ) BEUIHHKOB H . B . , O KOAUHecmeeHHOù oiieHKe CUAU 3eMAempHceuuH. H 3 B . A H C C C P , c e p . r e o t J ) H 3 . , 3 , 2 0 7 - 2 1 4 , ( 1 9 5 5 ) . ( 6 ) IHEBAJIHH H . B., O C8H3U MeDicdy snepeueù, óaAAbHOcmbw u lAyóunoù ovaia 3eMAempn- ceHUù. II3B. AH CCCP, cep. reo(J)H3. 4, 3 7 7 - 3 8 0 , ( 1 9 5 5 ) . ( 7 ) COJIOBBEB C . JI. H XL>KHEJIAFL3E 3 . A . , Oó u3Menenuu nomona ceucMuieCKOÙ ouepmu c paccmoHHueM. H3B. AH CCCP, cep. reocJ>H3. 5, 4 6 2 - 4 6 3 , ( 1 9 5 5 ) . ( 8 ) CABAPEHCKHH E . O . , PaBoma ceùcMunecKux cmamtuù u 3aòanu coeemcKOu ceùcMO- AOZUU. BecTHHK AH CCCP, 5, 4 0 - 4 3 , ( 1 9 5 5 ) . (9) EyH3 B. H . , O KAacctii/ìuKaijuu 3eMAempncemiù no SHepiuu ynpyiux BOAH, u3Ayuae- Mbix U3 ouaea. ,H,OKJI. AH T a f l » . CCP, 2 4 , ( 1 9 5 5 ) . (10) — O KAaccui/iuKaijuu 3eMAempnceHuu no ux cuAe Ha ocnoee UHcmpyMeumaAbHbix dauHbix. H 3 B . A H CCCP, cep. reotJ)H3. 1, 4 8 - 5 5 , ( 1 9 5 6 ) . ( U ) CABAPEHCKHH E . O . H J1>KHEJ7AA3E 3 . A . , O ceUcMunnocmu Eojibiuozo KaeiK. CCCP, 19, 3 - 1 2 , ( 1 9 5 6 ) . (13) — 06 ucnoAb3oeaHuu Memoda roAuifbina òah npuÓAUOiceHHOù oi\eHKU snepeuu6AU3Kux 3eMAempHceuuù. T p . M H C T . ceHCMOJlonra A H Ta,T>K. C C C P 1, 3 - 2 7 , ( 1 9 5 6 ) . ( 1 4 ) COJIOBBEB, C . J I . — O C6H3U suepiuu ti uwneHCueHocmu seMAemp/iceiiuii. H 3 B . A H CCCP, cep. reoc{>H3. 3, 3 5 7 - 3 5 9 , ( 1 9 5 6 ) . ( 1 5 ) CABAPEHCKHH E . ., OS ucKaoicenunx e xapmax ceucMunuocmu. H 3 B . A H C C C P , cep. reo(J)H3. 7, 745-754) ,1956). ( 1 6 ) KOHFLOPCKAH H . B. H n O C T O J I E H K O T. A . , CeÙCMUHHOCmb CCCP 3a 1955. H3B. A H CCCP, cep. reo., 06 iisynenuu ceùCMUHHoemu CCCP. ERONNETEHB C o B e T a n o CEHCMOJIORHH A H C C C P , 6 , 1 6 - 1 9 , ( 1 9 5 7 ) - ( 2 1 ) C o J i O B b E B C . J I . , HeKOtnopbie pe3yAbmambi npuMenenun luna/iu unmencuanocmu 3eMAempHceHUÙ Ha ceùCMUHecKUX cmanifUHX CCCP. G r y f l H H reo(J)H3HKa H MeTO- FLHKA ^ E X O C J I O B A I I K O H A H , 2 , 4 0 - 4 5 , ( 1 9 5 8 ) . ( 2 2 ) IIIEBAJIHH H . B . , HcnoAb3oeahue coomitouienun Meoicdy unmehcu6n0cmbw u óaAAtHO- cttibio 3eMAempHceHUÙ ÒAH oyeHKu ZAyòuHbi acmeHocifiepbi e paùoue Bpanna (Kap- nambi). C r y f l H a r e o ( J ) H 3 H K a H REOAETHKA M e x o c j i o B a i j K o ì i A H 2 , 8 6 - 8 7 , ( 1 9 5 8 ) . ( 2 3 ) K A p y c E . B . , rioiAOUfenue ynpyiux BOAH e eopHbix nopoòax npii cmatfUOHapHOM eo36yncdeHuu. H 3 B . A H C C C P , c e p . r e o(J)H3. 4 , ( 1 9 5 8 ) .