Suggestion f o r a new classification o f earthquakes according to the focal depth P . H E D E R V A R I ( * ) R i c e v u t o il 25 Marzo 196-1 SUMMARY. — I n this paper the A u t h o r suggests to regard the lower limit of the category of the so-called shallow earthquakes identical with the level of the regional isostatic compensation and the upper limit of the so-called deep shocks identical with the Byerly-discontinuity. RIASSUNTO. — In questa nota l ' A . suggerisce di identificare il limite inferiore della categoria dei cosi detti terremoti a profondita normale con il livello della superflcie di compensazione isostatica e il limite superiore dei cosi detti terremoti profondi con la discontinuity di B y e r l y . I n t h e g e o p h y s i c a l p r a c t i c e it is c u s t o m a r y t o c l a s s i f y t h e e a r t h - q u a k e s i n t o three d i f f e r e n t g r o u p s a c c o r d i n g t o t h e d e p t l i of l i y p o c e n t e r . I n a c c o r d a n c e w i t h this c l a s s i f y i n g w e distinguish shallow, i n t e r m e d i a t e a n d d e e p shocks r e s p e c t i v e l y . A c c o r d i n g t o t h e classical d i v i s i o n : T a b l e I . Denomination L i m i t s of focal depth, km Shallow Intermediate Deep 0 - 70 (sometimes 0-00) 70-300 300-750 ( * ) F . R . A . S . Vice-President of International Lunar Society. 3 7 0 P . I I E D E R V A I I I Some authors, e. g. Ritsema (2) and Honda (3) used a somewhat different classifying than that of in Table I . On the 6th figure of Ritsema's paper we find the following division of shocks: Table I I . Gro up Limits of focal depth, km I . 0-40 I I . 40-130 I I I . > 1 3 0 In his quoted paper Honda wrote the followings: " The earthquakes are classed here as shallow when the focal depth does not exceed 100 km, intermediate when it is from 100 km to 250 km, and deep when it exceeds 250 km " . There is no particular geophysical reason to maintain further the classical division of earthquakes (see in Table I ) . It seems to be more real if we connect the limits of focal depth with certain special geophysical levels in the Earth. Such levels may be the different discontinuities-, the Mohorovicic-surface, the Byerly- and Repetti-ones. However here appears a rather great difficulty. As it is well- known, the depth of the Mohorovicic-discontinuity is varying from place to place in accordance with the regional relief of the surface and the rule of isostasy. Although there are many data of the Earth's crustal thickness, due to gravitational and seismical explorations, nevertheless the number of useful values is as the present too small for a theoretical world-wide investigation. On the other hand the crustal thickness under oceanic territories is only 5-15 km or somewhat larger. If we connect the lower limit of the shallow shocks' category with the Mohorovicic-discontinuity, the greatest part of earthquakes over the oceanic territories (earlier pertaining to the shallow category), — would now belong into the group of the inter- mediate quakes. Hereby much of the regularities discovered concerning the shallow shocks all over the world would disappear. Naturally this were not desirable. Therefore ice suggest to regard the lower limit of the shallow shocks' category identical with the level of the regional isostatic N E W C L A S S I F I C A T I O N OF E A R T H Q U A K E S A C C O R D I N G . 3 7 1 compensation. According to the calculations of Gutenberg (4) the pres- sure between the surface and the depth of 50 km is as follows (See on Table HE): Table I I I . Depth, Pressure of column in bars Pressure difference in bars km a b c a-b a-c b-c 0 770 0 0 770 770 0 6 2340 1550 590 790 1750 900 11 3650 2800 2000 790 1050 860 15 4700 3910 3300 790 1400 610 20 6010 5220 4920 790 1090 300 25 7320 0680 0530 640 790 150 30 8780 8140 8150 640 630 — 10 40 11700 11380 11390 320 310 — 10 50 14630 14620 14630 10 0 — 10 a) l>) c) Pressure under a 3 km high mountain area; pressure under continental lowlands; pressure under a 6 km deep ocean. I t is to be seen that the difference of pressure in a depth of 50 km is almost zero. Therefore we may regard the depth of 50 km as the level of regional isostatic compensation. In accordance with these considerations the suggested new classi- fication of earthquakes regarding their focal depth is as follows: • Table I V . Category Limits of focal depth km S - R I C Surface, - level of regional isostatic compensation 0-50 R I C - B Level of regional isostatic compensation, - Byerly-discon- tinuity 51-413 B - R /?i/e)Zy-discontinuity, - Bepetti-discontinuity > 414 Table V . Date L a t . Long, (or approximate location) M T y p e Focal depth km Earlier class 1896 Jan. 19 Central Honsh u 8,1 S - R I C normal shallow 1896 March 4 •! •>. 8,3 1 S - R I C ? normal ? shallow ? 1896 May 5 ? * 8,3 « S - R I C ? normal ? shallow 1 1896 J une 15 N E Honshu 8,7 S - R I C normal shallow 1896 June 15 N E Honshu 8,5 S - R I C normal shallow 1896 June 15 N E Honshu 8,5 S - R I C normal shallow 1896 June 17 Central Honshu 8,5 1 S - R I C normal shallow 1896 Aug. 31 N E Honshu 8,5 S - R I C normal shallow 1897 Febr. 7 40 N 140 E 8,3 S - R I C normal shallow 1897 Febr. 19 38 N 142 E 8,3 S - R I C normal shallow 1897 Febr. 19 38 N 142 E 8,3 S - R I C normal shallow 1897 May 28 E Mediterran 8,0 R I C - B ? 1 intermediate '! 1897 J une 12 26 N 91 E 8,7 S - R I C 25 shallow 1897 Aug. 5 38 N 143 E 8,7 S - R I C normal shallow 1897 Sept. 20 6 N 122 E 8,6 S - R I C normal shallow 1897 Sept. 21 6 N 122 E 8,7 S - R I C normal shallow 1897 Oct. 18 12 N 126 E 8,1 S - R I C normal shallow 1898 Apr. 22 39 N 142 E 8,3 S - R I C normal shallow 1898 June 29 °l 8.3 1 S - R I C normal ? shallow ? 1899 Jan. 24 17 N 98 W 8,4 S - R I C normal shallow 1899 Sept. 4 60 N 142 W 8,3 S - R I C normal shallow 1899 Sept. 10 60 N 142 W 8,6 S - R I C normal shallow 1900 Jan. 20 20 N 105 W 8,3 S - R I C normal shallow 1900 July 29 10 S 165 E 8,1 S - R I C normal shallow 1900 Oct. 9 60 N 142 W 8,3 S - R I C normal shallow > > > > © > > > > > > > > fe 2 > > > > > > > © jo 1 -= 1 a g a S _o S t — j — ; £ w co CO CO "al CO CO 'cO H i H o CO CO & CO & H i CO 57 'r t © o 'cs o ci C3 CC "3 C Z - g lO § § £ r ' co £ r ^ £ £ g r £ g g UO § o o £ 2 io 2 § lO "" c lO o c c § o 2 _ — O C2 O - - O O rj O 5 6 6 o «« o s s rJ l CC.D̂ CCPH M s Z fl 2H p? co « o o P5 U lV lT fl X fl t> CO CO i> ^ CO CO CC CC CO CO cc CO 00 GO 00 GO 00 oc* X 00 00 GC X X 00 oo" cc cc cc 00 oo" C C oc 00 00 CO bb — g o _ l> r 1 p - K*" i— ci O _ o GO (M X CO I- (M 00 _ cq ac o Z D CO cs ^ (M CO CO C5 I- © CO lO t> >0 (M CS X CS t- cc (N r-H CO CO CO CO > > bb ti > O ^ ti: d t'c tl O o © o £1. w 38% s 74% W 45% s 73V4 W 373/„ S 393/4 S 443/. N 73 W 74i/2 W 149% E M 8,0 8.4 8,3 8,0 8,25 8,0 8,3 8 , 0 8,7 8,25 8,0 8.5 8,5 8,3 8,0 8,25 8,9 8,0 T y p e S-RIC S-RIC RIC-B S-RIC S-RIC S - R I C S-RIC S-RIC RIC-B RIC-B S-RIC S-RIC S-RIC S-RIC S-RIC S-RIC S-RIC RIC-B Focal deptli km normal normal 60 normal normal normal normal normal 75 60 normal normal normal normal normal normal normal 60 Earlier class shallow shallow shallow shallow shallow shallow shallow shallow intermediate shallow shallow shallow shallow shallow shallow shallow shallow ( * * ) shallow ( * ) Great Chilean earthquake-sequence. ( * * ) According to Rotlie. 3 7 8 P . I I E D E R V A R I L a s t l y , w e show the n e w classification in t h e case of t h e greatest e a r t h q u a k e s ( M > 8,0) b e t w e e n J a n u a r y 1896 a n d O c t o b e r 1963. T h e m a g n i t u d e - d a t a w e r e d e t e r m i n e d b y G u t e n b e r g (5), R i c h t e r ( 6 ), Galano- poulos ( 7 ), E a t o n ( 8 ), tlie Bulletin of the Seismological S o c i e t y of A m e - rica ( 9 ) and U S Coast a n d G e o d e t i c S u r v e y (1 0) r e s p e c t i v e l y . R E F E R E N C E S (*) Bulletin of the Seismological Society of America, 44, 4, (1954). (2) RITSEMA A . R., The Seismicity of the Sunda Arc in Space and Time. « Indonesian Journal for Natural Science », 1-3. January-June, (1954). (3) HONDA II., The Mechanism of the Earthquakes. Publications of the Dominion Observatory, Ottawa, X X . 2, (1957). ( ' ) GUTENBERG B., Geophysical Data Implied in Isostatic Calculations. Di- vision of the Geological Sciences, California Institute of Technology, 675. Contribution. (5) GUTENBERG B., Great Earthquakes 1896-1903. Transactions of American Geophysical Union, 37. 5, (1956). — Major and Great Earthquakes of 19o7 and 19oS. « Bulletin of the Seismological Society of America », 49, 50, (1959, 1960). — Seismicity of the Earth. Second edition. Prin- ceton, 1954. (Together with Richter, C.F). ( 6 ) RICHTER C. P . , Elementary Seismology. San Francisco, 1958. ( ' ) GALANOPOULOS A . G., A Catalogue of Shocks with To JI VI or .1/ JS 5 for the Years 1S01-1958. Athens, 1960. (8) EATON J. P . , TAKASAKI K . J., Seismological Interpretation. . . « Bulletin of the Seism. Soc. A m . i, 49. (1959). (9) Bulletin of the Seismological Society of America, 43-52. (1953-1962). (10) Preliminary Determinations of Epicentres, 1962 and 1963. U.S. Dept. of Commerce Coast and Geodetic Survey.