E v i d e n c e f o r t h e s e a t o f t h e s t r a i n - p r o d u c i n g f o r c e s A . G . G A L A N O P O C L O S R i c e v u t o il 1 S e t t e m b r e 1965 SUMMART. — E v i d e n c e is p r e s e n t e d w h i c h f a v o u r s t h e t h e o r y t h a t t h e s e a t of t h e s t r a i n - p r o d u c i n g forces is in t h e l o w - r i g i d i t y l a y e r n e a r t h e t o p of t h e m a n t l e . R I A S S U N T O . — L a n o t a serve a c o n f e r m a r e la t e o r i a secondo la quale le f o r z e d e f o r m a n t i v e n g o n o localizzate nello s t r a t o a b a s s a r i g i d i t à , pros- simo al m a n t e l l o s u p e r i o r e (sotto la superficie di Mohorovicic). I N T R O D U C T I O N . B à t h and D u d a (1964) in an effort to improve Benioff's method (1951a), being developed for strain release studies, were led to the con- clusion t h a t the earthquake volume V, identifìed with the total after- shock volume, increases with magnitude M, according to the following equation : log V = 9.58 + 1.47 M . Por the aftershock area 8, B à t h and Duda derived a new equation: log 8 = 4.95 + 1 . 2 1 M , which improves a previous relation: log 8 = 5.99 + 1.02 M , found by Utsu and Seki (1955). Considering t h a t V = LWE and 8 = LW, where L, and H are respectively, the length, the width and the vertical extent of aftershock zone in cm, we m a y immediately 4 0 0 A . G . G A L A N O P O U L O S derive f r o m the two relations given b y B à t h and D u d a for t h e earth- quake volume a n d the aftersliock zone, t h e following expression: log H = 4 . 6 3 + 0 . 2 6 M . The equation derived implies t h a t for earthquakes of m a g n i t u d e 7 . 7 to 8 . 7 t h e vertical e x t e n t of aftersliock zone reaches to a t least 43 to 78 k m depths. Taking this for g r a n t e d a n d another v e r y interesting result of Bàtli's and D u d a ' s investigation t h a t " The ratio of fault piane area to the vertical section through the aftershock zone, i.e. F/LH, increases with magnitude, approaching unity for the largest shocks " , it m i g h t be concluded t h a t t h e middle of t h e f a u l t piane of t h e great shocks is - a t least in most cases — immediately below t h e Mohorovieic diseontinuity, i.e. in t h e low-rigidity layer near t h e top of t h e m a n t l e . This impli- cations is strongly supported b y t h e f a c t t h a t ali shocks of m a g n i t u d e 5s 73/4 occurred in t h e area of Greece are of i n t e r m e d i a t e focal d e p t h . The Alaskan e a r t h q u a k e of March 27, 1964, with Richter magnitude of 8 . 4 to 8 . 6 , h a d a focal d e p t h of a b o u t 50 k m . I n a recent investi- gation, F . Press (1965) was led t o tlie conclusion " on the basis of the very graduai reduction of vertical deformation with distances that the faidt could not have extended to depths as sliallow as 50 km and that 100 to 200 km are more likely values. The fault probably carne to within 15 to 20 km of the surface " . Thus t h e vertical e x t e n t found for the f a u l t of t h e Alaskan e a r t h q u a k e exceeds t h e values previously found for other earth- quakes. According to Press " The larger magnitude associateti with the Alaskan earthquake (31 — 8.4) as compared with the others may partially explain the difference ". The f a c t t h a t m a n t l e surface waves and free oseillations are exeited only b y the larger earthquakes strengthens tlie notion t h a t the p r i m a r y faulting associated with these shocks extends f a r below t h e e a r t h ' s crust. Another evidence are t h e empirical relations of I 0 to ili" derived for Constant depths of foci b y V. Karnilc (1964): h = 3 — 5 k m , il/ = 0 . 5 6 I 0 + 0 . 4 5 h = 6 — 10 k m , il/ = 0 . 5 6 Io + 0 . 8 3 li = 11 — IS k m , M = 0.57 Io + 0 . 8 5 li = 19 — 30 k m , il/ = 0 . 5 0 I 0 + 1 . 7 5 Ti = 31 — 50 k m , il/ = 0 . 4 0 /„ + 2 . 3 6 li = 51 — 82 k m , M = 0 . 2 9 L + 3.77 j , , 4 h = 83 - 1 5 0 k m , M = 0 . 3 3 /„ + 3 . 2 3 | M l H W l t h ° U t ^ E V I D E N C E F O R T I I E S E A T OF T H E S T R A I N - P R O D U C I NO F O R C E S 405 The relations derived for M = MLH + ~ÌMH w i t h o u t d e p t h corrections, ~òM*, have approximately a common intersection near I 0 = X I and M = 7. This implies t h a t in case of shallow shocks t h e m a x i m u m in- tensity over observed, I 0 = X I , is a t t a i n e d b y shocks of magnitude 7 ; in other words there are no shallow shocks with m a g n i t u d e greater t h a n 7. D A T A U S E D . I n the following table we give ali shallow and intermediate earth- quakes of magnitude 51/2 occurred in the area of Greece during t h e 120-years interval, 1843-1962, for which period t h e available d a t a were proved to be fairly homogenous. I n t h e intermediate earthquakes we have included ali earthquakes of focal d e p t h > 10 k m . During the time i n t e r v a l considered there were released: 454 shallow shocks with M > 5Va, 210 with M ;:> 6, 83 with M > 6V2, 25 with M > 7 and 12 with M > 71/2- During the same period, 1843-1962, there were released: 74 i n t e r m e d i a t e shocks with M > 51/2, 48 with M ^ 6, 39 with M > 6Va, 17 with M > 7, 11 with M ':> 71/., and 7 with M 5> 8. The d a t a given above m a y be summarized as follows: M 5 V. 6 6 V, 7 7 V. 8 Ni 3 .78 1 .75 0 .69 0 .21 0 .10 — N2 0 .62 0 .40 0 .325 0 .14 0 .092 0 . 0 5 8 log Ni 0 .58 0 .24 — 0 .16 — 0 .68 — 1 .00 — log — 0 .21 — 0 .40 — 0 .49 — 0 .85 — 1 .04 — 1 . 2 3 where NLT N„ is respectively the number of shallow and intermediate shocks of m a g n i t u d e M or greater per one year. These values flt r a t h e r closely t o : log N i = [— 1 . 1 3 + 0 . 8 2 (8 — 3/)] ± 0 . 0 5 = — 1 . 4 3 ± 0 . 0 6 + + (0.82 ± 0.02)(8 — M) log N„ = [— 1 . 2 3 + 0.42 (8 — ili")] ± 0 . 0 6 = — 1 . 2 3 ± 0 . 0 3 + + (0.42 ± 0.01)(8 — M). The logarithm of t h e ratio of the number of sinaller shallow shocks to t h a t of intermediate shocks in the area considered is approximately 3 . 0 0 ; this makes the ratio of numbers a b o u t 1000: 1. Ilowever, t h e logarithm of the ratio of the n u m b e r of shallow shocks of m a g n i t u d e S and over, to t h a t of intermediate shocks is approximately - 0 . 2 0 ; this makes t h e ratio of numbers about 0.63:1, i.e. roughly 6 0 % of shocks 400 A . G. G A L A N O P O U L O S T a b l e I - C A T A L O G U E o r S H A L L O W E A R T H Q U A K E S O F M A G N I T U D E > 5 1 / 2 O C C U R R E D I N T U E A R E A O F G R E E C E D U R I N G T H E P E R I O D 1843-1962. No D a t e M a g n i t u d e a d o p t e d No D a t e M a g n i t u d e a d o p t e d 1 1843, S e p t . 5 6 V« 51 March 6 0 74 2 Oct. 18 6 3U 52 March 13 5 Va 3 1845, J u n e 23 6 7 4 8 53 March 25 5 7 4 Oct. 11 6 7 4 8 54 D e e . 4 6 3U 5 1846, J u n e 10 7 2 55 1867, J a n . 27 6 7 i 6 J u n e . 13 6 V« 56 M a r c h 7 6 7 7 1848, — — 6 7« 57 March 30 6 v 4 8 1851, J a n . 20 6 7« 58 Oct. 22 5 9 9 F e b r . 28 6 *U 59 1868, A p r . 20 6 74 10 Oct. 12 7 ì 60 M a y 3 5 V . 11 Oct. 17 6 3/4 61 Oct. 3 5 6 12 D e e . 29 6 l/« 62 1869, M a r c h 18 5 Va 13 D e e . 29 6 l/« 63 A u g . 14 6 74 14 1852, A u g . 26 6 Vi 64 S e p t . 1 6 74 15 1853, A u g . 18 7 2 65 Dee. 1 7 V» 16 1854, J u l y 30 6 6 66 Dee. 28 6 9 17 1856, N o v . 13 6 au 67 1870, A u g . 1 7 V. 18 1858, F e b r . 21 7 V . 68 S e p t . 28 6 2 19 A p r . 5 6 V 4 69 1871, J a n . 22 6 v« 20 S e p t . 20 6 9 70 A p r . 8 6 V 4 21 Oct. 10 6 8 71 J u n e 7 6 Vi 22 1859, M a r c h 13 6 V* 72 Oct. 8 6 9 23 A u g . 13 5 V» 73 1872, F e b r . 11 7 V. 24 A u g . 21 6 4 74 1873, J a n . 31 6 6 25 S e p t . 12 6 V* 75 J u l y 25 « 74 26 1860, A p r . 10 7 V . 76 Oct. 25 6 1 27 A p r . 15 5 V . 77 1874, J a n . 17 5 Va 28 A p r . 16 6 V 4 78 M a r c h 18 5 V 2 29 M a y 16 6 74 79 N o v . 16 7 3 30 A u g . 6 5 V . 80 1875, A p r . 24 5 Va 31 1861, D e e . 26 7 V . 81 J u l y 7 6 74 32 1862, M a r c h 14 7 V . 82 1876, J u n e 26 6 v« 33 Oct. 4 6 3/4 83 1877, J u l y 2 5 Va 34 1863, A u g . — 6 v« 84 Oct. 13 5 V . 35 1864, J u n e 14 5 l/« 85 Oct. 13 o V , 36 J u l y 17 5 l/. 86 1880, J u l y 29 6 6 37 A u g . 21 5 V . 87 1881, A p r . 3 6 3 38 1865, J u l y 23 6 6 88 1883, Oct. 15 6 4 39 Oct. 10 6 V* 89 1885, F e b r . 18 5 Va 40 Oct. 11 5 V. 90 M a r c h 28 6 0 41 N o v . 11 6 v« 91 Dee, 14 5 Va 42 1866, J a n . 2 6 7 4 8 92 1886, S e p t . 4 5 Va 43 J a n . 13 6 7 4 8 93 N o v . 27 5 Va 44 J a n . 31 6 1 94 1887, May 14 6 5 45 F e b r . 2 6 Vi 95 Oct. 3 6 3 46 F e b r . 6 6 74 96 1888, S e p t . 9 5 8 47 F e b r . 28 6 74 97 1889, A p r . — - 5 Va 48 M a r c h 2 6 74 98 A u g . 25 6 3 49 M a r c h 2 6 7 99 Oct. 25 6 9 50 M a r c h 2 5 7 100 1890, May 21 5 Va E V I D E N C E F O R T I I E S E A T O F T H E S T R A I N - P R O D U C I NO F O R C E S 405 T a b l e I (cont.) No D a t e M a g n i t u d e a d o p t e d No D a t e M a g n i t u d e a d o p t e d 101 1890, M a y 26 6 6 152 1907, A u g . 16 6 7 i 102 Dee. 14 6 5 153 1909, March 8 5 ìi 103 1891, M a y 11 6 1 154 M a y 30 5 9 2 104 J u n e 27 5 v . 155 J u l y 15 5 8 105 S e p t . 18 5 l/« 156 1910, Oct, 27 6 V* 106 1892, J a n . 9 5 5 157 1911, F e b r . 18 7 1 107 Dee. 27 5 7» 158 March 9 5 7 a 108 1893, J a n . 31 6 4 159 Oct. 22 5 1 / / 2 109 F e b r . 1 6 7 4 160 1912, F e b r . 13 7 0 110 F e b r . 9 5 9 161 F e b r . 15 6 74 111 Marcii 12 5 7 « 162 A p r . 1 5 7 a 112 A p r . 17 6 4 163 A p r . 19 5 7 a 113 M a y 23 6 1 164 A p r . 21 5 74 114 J u n e 14 7 7z 165 A u g . 9 7 74 115 Oct. 13 5 V . 166 1913, M a y 25 5 72 116 1894, A p r . 20 6 7 167 1914, Oct. 17 6 0 117 A p r . 27 6 9 168 Oct. 17 5 9 118 J u l y 26 6 74 169 1914, N o v . 23 5 7 2 119 1895, M a y 13 6 74 170 N o v . 27 6 1 120 M a y 14 6 74 171 Dee. 2 5 7 2 121 M a y 14 7 V . 172 1915, J a n . 27 6 0 122 M a y 15 6 4 173 J u n e 4 5 7 123 J u n e 21 6 74 174 A u g . 7 6 3 124 A u g . 6 6 74 175 A u g . 10 6 1 125 A u g . 19 6 74 176 A u g . 10 6 1 126 S e p t . 5 6 7 4 177 A u g . 11 6 4 127 1896, F e b r . 10 5 V a 178 A u g . 11 5 5 128 F e b r . 10 6 7 4 179 A u g . 19 5 7 129 F e b r . 10 6 7 4 180 1916, S e p t . 27 5 5 130 F e b r . 11 5 7 a 181 1917, March 14 6 74 131 F e b r . 11 6 7 4 182 March 26 5 7 2 132 March 18 6 7 4 183 S e p t . 23 5 7 2 133 Oct. 27 6 2 184 1918, J a n . 17 5 7 2 134 N o v . 13 5 V a 185 S e p t . 11 5 7 2 135 1897, F e b r . 12 6 1 186 N o v . 20 5 7 2 136 1897, J u n e 30 5 V a 187 1919, F e b r . 24 5 7 137 1898, F e b r . 11 5 7 2 188 Oct. 25 6 0 138 1898, J u n e 2 6 4 189 N o v . 18 5 7 4 139 J u l y 31 6 2 190 1920, Oct. 18 6 74 140 N o v . 9 6 0 191 Oct. 21 5 9 141 1899, J a n . 22 6 7 192 N o v . 15 6 1 142 1899, S e p t . 20 6 7 4 193 N o v . 25 5 7 2 143 1902, J u l y 5 6 6 194 N o v . 26 6 4 144 N o v . 5 5 7 . 195 N o v . 28 5 7 2 145 1904, A p r . 4 7 7 a 196 N o v . 29 6 2 146 1905, J u n e 1 5 7 2 197 Dee. 18 6 7 4 147 J u n e 3 6 0 198 D e e . 23 6 74 148 Oct. 8 6 4 199 1921, March 30 5 5 149 N o v . 8 7 0 200 J u l y 6 5 5 150 1906, M a r c h 3 6 74 201 1921, S e p t . 13 5 5 151 1906, S e p t . 28 6 74 202 Oct. 21 6 7 4 400 A . G . G A L A N O P O U L O S T a b l e I (cont.) No D a t e M a g n i t u d e a d o p t e d No D a t e M a g n i t u d e a d o p t e d 203 1922, A u g . 11 5 *U 254 M a y 11 o 74 204 1923, J a n . 7 6 1 / / 4 255 A u g . 17 6 . 3 205 M a y 6 6 3 256 1934, F e b r . 4 5 Va 200 D e e . 5 5 7 257 F e b r . 21 1/ O / „ 5 3U « 74 207 D e e . 27 5 V . 258 N o v . 21 1/ O / „ 5 3U « 74 208 1925, F e b r . 7 5 72 259 1935, J a n . 4 1/ O / „ 5 3U « 74 209 1926, M a r c h 18 6 9 260 J a n . 4 5 . 8 210 M a r c h 18 5 72 261 J a n . 4 6 . 0 211 March 19 5 1/ / 2 262 F e b r . IS 5 . 5 212 Dee. 17 6 2 263 M a r c h 30 6 . 0 213 D e e . 17 6 5 264 M a y 2 5 7 a 214 1927, M a r c h 24 5 V. 265 N o v . 7 6 . 0 215 J u n e 30 5 5 266 1936, A p r . 8 6 . 0 216 1928, J a n . 23 5 5 267 A p r . 15 5 . 7 217 Alarcli 31 6 74 268 1937, J a n . 2 5 V a 218 A p r . 18 6 74 269 M a y 23 5 . 7 219 A p r . 18 5 72 270 J u l y 6 6 . 0 220 A p r . 22 6 8 271 1938, J a n . 30 5 . 9 221 A p r . 25 5 72 272 F e b r . 10 5 »/« 222 A p r . 28 5 72 273 March 11 5 . 8 223 D e e . 10 5 6 274 M a r c h 13 5 . 7 224 1930, J a n . 23 5 7 275 May 12 5 V a 225 J a n . 28 6 74 276 J u n e 20 5 . 7 220 F e b r . 23 5 7 277 J u l y 2 5 . 8 227 M a r c h 31 6 I 278 J u l y 20 5 V4 228 A p r . 17 5 8 279 1939, M a y 20 5 V 2 229 J u n e 25 5 / 2 280 J u n e 23 5 V a 230 N o v . 21 6 0 281 A u g . 9 6 . 1 231 N o v . 21 6 74 282 S e p t . 22 0 V a 232 D e e . 2 6 74 283 1940, J a n . 6 5 . 6 233 1931, J a n . 4 5 6 284 F e b r . 23 5 V a 234 M a r c h 7 6 0 285 F e b r . 29 6 . 6 235 M a r c h 8 6 74 286 1941 , March 1 o V . 236 J u l y 12 5 8 287 May 14 5 V a 237 1931 , S e p t . 13 6 0 288 M a y 16 5 V a 238 N o v . 15 5 7 a 289 May 23 6 . 0 239 N o v . 23 6 4 290 May 28 5 V a 240 1932, M a r c h 9 5 7 2 291 J u n e 24 5 V a 241 A p r . 27 5 7 2 292 J u l y 13 5 7 4 242 May 14 5 7 2 293 J u l y 23 5 !/ ° i a 243 J u n e 29 5 7 2 294 S e p t . 1 5 1 / ° 12 244 S e p t . 26 6 9 295 Dee, 13 6 . 0 245 S e p t . S e p t . 26 6 1 296 1942, J u n e 16 5 . 6 240 S e p t . S e p t . 29 6 74 297 A u g . 27 5 V a 247 S e p t , 30 5 V . 298 S e p t . 1 5 . S 248 Oct. 23 5 7 2 299 Oct. 28 6 . 0 249 N o v . 1 5 7 2 300 Oct. 28 6 . 0 250 1933, J a n . 5 5 7 * 301 1943, J u l y 23 5 74 251 M a r c h 14 5 6 302 1944, M a r c h 14 5 . 9 252 A p r . 28 5 i/ 303 J u n e 25 6 . 0 253 May 8 6 1 2 304 J u l y 30 5 . 6 E V I D E N C E F O R T I I E S E A T O F T H E S T R AI N - P R O D U C I N O F O R C E S 4 0 5 T a b l e I (cont. ) No D a t e M a g n i t u d e a d o p t e d No D a t e M a g n i t u d e a d o p t e d 305 1944, Oct. 6 7 . 2 356 A u g . 12 6 . 2 306 Oct, 7 5 Va 357 A u g . 12 5 V» 307 1945, Sept, 2 6 Va 358 A u g . 12 6 . Ò " 308 1946, A p r . 12 5 Va 359 A u g . 12 5 . 5 30!) A p r . 16 5 . 5 360 Aug. 13 5 »/» 310 1947, M a r c h 21 5 V, 361 S e p t . 5 5 . 8 311 A p r . 12 5 . 7 362 S e p t , 14 5 . 7 312 J u n e 1 5 Va 363 Oct. 10 5 Va 313 J u l y 21 5 . 5 364 Oct. 21 5 . 7 314 Aug. 30 6 . 3 365 Oct. 21 6 . 5 315 Oct. 6 7 . 0 366 N o v . 28 5 V- 316 N o v . 29 5 Va 367 Dee, 28 5 Va 317 Dee. 9 5 . 5 368 1954, J a n . 2 5 72 318 1948, M a r c h 29 5 Va 369 March 8 5 Va 319 March 29 5 . 9 " 370 A p r . 17 5 Va 320 A p r . 22 6 . 4 371 A p r . 30 7 . 0 321 M a y 26 5 . 5 372 May 1 5 Va 322 J u n e 30 6 . 4 373 1954, May 3 5 Va 323 Oct. 10 5 . 8 374 May 4 5 . 7 324 Oct. 18 5 . 5 375 May 4 5 . 6 325 1949, J a n . 4 5 . 7 376 May 25 5 . 7 326 J u n e 17 6 . 7 377 J u l y 18 5 Va 327 J u l y 23 6 . 8 378 A ug. 3 6 . 0 328 1950, S e p t . 23 5 */« 379 Dee. 23 5 . 9 329 1951 , A p r . 5 6. 1 380 1955, J a n . 3 5 . 8 330 A u g . 24 5 . 5 381 M a r c h 28 5 74 331 A u g . 31 5 382 A p r . 13 6 . 0 332 A u g . 31 5 . 6 383 Apr. 19 « 7 i 333 Oct. 1 5 7a 384 A p r . 21 6 . 0 334 1952, M a r c h 19 5 3/., 385 J u n e 2 5 Va 335 Oct. 5 5 3/4 386 J u l y 9 5 Va 336 Oct. 10 5 V, 387 J u l y 16 6 . 8 337 Oct. 13 5 7* 388 1956, J a n . 6 5 . 7 338 Dee. 31 •r> 74 389 M a y 15 5 . 6 339 1952, Dee. 31 5 ^4 390 May 15 5 . 8 340 1953, J a n . 7 5 . 5 391 May 18 5 . 8 341 F e b r . 7 6 . 0 392 J u l y 9 7 7a 342 M a r c h 18 7 . 2 393 J u l y 9 0 3/4 343 March 18 5 . 6 394 J u l y 9 5 . 7 344 M a y 2 5 Va 395 J u l y 9 5 . 0 345 J u n e 3 5 3U 390 J u l y 9 5 Va 346 J u n e 13 5 V2 397 J u l y 9 5 . 5 347 J u n e 18 5 V2 398 J u l y 10 5 7 4 348 J u l y 22 5 72 399 J u l y 10 5 V4 349 A u g . 9 6 . 5 400 J u l y 22 5 . 5 350 A u g . 11 6 . 8 401 J u l y 30 5 . 6 351 A u g . 11 5 72 402 J u l y 30 6 . 0 352 A u g . 11 5 72 403 J u l y 30 5 . 6 353 A u g . 12 5 Va 404 A u g . 16 5 . 5 354 A u g . 12 7 . 2 405 Sept, S e p t . 6 5 . 7 355 Aug. 12 5 . 6 406 Sept, S e p t . 16 5 Va 4 0 6 A . CT. G A L A N O P O U L O S T a b l e I (cont.) No D a t e M a g n i t u d e a d o p t e d No D a t e M a g n i t u d e a d o p t e d 407 1956, N o v . 2 5 74 431 1958, J u n e 5 5 V a 408 N o v . 20 5 V . 432 J u l y 15 5 72 409 1957, J a n . 23 5 7 2 433 J u l y 17 5 7 4 410 F e b r . 19 6 . 0 434 A u g . 27 6 . 5 411 M a r c h 8 0 72 435 S e p t . 2 5 V 2 412 M a r c h 8 6 74 436 S e p t . 4 5 V i 413 M a r c h 8 5 . 6 437 1959, A p r . 25 6 74 414 M a r c h 8 6 . 0 438 A p r . 25 5 . 6 415 M a r c h 28 5 . 7 439 May 14 « V a 416 A p r . 26 6 . 2 440 J u n e 10 5 74 417 M a y 21 5 . 7 441 A u g . 16 5 V a 41S M a y 29 5 . 5 442 A u g . 17 6 . 0 419 Oct. 5 5 72 443 S e p t . 1 6 V 4 420 Oct. 30 5 . 7 444 Oct. 5 5 V a 421 Oct. 30 5 74 445 Oct. 7 5 74 7 . 0 422 N o v . 26 5 . 7 446 N o v . 15 5 74 7 . 0 423 N o v . 26 5 . 6 447 N o v . 19 5 V a 424 N o v . 27 5 . 9 448 Dee. 1 5 3/4 425 1958, J a n . 2 5 . 8 449 1960, F e b r . 23 5 V a 426 J a n . 16 5 . 7 450 M a r c h 12 5 . 6 427 M a r c h 15 5 . 6 451 M a y 26 6 V a 428 A p r . 3 5 74 452 J u l y 13 5 V a 429 M a y 3 5 72 453 1961, Oct. 2 5 V a 430 M a y 9 5 74 454 1962, J a n . 26 6 V « of magnitude 8 a n d over oceurred in t h e area of Greece are of intermed- iate focal depth. The regression equations for shallow a n d intermed- i a t e sliocks defìne two curves crossing each other a t M = 71/». Using t h e d a t a given in Table I and I I and applying t h e relations: log E1'* = 5 . 9 + 0 . 7 5 M , a n d S «a SE1/» , where E is the seismic wave energy, M tlie e a r t h q u a k e m a g n i t u d e and S t h e strain release in units IO11 (ergs)1^, we fìnd t h a t the average strain release per shallow and intermediate earthquake, i.e. 8/N, for t h e period considered, amounts to 0 . 4 and 2 . 1 , respectively. F o r t h e 60-years interval, 1903-1962, the corresponding values are 0 . 3 and 1 . 0 . Thus t h e average strain release per e a r t h q u a k e of intermediate foca\ depth is 3 to 5 times higher t h a n t h a t per shallow e a r t h q u a k e . E V I D E N C E F O R T I I E S E A T O F T H E S T R A I N - P R O D U C I NO F O R C E S 405 T a b l e I I - C A T A L O G U E O F E A R T H Q U A K E S O F M A G N I T U D E 5 > 5 1 / 2 W I T H F O C A L D E P T I L > 4 0 K M O C C U R R E D I N T I I E A R E A O F G R E E C E D U R I N G T H E P E R I O D 1 8 4 3 - 1 9 6 2 . No D a t e M a g n i t u d e a d o p t e d No D a t e M a g n i t u d e a d o p t e d 1 1846, March 28 8 . 1 38 1935, F e b r . 25 6 7 , 2 1856, Oct. 12 8 . 6 39 March 18 6 V4 3 1862, J u n e 21 6 . 8 40 1936, A p r . 28 5 3U 4 1863, A p r . 22 8 . 5 41 A u g . 8 5 3 J 4 5 1867, F e b r . 4 7 . 9 42 1937, D e e . 16 6 V- 6 S e p t . 20 7 . 6 43 1938, J a n . 16 5 Va 7 1869, A p r . 18 6 . 9 44 J u n e 3 5 3 ! 4 8 1886, A u g . 27 8 . 4 45 S e p t . 18 6 Va 9 1887, J u l y 17 7 . 7 46 1939, Sept, 20 6 Va 10 1897, May 28 7 . 6 47 1942, May 9 5 3/4 11 1903, A u g . 11 8 . 3 48 M a y 21 5 7 a 12 1904, A u g . 11 8 . 0 49 J u n e 21 6 V4 13 1908, May 17 6 3/4 50 1943, J a n . 7 5 Va 14 1910, F e b r . 18 7 0 51 F e b r . 14 6 0 15 A u g . 21 « Va 52 J u n e 27 5 74 16 1911, A p r . 4 7 . 0 53 Oct. 16 6 V, 17 1912, J a n . 24 6 3U 54 1944, J a n . 5 5 . 7 18 1913, S e p t . 30 5 3U 55 May 27 6 V 4 19 1918, J u l y 16 6 Va 56 A u g . 9 5 Va 20 1922, A u g . 13 6 3U 57 1946, A p r . 5 6 0 21 N o v . 11 5 . 7 58 1947 , J u n e 4 6 0 22 1923, A u g . 1 6 . 7 59 J u l y 7 5 Va 23 1925, J uly 6 0 Va 60 1948, F e b r . 9 7 . 1 24 1926, J u n e 26 8 . 3 61 J u l y 24 6 Va 25 J u l y 5 5 Va 62 S e p t . 11 6 5 26 A u g . 30 7 . 0 63 1952, Dee. 17 6 3 / 4 27 S e p t . 19 6 !/ ° /4 64 1953, F e b r . 14 5 3 ! 4 28 1927, J u l y 1 6 . 9 65 J u n e 23 5 3U 29 1929, M a r c h 27 E 3 / ° li 66 1957, A p r . 24 7 0 30 N o v . I I 5 . 7 67 A p r . 25 7 V 4 31 1930, F e b r . 14 6 3 ! 4 08 1958, J u n e 30 6 Va 32 M a r c h 6 5 7 . 69 M a y 27 5 . 6 33 March 6 6 . 0 70 N o v . 15 5 74 34 1931, J u n e 30 5 Va 71 1960, N o v . 4 5 . 8 35 1932, A u g . 15 5 Va 72 N o v . 11 5 . 7 36 1933, A p r . 23 6 3 / 4 73 1961, Mav 23 6 Va 37 1934, N o v . 9 6 3 / 4 74 N o v . 28 5 Va D I S C U S S I O N . A s s u m i n g w i t h B à t h a n d D u d a (1964) t h a t " the mairi difference between large and small earthquakes is not to be found in the strain but 400 A . G . G A L A N O P O U L O S in the total volumes involved ", it might be possible to t h i n k t h a t the higher average strain release per intermediate e a r t h q u a k e is due to the larger ability of deformation of t h e low-rigidity layer of t h e u p p e r mantle. However, eonsidering t h a t a large a m o u n t of deformation in weak layers is rei i e ve d by plastic flow, i.e. t h a t the possibilities of strain Storage in the upper m a n t l e are not very good (Bàth and D u d a , 1963/III), t h e pronounced m a x i m u m of average strain release per e a r t h q u a k e a t 50 to 150 k m focal depths might be due to a combination of stronger strain accumulation and greater seismic gain ratio. The greater " seismic gain ratio " , i.e. tlie greater ratio between seismic energy a n d elast-ic strain energy in the upper mantle, is evidenced by the very small n u m b e r of aftershocks. Owing to tlie lower breaking strength of t h e low- velocity layer of t h e upper hiantle t h e strain àccumulated in a very large volume is probably completely relieved in almost one e a r t h q u a k e . A stronger strain accumulation a t 50 to 150 k m depths combined with a smaller ability of strain S t o r a g e implies t h a t t h e seat of t h e strain producing forces is in t h e upper m a n t l e . I t is reasoned therefore t h a t eonvection currents are probably t h e prevailing agent in producing a strained region in the E a r t h ' s interior. This evidence is fairly corrobo- rated b y t h e f a c t t h a t in Greece t h e perioda of greater seismic activity are initiated b y intermediate shocks, i.e. t h e seismic activity in t h e area of Greece is induced b y processes occurring under the E a r t h ' s crust (Galanopoulos, 1956). L a t e r on Blot (1963) was led to t h e sanie conclusion for other re- gions of t h e globe where intermediate a n d deep foci exist: Southern Pacific, America, J a p a n , Indonesia, I n d o - K u s h . . . According to Blot " The earthquakes of very grcat magnitude are preceded by deep earth- quakes of a magnitude superior to 7, or by several deep earthquakes of a lesser magnitude ". Recently L. Don Leet a n d Florence J . Leet (1965), based 011 other d a t a were led with a different reasoning to believe t h a t " the fundamental cause of earthquakes is movement in the soliqucous mantle ", and even more t h a t " ivhen computational procedures are furtlier refined, it may turn out that ali earthquakes are basically mantle raptures, even when some displacements are coupled to the surface ". Benioff (1951b) has already p u t f o r w a r d evidence t h a t e a r t h q u a k e s of magnitude exceeding 8, m a y not be entirely independent events, b u t m a y be related in some forni of global stress-system. This suggestion is v e r y well understood in t h e assumption t h a t t h e seat of t h e strain producing forces is in t h e upper mantle. E V I D E N C E F O R T I I E S E A T OF T H E STR AI N - P R O D U C I NO F O R C E S 405 A C K N O W L E D G M E N T S . T h e r e s e a r c h r e p o r t e d i n t h i s d o c u m e n t h a s b e e n s p o n s o r e d b y t h e «Air F o r c e Office of S c i e n t i f i c R e s e a r c h » u n d e r c o n t r a c t A F (>l(052)-803 t h r o u g h t h e « E u r o p e a n Office of A e r o s p a c e R e s e a r c h ( O A R ) », « U n i t e d S t a t e s A i r F o r c e », a s p a r t of t h e A d v a n c e d R e s e a r c h P r o j e c t s A g e n c y ' s P r o j e c t Vela Uniform. T h e a u t h o r w o u l d l i k e t o e x p r e s s b i s s i n c e r e t h a n k s t o D r . B . C. P a p a z a c h o s f o r r e a d i n g c r i t i c a l l y t h e m a n u s c r i p t of t h i s p a p e r . R E F E R E N C E S B A T H M . a n d D U D A S. J . , Earthquake Volume, Fault Piane Area, Seismic Energy, Strain, Deformation and Related quantities, " A n n a l i di Geo- fìsica " , 17, 353-368, (1964). B A T I I M . a n d D U D A S. J . , Strain Release in Relation to Forni Depth, " Geo- fìsica P u r a e A p p l i c a t a " , 56, 93-100, ( 1 9 6 3 / I I I ) . B E N I O F F F . , Earthquakes and Rock Creep, P a r t 1: Creep Gharacteristics of Rocks and the Origin of Aftershoclcs, " Bull. Seism. Soc. A m e r . " , 41, 31-62, (1951a). B E N I O F F I I . , Global Strain Accunmlation atul Release as Revealed by Great Earthquakes, " Bull. Geol. Soc. Ara. " , 62, 331-338, (1951b). B L O T C . , Deep Origin of Shallow Earthquakes and Volcanic Eruptions. Ab- s t r a c t s u b m i t t e d t o t h e IIJGG General A s s e m b l y in Berkeley, 19-31, A u g u s t 1963. D O N L E E T L . a n d F L O R E N C E J . L E E T , The Earth's Mantle, " Bull. Seism. Soc. Ani. " , 55, 3, 619-625, (1965). G A L A N O P O U L O S A., The Seismic Efficiency of Greece, " P r a k t . of A t h e n s Aca- d e m y " 31, 368-375, (1956). K A R N I K V . , Seismicity of Europe. P r o g r e s s R e p o r t I V p r e p a r e d for t h e c.onference of t h e E u r o p e a n Seismological Commission in S e p t e m b e r 1964. M i m e o g r a p h e d , 1-22 (Tokio, 1964). P R E S S F . , Displacements, Strains, and Tilts at Teleseismic Distances. " J o u r . G e o p h . R e s . " , 70, 10, 2395-2412, (1965). U T S U T. a n d S E K I A., Relation between the Area of the Aftershock Region and the Energy of the Mairi Shock, " J o u r . Seism. Soc. J a p a n " , Ser. I I , 7, 233-240, (1955).