SHAPE AND STRUCTURE OF OCEAN BASINS 9 



It has been known for about 70 years (Rudolph, 1887, 1895) 

 that the Mid-Atlantic Ridge is a seismically active belt. Since the 

 introduction of seismographs, epicenters have been located in 

 increasing numbers and with gradually increasing accuracy. The 

 seismicity of ocean areas has been discussed by various authors 

 (Tams, 1922, 1927, 1928, 1931; Rajko and Linden, 1935; Mush- 

 ketov, 1935; Heck, 1938; and Rothe, 1954). In their classic work 

 Seismicity of the Earth (1949) Gutenberg and Richter discussed 

 three oceanic active earthquake belts, the Arctic Belt, the Atlantic 

 Belt, and the Indian Ocean Belt, and showed a strong correlation 

 of seismicity with mid-ocean rises. But even at that time submarine 

 topography was imperfectly known, particularly in the southern 

 hemisphere, where sensitive seismographs also were few. The 

 number of shocks that could be located was small and the precision 

 of location was poor, so the continuity, and even the trend, of the 

 seismic belts was obscure in several critical areas, e.g., between the 

 South Sandwich Islands and Africa. During recent years epicenter 

 location has greatly improved, so that the continuity, position, 

 and linear character of a belt of shallow-focus earthquakes follow- 

 ing the line shown in Fig. 4 is beyond doubt. 



On all expeditions of Columbia University's Research Vessel 

 Vema, the tracks have been planned to permit crossing the critical 

 parts of the ridge as many times as possible. On every crossing the 

 ridge has been found where it was expected, with its median rift 

 accurately following the mid-oceanic earthquake belt. 



This has been found true so many times that now we may say 

 with certainty that the broad (up to 1000 miles wide) ridge can be 

 traced by following the narrow median earthquake epicenter belt. 

 The width of the belt is not appreciably greater than that attrib- 

 utable to epicenter location uncertainty. 



Recently Vema has crossed the earthquake belt in many places 

 where the existence of a ridge had never been considered as a 

 possibility by most geologists, but in every case the ridge and rift 

 were found (Ewing and Heezen, 1960). 



The earthquake belt extends into continental areas in several 

 places, permitting a more detailed examination of many of the 



