1088 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1957 



seismic belt is of secondary importance when compared with the intense 

 seismic belt associated with the deep trenches of the Pacific.) 



There is a second belt which extends from the Azores to the Iberian 

 Peninsula, and there is a major earthquake belt which follows the \\'('st 

 Indies Island Arc along part of the western boundary of the Atlantic. 

 In addition to these belts there are a few scattered earthquakes around 

 the margin of the ocean basin. 



The locations of earthquakes in the ocean are in general poorly deter- 

 mined, and an accuracy of ±|° ('^SO miles) is about all that is ever 

 claimed. Within this measurement accuracy, all quakes in the central 



VERTICAL EXAGGERATION 40:1 



^ 500 

 O 1000 

 f 1500 

 U-2000 

 2500 



SEISMIC BELT 

 MID-ATLANTIC RIDGE 



ATLANTIS 

 SEAMOUNT 



ALONG 30° N LATITUDE 



Fig. 25 — Profile of Mid-Atlantic Median Rift showing maximum limits of 

 seismic belt. 



part of the Atlantic fall in the Mid-Atlantic rift zone. Thus, there is a 

 strong probability that all quakes on the Mid-Atlantic Ridge fall in 

 this median trench. (Earthquakes of small magnitude are often detected 

 when the signals are not sufficiently clear to allow a position to be de- 

 termined. There are undoubtedly other quakes that are altogether un- 

 detected.) All cables to Europe must cross this rift zone and thus will 

 undoubtedly undergo earthquake shocks from time to time. There is no 

 positive evidence that quakes have broken telegraph cables where they 

 cross the rift. There are, however, several cases where the telegraph 

 cables have parted in or near the rift. 



In the western Atlantic there have been recorded only six earthquakes 

 that fall outside the belts described above. Two of these quakes occurred 

 on the Bermuda Rise well away from cable routes. One quake occurred 

 on the continental slope south of Newfoundland (the famous destructive 

 Grand Banks earthquake); a second small shock nearby apparently 

 caused no damage to cables. Two more quakes were centered off the 

 Labrador Coast. Many cases have been recorded along the Pacific coast 

 of Central and South America where the cables failed in deep water 

 following an earthquake. 



