Amplitude Distribution of Storm Microseisms 



37 



storm moved off the coast of Labrador. This 

 storm was undoubtedly the indirect cause of 

 the microseisms shown in Figure 2, some of 

 which were received as far away as Fairbanks, 

 Alaska, but not at Bermuda, which is much 

 closer. Normal microseisms east of the Rocky 

 Mountains had periods of 5 sec. or less. The 6 

 sec. microseisms first began to appear at mid- 

 continent stations about an hour before the 

 time shown in Figure 3 and about 8 hours after 

 the center of the storm had moved oceanward 

 from Labrador, at which time storm-generated 

 swells on the northwest limb had time to ap- 

 proach the Labrador coast. The microseisms 

 died out about 15 hours later after the storm 

 had moved oceanward off Greenland. 



Case II. The source of heavy microseisms 

 shown in Figure 4 is undoubtedly a violent 

 storm accompanied by 75-knot winds on its 

 northwest limb, moving oceanward off New- 

 foundland. An amplitude buildup accompanied 

 by 6-6V2 sec. periods began at Chicago a few 

 hours after the storm center had left New- 

 foundland. Heavy microseisms continued un- 

 til the storm center had moved off the east 

 shore of Greenland and after the winds off 



Labrador had shifted from onshore to offshore. 

 The storm continued violent off Greenland long 

 after the continental microseisms had died 

 down. During this time 6+ sec. microseisms 

 were also recorded at Sitka along with 4 J /2 

 sec. periods from a local source. 



Case III. Figure 6 shows two possible 

 sources, one in Davis Strait and the other mov- 

 ing inland from the Pacific off Alaska. The 

 Davis Strait source perhaps was responsible 

 for much of the 6± sec. normal background 

 over the continent. 



While the Pacific source was moving off- 

 shore along the Alaska Peninsula, local 7 sec. 

 microseisms were generated, reaching Fair- 

 banks and Sitka but not in force inland. Then 

 after the storm center had moved inland, 8-8 y 2 

 sec. microseisms began to appear at all con- 

 tinental stations as shown in Figure 5. 



This was a rapidly moving storm. If it 

 were responsible for the 8+ sec. microseisms 

 that were recorded on a continent-wide basis, 

 of which in my mind there is no doubt, the 

 storm moved ahead of the mechanism that 

 actually produced the microseisms. 



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Figure 5. Storm microseism from a west coast source recorded at representative North American 



stations . 



