38 



Symposium on Microseisms 



Case IV. Figure 7 shows (a) a violent 

 storm, the center a few hundred miles off the 

 North Pacific coast, and (b) a storm moving 

 northeast across the Great Lakes, then along 

 the St. Lawrence Valley. Two Greenland lows 

 appeared earlier but dissipated and are of no 

 importance to us. The Great Lakes low moved 

 north across eastern Labrador into the Davis 

 Strait. The Pacific storm moved very slowly 

 toward the coast. While it was most violent 

 on late January 20, continental microseisms 

 were normal. While it was still violent, but 

 abating slightly, 7 sec. microseisms appeared 

 on a continent-wide basis at a time before the 

 eastern storm had a chance to produce them. 

 While the Pacific storm having abated some- 

 what was still off the coast the center just a 

 hundred or so miles from Sitka, and also at the 

 time the eastern low was in Davis Strait when 

 it should have been producing the observed mi- 

 croseisms if it were the generating medium, 

 continent-wide microseisms, including those at 

 Sitka, had returned to normal. 



Case V. At first glance this seems an in- 

 definite source, except that heavy 8'/2-9 sec. mi- 

 croseisms appeared suddenly at Sitka and Col- 

 lege, Alaska. At nearly the same time 8Vj»-9 

 sec. microseisms with almost parallel amplitude 

 buildups appeared also at Chicago and Colum- 

 bia. The source must have been on the west 

 coast because of the large amplitudes at Sitka 

 and College. 



In Figure 8 the contours represent the ele- 

 vation of the 1000 mb. surface in intervals of 

 200 ft. This is equivalent to about 7 mb. 

 where the intervals on earlier illustrations 

 were 3 or 5 mb. The larger map shows a fair- 

 ly heavy storm over the Aleutian Islands but 

 which abated before it reached the mainland. 

 But the front from this storm reached the en- 

 tire Alaskan coast almost simultaneously 36 

 hrs. later and about 4 hrs. before the 8V-2+ sec. 

 microseisms began to appear. 



This storm, in my opinion, supports the 



7.7 - 8.5 

 MICROSEISM PERIOD 



Colleqe-- 



- -Chicago- 



"5" 



Bozeman 



DECEMBER 1949 



10 



Figure 6. Weather charts 7-9 December 1949 showing a storm moving inland from the coast of 

 Alaska, and a time -per iod- amp litude graph of microseisms recorded at certain North American 

 stations. This storm is believed to have been responsible for the microseisms shown in figure 

 5, but the microseisms appeared after the storm had moved inland. Interval 5 millibars. 



