Amplitude Distribution of Storm Microseisms 



39 



ocean swell viewpoint of microseism genera- 

 tion. There seems to be no other source of 

 these microseisms except possibly the cold front 

 moving off the Atlantic coast which produced 

 4 or 5 sec. microseisms that were also re- 

 corded at Chicago. 



In the five cases presented here the micro- 

 seisms under discussion were more nearly as- 

 sociated either with the high winds on the 

 shore-directed limb of cyclonic storms hours or 

 days before the microseisms appeared, or with 

 the front from the storm sweeping inland or 

 oceanward. Certainly there is no direct as- 

 sociation between the microseisms and the 

 storm center, or even the area in the storm 

 about the center associated with a given wind 

 velocity. In one case presented here the storm 

 area had dissipated before the appearance of 

 the microseisms. 



The ocean swell idea of microseismic gen- 

 eration has the best support. If these five 

 cases and also some of the examples of hurri- 

 canes presented by Gilmore are examined more 

 critically, microseismic generation could very 

 conceivably result in part from swells driven 

 before winds on one limb of the storm meet- 

 ing swells from oppositely directed winds of 

 an earlier storm, or even the same storm if it 



has small area and is moving fast enough in 

 accordance with the idea advanced by Deacon. 



Other case histories of microseisms which 

 were recorded during the past three seasons 

 could have been used instead of those which 

 were presented here, and would have told the 

 same story. At no time could near-the-con- 

 tinent-generated microseisms be identified in 

 force at Honolulu, Bermuda, or San Juan, in- 

 dicating that the North Pacific or North At- 

 lantic basins are effective absorbing media. On 

 the continent, absorbtion across the western 

 mountains is probably greater than across the 

 central plains area, but the mountains cer- 

 tainly do not cut off the longer period micro- 

 seisms. The Sierra Nevada, or more prob- 

 ably the Central Valley of California, are pos- 

 sible exceptions. 



If the data presented in this paper are 

 correctly interpreted and if the conditions out- 

 lined are general, it is inconceivable that strong 

 microseisms on the continent can have their 

 source far at sea. 



This is supported by Gilmore's paper. On 

 page — he states that two hurricanes 600 miles 

 offshore send microseisms to Bermuda (prob- 

 ably from its Bermuda directed NE limb) but 

 not to the nearest continental station 600 miles 



Figure 7 



Figure 7. Weather charts 20-23 January 1951 showing a violent storm off the west coast of 

 North America and t ime - ampl i tude graph of 7 sec. microseisms recorded at Sitka and Chicago 



during the same interval. 



