Amplitude Distribution of Storm Microseisms 



49 



microseisms which match very well the times 

 of beginning, maximum and termination of the 

 ocean waves. It is suggested that these are the 

 microseisms generated by the relatively strong 

 pressure variations or impulses resulting either 

 from the waves in shallow water or the surf 

 action on the shore. 



This should be contrasted with the very in- 

 tense microseisms that develop almost as soon 

 as a cold front passes seaward from land, as 

 shown in Figure 10. In this case too, strong, 

 onshore winds preceded the cold front which 

 must have generated high waves. It may be 

 safely assumed that in all the above cases, high 

 ocean waves were general along the New Eng- 

 land coast. If microseisms in this area are ever 

 generated from standing waves resulting from 

 reflection and interference of ocean waves, they 

 should have been prominent in these and other 

 similar cases. It may be argued that the shal- 

 low-water area of the strong progressive waves 

 was too small for effective microseism genera- 

 tion through any shallow-water effect. But 

 fronts and post-frontal disturbances produce 

 microseisms when over as small or smaller a 

 water area in the same locality. Any weak sec- 

 ond order effect from interference between on- 

 shore waves and waves that may be set up by 



the offshore-moving front would be limited to 

 the same small area. Since there would be a 

 stiking difference between the periods of the 

 two sets of waves and since an angle of greater 

 than 10° occurs between ocean waves from the 

 south and fronts from the west, the develop- 

 ment of such an effect seems even less likely. 



It has been argued that the bottom in the 

 vicinity just described (off New England) is 

 for some reason not conducive to microseism 

 generation as an explanation of the negligible 

 effect of strong ocean waves. But at other times 

 the same bottom seems quite conducive to mic- 

 roseism generation by fronts or offshore cold 

 winds of such a limited extent that no other 

 area could possibly have been involved. 



Two additional cases will be given now in 

 which the sea surface off northern New Jersey 

 (and presumably southern New England) was 

 calm preceding and during the generation of 

 strong microseism storms. 



Case 1. Analyzed wave records of the 

 Beach Erosion Board made from a bottom pres- 

 sure gauge off Long Branch, New Jersey, give 

 sea conditions as "calm" from February 12 to 

 15, 1952, with surface heights of 0.4 to 1.0 ft. 

 and 9 to 10 seconds period for February 11. 



7/1-1 0-.-. 



7/2-02 



7/2-10 •' 

 12 



14 



16 



18 

 20 



22 



24 



7/3-02 



04 



06 

 08 



» 1 *~r * •' h • ■ •- 



Figure 9B. Weston microseisms of July 1-3, 1946, recorded by long-period vertical. 



