Can Sea Waves Cause Microseisms 



91 



nounced period. The slight increase in micro- 

 seisms during the wave activity can be inter- 

 preted as being associated with the swell rather 

 than being generated directly under the storm 

 for these reasons: 



(1) No simultaneous increase in micro- 

 seisms occurred in Washington. 



(2) Microseisms generated under the 

 storm should also have shown in- 

 creased activity before the arrival of 

 swell. 



According to the Longuet-Higgins theory, 

 a standing-wave pattern is required to transfer 

 the water wave energy to microseisms. A 

 standing wave pattern can conceivably be es- 

 tablished upon reflection of the incoming swell 

 by a sufficiently steep coast. The low level of 

 microseismic activity during the swell from 

 "Easy" would indicate, if the Longuet-Higgins 

 theory is of importance, that the reflected wave 

 energy along the Florida coast is very small. 

 Because of the very gradual slope of the shore 

 along Florida one would indeed expect low 

 reflections. 



The fact that no microseisms of any con- 

 sequence were recorded during the period this 

 intense storm remained over deep water indi- 

 cates either one of two things : (1) microseisms 

 were not generated by any method or (2) the 

 generated microseisms were almost completely 

 attenuated before reaching the continent. The 

 data obtained by NRL is unable to resolve 

 which of these two factors is the important one. 

 Carder (1951) has presented evidence to 

 indicate that the attenuation of microseisms 

 propagated through the floor of the Western 

 Atlantic is much greater than the attenuation 

 over continental land masses. If attenuation 

 is the important factor, then the attenuation 

 may vary with the nature of the ocean floor 

 and thus the results could be different in vari- 

 ous parts of the world. Darbyshire (1950), 

 Banerji (1935) and others have pre- 

 sented evidence that microseisms are generated 

 in deep water and have been recorded at distant 

 points in the case of storms over the Eastern 

 Atlantic, the mid-Bay of Bengal, and the Paci- 

 fic. In view of these observations which con- 

 trasts with the observations in the Western 

 Atlantic it may be inferred that attenuation is 

 a much greater factor in the Western Atlantic 

 than in certain other parts of the world. 



It is of interest to point out the fact that 

 longer period (7.5 to 8.0 second) mocroseisms 

 are evident on curves A and B, Figure 4, as 

 occurring at Washington and Orlando on the 

 morning of 12 September. The records of 

 these microseisms were nicely formed and of a 

 regular nature. The simultaneity in time and 

 period of these microseisms at Washington and 

 Orlando would indicate a common area of 

 generation. The fact that the storm at this 



particular time was dissipating itself over the 

 shallow areas off the coast of Newfoundland is 

 further evidence that a storm moving from 

 deep water to shallow water begins to generate 

 microseisms. Intense winter microseisms are 

 frequently observed when low-pressure areas 

 move over this portion of the North Atlantic. 



Let us consider Figure 3 which shows sim- 

 ultaneous data on wave and microseismic acti- 

 vity obtained during hurricane "How." The 

 wave gage was fortuitously placed in a strate- 

 gic location slightly to the north of the area 

 where the storm entered the Atlantic. We 

 may therefore assume that, if waves are re- 

 sponsible for the generation of microseisms, the 

 waves as measured at this time should yield the 

 best possible correlation inasmuch as the waves 

 were confined to the water areas near the 

 gages. Let us therefore compare the water 

 wave amplitude and the position of the storm. 

 We note an abrupt increase in wave amplitude 

 during the early morning of 2 October, reach- 

 ing a maximum about 1200 and dropping off 

 abruptly about 2000. Referring again to 

 Figure 1 we see that the forward part of the 

 storm entered the Atlantic in the morning of 

 2 October with strong winds blowing from 

 south-southeast and bringing waves toward 

 Cocoa Beach. At about 1200 the center of the 

 storm moved into the Atlantic and by 2000 the 

 winds in the trailing part of the hurricane were 

 from the north, thus effecting a reversal of 

 wind as it existed 20 hours previously over this 

 area. This reversal of wind is evident on the 

 wave records by a rather abrupt decrease in 

 wave amplitude. On Figure 3 we see from 

 curve C that the maximum microseisms oc- 

 curred just after the wind reversal. From 

 curves D and E we observe that during the 

 period when the water wave activity was con- 

 fined to an area near the wave recorder the 

 period of the water waves was closely two 

 times the period of the microseisms. It should 

 also be pointed out that the magnitude of the 

 arbitrary units used as a measure of microseis- 

 mic amplitude on the Orlando records during 

 "Easy" and "How" are the same. It is appar- 

 ent that, although the height of water waves 

 recorded during the two storms is about the 

 same, the amplitude of the microseisms during 

 "How" was five or six times as large as the 

 amplitude during "Easy" and in the case of 

 "How" the amplitude was very outstanding 

 above the normal background. 



From the above facts one may make the 

 following interpretations : 



(1) The correlation between one half the 

 period of the waterwave and the 

 period of the microseisms during 

 "How" lends support to the Longuet- 

 Higgins theory. 



(2) The reversal of wind and the setting 

 up of waves in a direction more or 



