Storm and Surf Microseisms 



95 



vicinity of the storm, an oceanographic distur- 

 bance is established which radiates through the 

 ocean in the form of ocean swell until it reaches 

 a continental boundary — be it continental 

 shelf or actual coast. There, the disturbance is 

 transmitted to the solid continental block on 

 which is erected the microseismic recorder. It 

 should be noted that this definition of surf 

 microseisms in no way adopts the usual defini- 

 tion of surf — the breaking of the sea against 

 the coast. The mechanism by which the micro- 

 seisms are generated is undefined in both cases. 

 The distinction is merely whether the initial 

 generation is near the storm or at a consider- 

 able distance from the storm. 



Let us examine some of the evidence favor- 

 ing the one theory or the other in much the 

 same way that we in Naval Operations ex- 

 amined it. 



For a number of years, Gilmore used the 

 tripartite method for storm tracking. The 

 fact that he was unable to track certain storms 

 he attributed to the presence of geological bar- 

 riers in the ocean bottom which refracted and 

 reflected the microseismic disturbance to the 

 extent that the direction from which the micro- 

 seisms apparently came bore no simple rela- 

 tionship to the direction of original propaga- 

 tion. On scientific and theoretical grounds, I 

 am not in a position to contest this theory — as 

 indeed I am not in a position to contradict any 

 of the prevalent theories. Two things seem 

 apparent, however, in reviewing the earlier 

 Gilmore work. First, if the ocean bottom is 

 really as discontinuous as is indicated, success- 

 ful tracking of typhoons and hurricanes would 

 appear to be improbable. A half-century of 

 storm tracks indicate that those are so non- 

 reproducible that it would be rare for two to 

 follow the exact same path. Newly apparent 

 barriers would be appearing all the time and no 

 large confidence factor could be given to micro- 

 seismic storm tracking. 



The second thought concerning the results 

 must probably be labelled a more-or-less philo- 

 sophical one. Time and again it has been 

 demonstrated in the history of science that 

 when an accepted theory results in practice in 

 more exceptions to the rule than cases which 

 follow the rule, the theory has been inadequate 

 or incorrect. Certain geological barriers are 

 well established but as more and more unsus- 

 pected ocean barriers appear, the possibility 

 that some additional factor not taken into ac- 

 count by Mr. Gilmore becomes more plausible. 



It seems possible that the unknown factor 

 might be surf microseisms. On the other 

 hand, of course, the ocean barrier theory may 

 be entirely sound. 



The work at the Lamont Geological Ob- 

 servatory would indicate also that microseis- 

 mic disturbances are produced only in the vicin- 

 ity of the storm. Three dominant arguments 



are presented. The first is a number of instan- 

 ces when microseismic level was high and ob- 

 served swell was low. The second is the reverse 

 of this picture — observed swell was high but 

 microseismic response was low. The third in- 

 volves cold frontal passages off the east coast 

 of the United States when the seismometers 

 did not record the relatively strong winds pre- 

 ceding the passage of the front off the coast but 

 began to respond only when the front itself 

 progressed over water. Lower wind velocities 

 behind the front did not apparently affect the 

 microseisms adversely. 



Kammer and Dinger, on the other hand, 

 indicate that microseisms occur when the storm 

 affects shallow water and that the magnitude 

 of response to such "surf action" is such as to 

 mask any direct storm response. 



An individual not directly involved in mic- 

 roseismic research must accept all of the pub- 

 lished data as valid. He must make a choice 

 between two courses of action. He must either 

 abandon further consideration of the problem 

 until the experts reach agreement or he must 

 attempt to derive some logical explanation 

 which will provide consistency in apparently 

 divergent findings. Abandoning the field to 

 the experts would undoubtedly be more dis- 

 creet. Unfortunately, as administrators of the 

 Navy Microseismic Program, that would im- 

 mediately involve withdrawal of Naval Aero- 

 logical support. A less wise but more practical 

 solution is to attempt to resolve the differences. 



As a starting point, let us adopt the as- 

 sumption that microseisms can only be pro- 

 duced when two trains of swell intersect and 

 produce stationary or quasi-stationary waves. 

 This phenomenon may occur in the vicinity of 

 the storm or along a coast. Simple surf or 

 shore pounding would, therefore, not produce 

 microseisms. The absence of microseisms in 

 some of the cases when surf measurements 

 showed high waves would thus be explained. 



The conventional cold front is preceded by 

 southwesterly winds which over a water area 

 would be persistent enough to produce apprecia- 

 ble swell. Following the cold front, the winds 

 are northwesterly. The interaction of the swells 

 produced by these winds and those produced 

 ahead of the front may well produce micro- 

 seisms. The cold front case might be ex- 

 plained in this way. 



Hurricanes paralleling the eastern coast of 

 the United States could be expected to produce 

 large areas of swell which would first reach the 

 continental shelf and then secondarily produce 

 coastal surf. The time lag observed by Donn 

 might conceivably be the result of the time lag 

 between the primary effect and the secondary 

 effect. Thus microseismic activity would build 

 up before the observed waves at the beach. 

 Donn's observations are mainly those taken 



