AMPLITUDE DISTRIBUTION OF STORM MICROSEISMS 



Marion H. Gilmore 

 U. S. Naval Air Station, Miami, Florida 



Abstract — The work of the Microseismic Re- 

 search Project has been carefully reanalyzed 

 in an effort to determine as many facts as pos- 

 sible concerning the nature of microseisms and 

 their possible operational value in detecting 

 and tracking severe tropical storms. From 

 these data there is no longer any doubt that 

 dominant group microseisms are generated by 

 various types of meteorological disturbances 

 over the oceans. It is still impossible to track 

 storms by means of cross bearings from tri- 

 partite stations. A new method has been de- 

 veloped by the Navy Microseismic Research 

 Project that permits accurate tracking of 

 storms that are far from land. This micro- 

 ratio technique of storm tracking is entirely 

 independent of microseismic bearings from 

 tripartite stations, or geology of the earth's 

 crust, or theories concerning microseism gen- 

 eration ; it is dependent only upon the ampli- 

 tude of microseisms actually recorded from 

 a storm at sea. It is possible, by the use of 

 special microseismic charts, to detect, to track, 

 and to determine changes in the intensity of a 

 storm when it is within range of three or more 

 microseismic stations. 



The Problem of Recording Storm Microseisms 



— The Naval Aerological Service initiated the 

 Microseismic Research Project in 1943 with 

 one major objective, which was to determine 

 if severe tropical storms could be detected and 

 tracked by recording changes in the amplitude 

 and period of microseisms. Rapid progress is 

 now being made in solving this problem. The 

 many new data obtained over a period of eight 

 years by recording microseisms generated by 

 several hundred tropical storms in the Pacific 

 and Caribbean aid in verifying certain theories 

 concerning the origin and method of propaga- 

 tion of storm microseisms. However, this pa- 

 per will present only the details of the new 

 Micro-Ratio technique of storm tracking and 

 give facts and figures showing the degree of ac- 

 curacy obtained by these methods in forecast- 

 ing tropical storm movements. 



The Microseismic Research Project uses 

 Sprengnether type, horizontal component, elec- 

 tromagnetic seismometers with natural periods 

 of approximately 7.0 seconds. Both the seis- 

 mometer and the galvanometer are critically 

 damped and of exactly the same period. In 

 order to standardize the work at each station 



the magnification is held rigidly to 5,000 for 

 ground motion with periods between 3 and 5 

 seconds, and each seismometer is orientated in 

 a N-S direction. A trace up on each record 

 represents an east movement of the ground. 

 There are now 24 such instruments in opera- 

 tion at three single and six tripartite stations 

 in the southeastern United States, the Carib- 

 bean, the North Atlantic and the Western 

 Pacific. The data derived from records made 

 on properly calibrated seismometers are trust- 

 worthy in all respects, as in Figure 1, which 

 shows similar microseisms recorded at Ber- 

 muda on three different days from instruments 

 one-half mile apart. This high degree of stand- 

 ardization is maintained at each station and 

 is the same year after year. 



There are many classes of microseisms re- 

 corded on seismographs but the type generally 

 called "Group Microseisms," Figure 1, is the 

 only class discussed in this paper. Macelwane 

 [1951] says: "These microseisms are regular 

 in wave form and appear in a succession of 

 groups of a few large waves, each with short 

 intervals of slight motion between the groups. 

 They do not appear at all times but in dis- 

 creet sequences which may last for a period 

 of hours or days, building up to a maximum and 

 dying down again. Such a sequence has come 

 to be known as a microseismic storm." There 

 is no longer any doubt that these storm micro- 

 seisms are generated by various types of me- 

 teorological disturbances, but there is yet no 

 complete agreement on the exact manner in 

 which energy derived from the storm is trans- 

 ferred from the storm to the ground. How- 

 ever, it may be pointed out here that the newly 

 developed technique of using the amplitude of 

 microseisms to detect and track tropical 

 storms is valid regardless of the method of gen- 

 erating storm microseisms. 



Group microseisms are always recorded at 

 each Navy seismograph station as soon as a 

 generating source, such as a hurricane or cold 

 front, comes within range of a station. Since 

 this has happened hundreds of times in the past 

 eight years at one or more of the microseismic 

 stations it is now possible to describe certain 

 outstanding characteristics of storm micro- 

 seisms. These facts are all the more note- 

 worthy because they also direct attention to 

 the important problem of "How microseisms 



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