Amplitude Distribution of Storm Microseisms 



55 



Both the Woods Hole Oceanographic Institu- 

 tion and the Beach Erosion Board of the Corps 

 of Engineers, U. S. Army, generously made 

 available a large supply of wave records and 

 wave record analyses. 



Records from U. S. Navy tripartite sta- 

 tions were obtained from the Microseismic Re- 

 search Project at the Hurricane Weather Cen- 

 tral in Miami through the generous coopera- 

 tion of M. A. Gilmore. The writer is deeply 

 grateful to all the institutions and individuals 

 who have so cooperated. 



Discussion from the Floor 



Byerly asked about the fact that the isointen- 

 sity lines were drawn directly across the Flor- 

 ida peninsula. In private discussion later Gil- 

 more answered the question and said that while 

 such had not been observed, he thought it was 

 because storms with 90 knot winds did not oc- 

 cur over the peninsula, but he thought that 

 microseisms would be generated if they did. 

 Press asked about the periods, and Gilmore re- 

 plied that the periods are generally the same 

 for the same location. Gilmore. Storms of 

 different intensities in the same area produce 

 different microseisms at the same station. The 

 larger storms will always produce the larger 

 microseisms. In some areas a 90-knot storm 

 will not produce microseisms at a particular 

 station, whereas a storm of larger intensity 

 will produce larger than normal microseisms at 

 that station. 



(Peoples pointed out that the micro-ratio 

 lines might well indicate the geology of the 

 region. Dinger asked if Gilmore 's method had 

 been applied to hurricane "Easy" of 1951. Gil- 

 more replied no. Ramirez asked how accurate- 

 ly the wind velocities are known.) Gilmore. 

 It is very difficult for the forecaster to estab- 

 lish the true velocity of hurricane winds. The 

 best that he can do is to get an average velocity 

 of the wind which may be, and often is, as 

 much as ten or more knots above or below his 

 estimate. 



(Melton asked if the tripartite stations are 

 still running and do they always show the same 

 errors. Gilmore replied yes, and probably no. 

 Bath asked if Gilmore had plotted amplitude- 

 period ratio lines. Gilmore replied, in only a 

 few cases.) van Straten. For some time, I 

 have been concerned about the terms by which 

 tropical storms are described. One hears dis- 

 cussion and comparison of "90-knot storms" or 

 "120-knot storms." Actually, the magnitude 

 of the wind at the center of the storm is only 

 one factor in the description of a storm. The 



area under the influence of strong winds seems 

 another significant factor. 



In order to determine what factors might 

 be related to microseismic generation, I re- 

 quested that Fleet Weather Centrals Guam and 

 Miami plot three factors concerning a storm 

 against time: (1) center wind speed, (2) area 

 enclosed by the highest closed isobar, (3) area 

 enclosed by the 50-knot isovel. 



The initial reports indicate that the max- 

 ima in microseismic amplitude correspond 

 closely to the area enclosed by the 50-knot iso- 

 vel. This correspondence is much greater than 

 that attempting to relate center wind speed 

 with microseismic amplitude. 



Press. As Dr. van Straten showed this morn- 

 ing, microseisms are affected not by conditions 

 at the very center of the storm, but by condi- 

 tions over an area enclosed by a given isovel. 

 For this reason, statements concerning the 

 presence or absence of the microseisms as a 

 function of the position of the center of the 

 storm are rather dangerous, especially for cy- 

 clones extending over areas as large as those 

 considered by Dr. Carder. If one adds to this 

 the very sharp effect of barriers, one might in- 

 terpret Dr. Carder's results differently. It 

 may be possible to come up with a different 

 interpretation. 



(Press also pointed out that earthquakes 

 indicate a barrier off the California coast. 

 Gutenberg pointed out that a hurricane off the 

 lower California coast had given large micro- 

 seisms at Pasadena and Tucson, but none at 

 Santa Clara and Berkeley. Deacon pointed out 

 that we do not know very much about where 

 the actual wave interference may take place. 

 Longuet-Higgins pointed out with a small ori- 

 gin there was more attenuation close to the 

 source.) 



Longuet-Higgins. One cause contributing to 

 the apparently rapid attenuation with distance 

 of "hurricane" microseisms may be mentioned. 

 If the microseisms are surface waves spread- 

 ing out horizontally from the generating area, 

 their amplitude can be expected to decrease like 

 r _1/2 , where r is the distance from the center 

 (viscous dissipation and structural barriers be- 

 ing disregarded). Microseisms originating in 

 a small generating area, such as may be as- 

 sociated with a hurricane, and recorded near 

 the center, would decrease rapidly with r; but 

 microseisms from a large generating area, such 

 as an extra-topical cyclone, and recorded at 

 greater distances, wouid fall off less rapidly. 



