Amplitude Distribution of Storm Microseisms 



23 



DISTRIBUT ION AT MIAMI 



Figure 3. Hurricane tracks around Miami 



each other and for the beginning of first in- 

 crease in microseisms. From such data it is 

 possible to draw lines of equal microseismic 

 amplitude for a storm of any intensity around 

 a station where sufficient storm microseisms 

 have been recorded. The lines of equal ampli- 

 tude around Miami for a 90 knot storm are 

 shown in Figure 4. It will be noted that these 

 lines cross Florida and that some storms con- 

 tinued with 90 knot winds. Considerable perti- 

 nent information concerning a storm can be 

 obtained from such amplitude charts. It is 

 an early warning for any station because it is 

 impossible for a 90 knot storm to exist inside 

 the 10 mm line on seismometers standardized 

 by the Microseismic Research Project and not 

 cause the ground to move or shake sufficiently 

 to make microseisms of 10 mm in amplitude. 

 This, therefore, makes the seismograph a one 

 hundred per cent detector of severe tropical 

 storms, regardless of all other types of infor- 

 mation. Storms of greater than 90 knot winds 

 will register the 10 mm amplitude of micro- 

 seisms a greater distance from the station so 

 that such lines can then be drawn for storms 



of any intensity. After a storm comes within 

 range of Miami by crossing the 10 mm line, 

 its intensity can be determined by plotting 

 the position on the 70, 90, 110, or 130 knot am- 

 plitude chart. When a particular amplitude 

 chart agrees with the location of the storm and 

 the amplitude of the microseisms, then the in- 

 tensity on that chart is very close to the actual 

 intensity of the storm. Ten knots, more or 

 less in the intensity of a severe storm will make 

 little difference in its destructiveness to life and 

 property. 



The amplitude lines drawn in Figure 4 are 

 valid for storms moving in any direction in re- 

 lation to Miami. This suggests that the source 

 of microseisms cannot be along the coast or 

 continental shelf because, were this true, a 

 hurricane approaching Florida would undoubt- 

 edly cause larger microseisms than a storm 

 leaving Florida. The lines around Miami are 

 very similar to those around all other stations 

 in that none are concentric with the station in 

 the center. They tend to run close together 

 on one or two sides of a station and are more 



