Amplitude Distribution of Storm Microseisms 



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WHITING FIELD 



DRUU SPEED 1 H.M. PER SEC. 



CHERRY POINT 



MICROSEISMIC RESEARCH 

 PROJECT 



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DRUM SPEED '/ 2 M.U. PER SEC. 



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JACKSONVILLE 



DRUM SPEED ^ M. H. PER SEC. 



COMPARISON OF MICROSEISMS RECORDED 

 SIMULTANEOUSLY AT FOUR STATIONS 

 DURING AUGUST 1950 HURRICANE 



DRUM SPEED B K. U. PEC SEC. 



MIAMI 



Figure 5. Group microseisms simultaneously recorded at four stations 



illustrated in Figure 6 for a 1950 storm. Neith- 

 er aircraft nor ship reports were available for 

 a period of 42 hours from 7 September 1230 

 GCT. The forecasting agencies were left with 

 no alternative but to rely entirely on micro- 

 seismic data from Bermuda to make forecasts 

 of the movement and intensity of the hurri- 

 cane. Because of a large high pressure cell 

 building up to the north, upper right of Figure 

 6, a continued northward movement was un- 

 likely. An amplitude of 30 mm was recorded 

 at 1200 GCT, 7 September, lower right of Fig- 

 ure 6, at which time the storm was reported 

 by aircraft to have 100 knots of wind and its 

 position as reported was located between the 

 35 and 40 mm lines on the 90 knot amplitude 

 chart. That position and the 100 knot wind 

 agreed with the amplitude chart because the 

 100 knot storm was located just outside the 

 30 mm line of a 90 knot chart. A maximum 

 amplitude of 44 mm was reached at 0300 GCT 

 on 8 September and, within an hour, it de- 

 creased 6 mm. The storm could not move 

 north because of the high cell; it could not 

 move west without causing an increase in mi- 

 croseisms at Cherry Point. The only forecast 

 possible to make, from microseismic data, was 



that the storm was drifting slightly northward 

 and slowly filling. This forecast was fully 

 verified at 0630 GCT on 9 September, when the 

 storm was located by a radar plane and found 

 to be between the 45 and 50 mm lines, with 

 only 75 knots of wind. The microseisms were 

 only 37 mm in amplitude. The storm then 

 moved westward, causing Bermuda amplitude 

 to drop rapidly, while that at Cherry Point 

 started increasing. It might be pointed out 

 here that a 100 knot storm was within 600 

 miles of the coast for three days, yet no in- 

 crease in microseisms were recorded at Cherry 

 Point nor Jacksonville, despite the fact that 

 storm-produced swells had ample time to reach 

 the coast. 



The tracks of three typhoons passing through 

 the area between Guam, Manila and Okinawa 

 are plotted on map, Figure 7, together with 

 the corresponding amplitudes of microseisms 

 recorded at the three stations. Typhoon Rita 

 developed as a strong tropical depression late 

 in 1948, west of Guam and continued westward. 

 The Guam, Okinawa, and Manila amplitude 

 chart, lower left of Figure 7, shows a very slow 

 increase in microseisms until the 1200 GCT 



