50 



Symposium on Microseisms 



Figure 11 E is a simplified surface weather 

 chart for 1830, February 11, and shows a cold 

 front which passed seaward over the coast 

 some 3 to 4 hours earlier. The heavy broken line 

 shows the frontal positions 6 hours earlier, at 

 1230. The dot below the "P" marks the Pali- 

 sades station of the Lamont Geological Observ- 

 atory where the records A to D of Fig. 11 

 were made. "A" is from the seismically com- 

 pensated microbarograph (6) ; "B" is a record- 

 ing from the hot-wire microphone ; "C" is from 

 a vertical component seismograph uncompen- 

 sated for pressure changes and "D" is from the 

 Columbia vertical component electronic seismo- 

 graph. 



The microseisms on "D" can be detected 

 earliest at about the time of the 1830 weather 

 chart and have a period of 2.5 seconds. Earlier 

 studies (Donn 1951) with shorter-period in- 

 struments have detected microseism onset 

 when the front was much closer to the coast. 

 Both the sensitive microbarograph and the hot- 

 wire microphone show pressure changes of 



different, but relatively short-period, beginning 

 about 1500 to 1600, when the front had prob- 

 ably just passed the coast. This would place 

 the Palisades instruments in the cooler turbu- 

 lent zone just to the rear of the front. The exis- 

 tence of this zone has been confirmed by recent 

 findings of Roschke (1952). Since Palisades is 

 close to the coast these atmospheric conditions 

 must have passed seaward very soon there- 

 after, at just about the time of commencement 

 of the microseism storm. Much more severe 

 atmospheric disturbances commenced at Pali- 

 sades at 0200 probably in the main mass of cold 

 air following an unmarked secondary cold front 

 (which is actually shown on earlier weather 

 maps). The microseisms on "D" show marked 

 and continued increase as the turbulent cold 

 air passed seaward and continued for the fol- 

 lowing day, with calm sea conditions prevailing. 

 It is interesting to note that seismogram "C", 

 which is uncompensated for pressure changes 

 shows simultaneous microseisms and somewhat 

 longer-period pressure changes shown by long- 

 period, rather irregular waves which can be 



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Figure 10. Portion of Weston long-period vertical recorded for February 7-8, 1951, and charts 

 of related marine weather conditions. 



