Amplitude Distribution of Storm Microseisms 



45 



1230 



30 Sec 



20Sec 



"HjN 



Figure 6. Palisades resonant seismograph traces for May 16-17, 1952 and charts of the asso- 

 ciated marine cyclone development. 



on conventional records. This will be inves- 

 tigated by running the instruments simultane- 

 ously at both speeds. It is significant that 

 these peaks cannot be identified from trace to 

 trace, indicating the sharpness of the tuning. 

 This will be given more attention below. 



Figure 4 shows the records of the Benioff 

 vertical (B) and the Columbia vertical, (C), 

 both at Palisades together with a chart (A) 

 showing the 0030 and 0630 positions of the cold 

 front associated with the microseism storm. 

 (The dot below "P" on the chart marks the 

 position of the Palisades station; the dotted 

 line off the coast represents the 1,000 fathom 

 depth contour.) The earliest detectable activ- 

 ity on the Benioff, between 0100 and 0200, 

 shows microseisms of 1.4 to 1.6 seconds, with 

 2-second microseisms being recorded between 

 0200 and 0300. This suggests that the early 

 activity on the 2-second trace may be a re- 

 sponse to less than 2-second microseisms. It 

 is hoped that the 1.5-second instrument will 

 show greater magnification at this level and 

 even earlier response to fronts. Although the 

 Palisades vertical shows some discernible ac- 

 tivity of short-period at this time, it is of no 

 value for studying this situation. 



Case 2. May 6-7, 1952 — On Figure 5 A, 

 the 2-second trace shows a gentle amplitude 

 increase about 1900 GMT, on May 6. This cor- 

 responds within one to two hours with the off- 

 shore passage of a cold front in the neighbor- 



hood of the station. The other traces are at 

 background at the time. A more prominent 

 amplitude increase on the 2-second trace oc- 

 curs at 0650 on May 7. This corresponds al- 

 most exactly with the time of abrupt increase 

 of winds from the NW to force 4 and 5 over 

 local, shallow waters near the station. The 

 3-second trace shows a later increase corre- 

 sponding to the spreading of these winds over 

 a more extensive water area. Although the 

 Benioff (Figure 5 B) shows activity corre- 

 sponding in time to the latter event, no evi- 

 dence of the less intense earlier frontal micro- 



PROGRESO, YUC. 



W/nd l/e/oc/Yu 



Figure 7. Wind velocity recording for Yucatan 

 showing gustiness during the passage of a 

 s torm. 



