104 



Symposium on Microseisms 



The fact that in Southern California the 

 microseisms usually reach their maximum at a 

 time when the storm center has moved rather 

 far inland and when its intensity is decreasing 

 makes it easier to investigate their correlation 

 with the meteorological effects there than at 

 the east coast of North America where the 

 storm is usually moving out to sea and fre- 

 quently intensifying at the time of the micro- 

 seismic maximum. It also seems to be easier 

 at the west coast to distinguish between the 

 rather regular microseisms with periods from 4 

 to 10 or more seconds (Fig. 2B) and the irregu- 

 lar microseisms with periods near 4 seconds 

 (Fig. 2A). While microseisms of the regular 

 type usually increase and decrease rather 

 slowly and, during their maxima, have periods 

 of at least 6 seconds, the amplitudes of the ir- 

 regular microseisms frequently increase from 

 small values to large maxima within a few 

 hours with the periods remaining close to 4 

 seconds. This type of irregular microseisms 

 seems to be correlated with strong local winds, 

 especially after passage of cold fronts. Micro- 

 seisms with periods of about 2 seconds are 



frequently superposed (Fig. 2A), especially 

 during local rain. In the author's opinion, the 

 fact that the irregular microseisms with peri- 

 ods of about 4 seconds are frequently confused 

 with the regular microseisms (they do not al- 

 ways differ as much as the examples in Fig. 2) 

 is the main reason for the failure to reach a 

 conclusion as to the source of energy for these 

 microseisms in spite of extensive investigations 

 for about 50 years. 



Discussion 



J. Joseph Lynch, S.J. 

 Fordham University 



Dr. van Straten has presented a thought 

 provoking paper. She has courageously at- 

 tempted to reconcile two schools of thought on 

 the origin of group microseisms — the school 

 that holds that microseisms originate at the 

 center of a storm and the school that holds that 

 microseisms cannot originate at the center of a 

 storm over deep water but rather at some dis- 

 tance from the center in shallow water, as a 



Figure 7 



