474 J. E. Burbank — Microseisms Caused by Frost Action. 



Akt. XL. — Microseisms Caused by Frost Action ; by J. E. 



Burbank. 



In a paper on " Some Apparent Variations of the Vertical 

 etc."* the writer called attention to a class of minute earth 

 movements or microseisms of very small amplitude and irreg- 

 ular period varying from 8 seconds to 2 minutes. At that 

 time only a few cases had been identified. 



Recently an abstract of a paper by B. Gutenbergf has come 

 to my attention. In this abstract it is stated that the distri- 

 bution of frost in southwestern Europe up to about 60° ^N\ 

 Lat. and 30° E. Long, can be determined from the records of 

 the 100 kilogram pendulum of the Geophysical Institute at 

 Gottingen. The movement showed a well-defined daily period, 

 max. about 6 a. m. and min. about 3 p. m. and an amplitude 

 which on one occasion showed an earth movement in the 

 north-south direction as great as % millimeter on each side of 

 the position of rest. One would infer that these microseisms 

 sometimes occur when the ground at some distance is freezing 

 and thawing, while at Gottingen it was not frozen. It is diffi- 

 cult to understand how the expansion and contraction of the 

 surface layers in freezing and thawing can produce vibrations 

 or variations of level of sufficient magnitude to be recorded 

 more than a few kilometers beyond the frost zone. 



Cheltenham is so located that the approach of cold waves 

 and freezing of the ground can be studied several days before 

 they reach us, and often the zone of frost is only a short 

 distance, 100 to 200 kilometers, to the north of the station, 

 while the ground at Cheltenham is not frozen. Our pendulums* 

 are not as sensitive as those used hy Gutenberg, and a move- 

 ment of the earth particles of less than '02 millimeter would 

 not be recognized. 



An examination of our seismograms for a period of several 

 years past shows that whenever actual freezing or thawing of 

 the ground is taking place at Cheltenham these microseisms 

 are recorded as irregular tilts or movements of the pendulum 

 back and forth in a somewhat jerky and irregular manner. 

 The most common period is between 8 and 14 seconds, but 

 they frequently have a period as great as two minutes. The 

 amplitude increases with the intensity of the freezing or thaw- 

 ing, the usual range of motion of the earth particles being 

 between *02 and *10 millimeters. These microseisms are con- 



* This Journal, vol. xxx, Nov. 1910, p. 332. 

 f Physikalische Zeitschrift, 1910, pp. 1184-5. 

 \ See preceding paper. 



