470 J. E. Burbmik — One Phase of Microseismic Motion, 



Art. XXXIX. — One Phase of Microseismic Motion ; by 



J. E. BuRBANK. 



Seismologists generally include in the term microseismic 

 motion all pulsations and movements of the earth's crust which 

 are not attributable to earthquakes or to motion of a more or 

 less violent and abrupt nature. 



Microseisms may be due to local causes, as industrial opera- 

 tions and ordinary traffic, storms and waves on adjacent shores, 

 frost action, and possibly by wind, tide, and waves on distant 

 shores. The kind and number of microseisms recorded at any 

 place will naturally be limited by the adjustment and damping 

 of the pendulum and the nature of the record, as a photo- 

 graphic registering seismograph with high magnification will 

 record microseisms when a mechanically registering seismo- 

 graph would give only a smooth straight line. Moreover, with 

 mechanical registration the recording surface may not always 

 be uniformly coated with lampblack and hence will offer vary- 

 ing resistance to the lateral movement of the writing stylus. 

 The mechanical registration with low magnification offers a 

 distinct advantage in studying certain microseisms, since it 

 does not give such a large mass of detail, in which it is often 

 difficult to identify a particular type of motion. 



At the Cheltenham Magnetic Observatory we have been 

 studying the relation between microseismic motion and the 

 variations in atmospheric pressure since 1906. Our seismo- 

 graph is a two-component, 10 kilogram, horizontal pendulum, 

 of the Omori type, with mechanical registration and magnifi- 

 cation of ten times. The periods of the pendulums have been 

 kept between 24 and 29 seconds. With the seismograph oper- 

 ating under these conditions, only the more pronounced micro- 

 seisms are recorded, yet it is an interesting fact that during 

 nearly five years record there have been not more than 25 cases 

 of moderate barometric changes in connection with which mi- 

 croseisms might have been expected and were not found on 

 the seismograph traces. 



The microseisms accompanying atmospheric pressure vari- 

 ations have a remarkably regular wave-like motion which almost 

 always shows a rhythmical increase and decrease of amplitude 

 indicating interference. The waves occur in groups of from 

 6 to 12, and vary in amplitude with the intensity of the baro- 

 metric variation. The most pronounced cases indicate a move- 

 ment of the earth particles at this place amounting to about 

 0*05 millimeters on each side of their mean position. 



The results of our observations from September 1, 1906, to 



