Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 913 



blasts, trafl&c, various industrial activities, earthquakes, wind, surf, and 

 the like. They are largely responsible for the phenomenon known as 

 seismic unrest. Since there are, at almost every locality, well-defined 

 geologic formations that are capable of oscillation and impress their fre- 

 quency characteristics on microseismic and earthquake records, the 

 action of the ground and its constituent parts may be considered like that 

 of a seismograph and may be characterized by a single or by several 

 natural frequencies and damping ratios. Harmonics of such frequencies, 

 where recorded, are in the ratio of 1:3:5.^* The frequency characteristics 

 of the ground determine its response to external impulses, this reaction 

 being defined in seismology by the so-called "station factor." The pres- 

 ence of formation members capable of free oscillation accounts for the 

 observation of identical predominant frequencies in the microseismic 

 unrest in earthquake and in explosion records, and these usually agree with 

 the resonance frequencies excited by vibrators. Statistical analyses of 

 earthquake records have been made by various investigators to determine 

 predominant ground frequencies. Examples are the investigations carried 

 out recently by B. Gutenberg'* as part of a U. S. Coast and Geodetic 

 Survey earthquake research program in California. 



As stated before, the damage done by earthquakes and artificial vibra- 

 tions depends (1) on the djmamic response of a given building or structure 

 to the ground vibrations, and (2), although to a lesser extent, on the 

 reaction of the ground to incoming earthquake waves. The frequencies 

 of distant quakes are usually so low as to be completely out of resonance 

 with the natural ground frequencies. Therefore, such quakes rarely do 

 any damage;'^ besides, their amplitudes would probably be too small even 

 if resonance did occur. Greater damage may be expected from near- 

 quakes and industrial vibrations, since they have greater amplitudes and 

 their frequency is likely closer to the predominant ground frequencies. 

 In some instances it has been possible to correlate prevalent frequency 

 with formation thickness, which is approximately equal to one-quarter of 

 the transverse wave length. Hence, d w v/4/o where v is the velocity of 

 the transverse waves and /o is the natural frequency of free-layer oscilla- 

 tion. How earthquake damages to structures may be evaluated approxi- 

 mately when natural frequencies and damping of ground and structure are 

 known, will be discussed below. 



Related to the problem of recording free ground vibration is that of 



"R. Koehler, Nachr. Ges. Wiss. Goettingen, Math. Phys. Klasse 1, (2), 11-42 

 (1934). 



^» Koehler, Zeit. Geophys., 6(2), 123-126 (1930). 



^* U. S. Coast & Geod. Surv., Spec. Bull. No. 201, 163-224 (1936). 



^* Koehler, Nachr. Ges. Wiss. Goettingen, loc. cit. 



