930 EXPLORATION GEOPHYSICS 



building is located, a dangerous condition of resonance may be anticipated 

 during the earthquake and the insurance premium correspondingly should 

 be high. Furthermore, it is reasonable to expect that if the periods of 

 the ground, the building, and the earthquake are approximately equal, 

 a more dangerous resonance may be set up in the structure than would 

 be the case if these periods were substantially different. 



An evaluation of the earthquake-resistant properties of a particular 

 building or structure requires, therefore, a knowledge of: (1) the 

 dominant or most common periods of the earthquake waves in the area, 



(2) the natural period of the ground on which the building is located, and 



(3) the natural period of vibration of the building. 



The first of the above factors can be deduced from a statistical analysis 

 of earthquake records obtained at permanent seismological stations. Thus, 

 Professor B. Gutenberg deduced from an analysis of numerous records 

 that the following earthquake wave periods are most common in CaH- 

 fornia: 0.2 to Q.Z sec, 0.5 to 0.6 sec. and 1.0 sec. The second factor 

 (natural period of the ground) may be determined by an application of 

 seismic methods, as described in the section on soil dynamics investigations. 

 The third factor (natural period of vibration of the building) can be 

 determined by putting controlled vibrations into the building. The 

 oscillator used for this purpose is of the type previously described, except 

 that much smaller energy is necessary. The natural frequency of the 

 structure under test is determined by plotting a frequency-amplitude 

 curve for the building and noting the frequency corresponding to maxi- 

 mum amplitude.* 



To check buildings for damage, their periods must be redetermined 

 after a severe earthquake. If the period has increased appreciably, the 

 building probably has sufifered considerable structural damage which may 

 not be apparent always from a visual inspection of plaster or masonry 

 cracks. 



SEISMIC METHODS 

 UNITED STATES PATENTS 



859,123 Issued July 2, 1907. Johann Schiitte. "Instrument for Recording and 



Measuring Vibrations." 

 1,095,022 Issued April 28, 1914. Thomas O'Brien. "Seismograph." 

 1,194,376 Issued Aug. 15, 1916. H. J. Furber, Jr. "Apparatus for Ascertaining the 



Relative Locations of Distant Points." 

 1,227,114 Issued May 22, 1917. George A. Campbell. "Electrical Receiving, Trans- 

 lating, or Repeating Circuit." 

 1,240,328 Issued Sept. 18, 1917. Reginald A. Fessenden. "Method and Apparatus for 



Locating Ore Bodies." 

 1,406,445 Issued Feb. 14, 1922. C. A. Culver. "Photographic Receiving Apparatus." 

 1,451,080 Issued April 10, 1923. Ludger Mintrop. "Field Seismograph." 

 1,552,186 Issued Sept. 1, 1925. John A. Anderson. "Seismometer." 

 1,578,998 Issued Mar. 30, 1926. John J. Jakosky. "Shot-Firing System." 

 1,599,538 Issued Sept. 14, 1926. Ludger Mintrop. "Geological Testing Method." 



* Governmental agencies have measured the natural periods of several hundred 

 buildings and many water towers and dams. 



