914 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



rock-burst investigation. Rock bursts are miniature earthquakes of the 

 displacement type and result from the release of rock stresses. These 

 stresses may be due to natural orogenic forces, that is, folding or faulting, 

 or may be caused by the removal of rock material in underground minmg 

 operations. At times these rock bursts are of no small intensity; they have 

 been recorded by seismic stations several hundred miles away. Continu- 

 ous rock-burst records in areas subjected to faulting and mining may not 

 only help in the interpretation of the records of distant earthquake stations, 

 but may be expected to be helpful in predicting fault quakes, cave-ins, 

 and roof failures. This subject is discussed further in the section on strain 

 gauging (page 929). Statistical analyses of rock-burst records appear to 

 indicate a triggering effect of variations in atmospheric pressure. There 

 may also be a parallelism with sun spot cycles and bodily tides. There- 

 fore, the possibility of predicting earthquakes, at least those of the dis- 

 placement type, is probably not so remote as some seismologists appear 

 to believe. 



Most of the measurements of free ground vibrations published to date 

 have been made with regular seismic station equipment. For proposed 

 building sites, dam foundations, and the like, it is necessary to employ 

 recording mechanical or electrical vibrographs. The vibration produced 

 by traffic or industrial plants in the vicinity may be sufficient; otherwise, 

 blasting will be required. However, it is much more satisfactory to use a 

 vibrator and to record a complete response curve. In the interpretation 

 of the field data much help may be obtained from an investigation of models 

 of the surface formation." If the actual geologic conditions have been 

 duplicated with sufficient accuracy on the model (for instance, by securing 

 data on Ihe thicknesses and elastic characteristics of surface formations 

 from refraction surveys), and if the elastic properties of the model material 

 have been scaled down in keeping with dimensional analysis, field data 

 and model results will be in good agreement. 



B. Dynamic Testing 



In various fields of material testing it has been recognized for some time 

 that dynamic tests give better and more complete data on the properties 

 of materials than do static tests. The limitations of static methods are 

 obvious in the testing of soils in situ: with a given load, the area covered 

 is small; the lateral and vertical compression ranges are limited; and the 

 weight sizes that have to be used to obtain sufficient penetration become 

 impracticable even for moderate depth ranges. On the other hand, a 



^«C. Mainka, Forsch. & Fortschr., 14(28), 314-321 (1938). 

 " C. A. Heiland, A.I.M.E. Tech. Piibl. No. 1054, Feb., 1939. 



