372 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 19 31 



interesting in view of Gutenberg's adopted 50 kilometers. This 

 method becomes uncertain at distances greater than 200 kilometers 

 both because the ratio of depth to distance is becoming small and 

 because straight-line transmission can no longer be assumed. There 

 is still much work to be done on this problem. 



When we come to the central region of strong earthquakes, we 

 find conditions of great complexity. Observations by adequate 

 instruments are so few that we are driven to study earthquake effects 

 as the best guide to what has occurred, a method obviously defective. 

 Men under mental stress are not capable of making observations 

 with the impartial attitude of a machine. However, invaluable 

 information is obtained from such reports and when a phenomenon 

 is reported by many witnesses which seems to conflict with the known 

 principles of earthquake wave behavior, a careful investigation 

 should be made. As an illustration, in regions of deep alluvium 

 or of a moderately thick layer of soft material, earthquake waves 

 have been seen to pass over the ground in a manner similar to the 

 ground swell of the ocean. In the case of the California earthquake 

 of 1906, 16 persons in as many different localities reported seeing 

 such AA^aves. Similar reports were made in the case of the Porto 

 Rican earthquake of 1918, and there have been many other examples. 



At first thought this might seem not unreasonable, but it must 

 be remembered that while earthquake waves travel at least 2 miles 

 per second, the ocean swell rarely exceeds 65 feet p^r second. It 

 is therefore difficult to comprehend how we can see the waves pass 

 along. Crests of such waves have been reported as 2 feet above the 

 troughs, but, even if the phenomenon is genuine, the probability 

 is that the height does not exceed 6 inches. It will be interesting 

 to learn the facts by instrumental observation. In some cases a 

 series of parallel cracks in the earth have been found parallel to 

 the crest of the reported waves. Even then the waves may be an 

 illusion due to the effect of the earthquake itself on the observer. 



Railroad tracks sometimes afford evidence that strong forces have 

 been at work. In some cases the distortion appears to have been 

 due to shortening, but it is not clear how this shortening has been 

 brought about. No effort has been made to analyze the forces. 

 The directions taken by falling monuments in cemeteries bear 

 witness to the variations in the forces at work. Often the majority 

 will fall in the same directions, but the remainder will fall at an 

 angle with, or even at right angles to, the prevailing direction. 

 There are many striking examples of turning of a monument on 

 its base, or even of different parts of a monument by different 

 amounts, but without fall. 



