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University of California Publications. 



[Geology 



Naturally enough great efforts were made to explain the 

 Causes of earthquakes. The very crude notions in ancient times, 

 and among uncivilized people, have suggested the most extra- 

 ordinary ideas. Among the semi-civilized and barbarous races 

 we find the idea was prevalent that an animal of some kind 

 existed below the ground whose movements caused earthquakes ; 

 the different races selected different animals, according to their 

 tastes. In his interesting book on "Seismology," Professor 

 Milne writes: "In Japan it was supposed that there existed 

 beneath the ground a large earth-spider, or jisltin mushi, which 

 later in history became a cat-fish. ... In Mongolia, the earth- 

 shaker is a subterranean hog ; in India, it is a mole ; the Mussul- 

 mans picture it an elephant ; in the Celebes there is a world- 

 supporting hog; while in North America the subterranean 

 creature is a tortoise." There were other futile attempts to 

 explain earthquake phenomena during the period before the 

 development of science. Science, as has been said, is merely 

 systemized knowledge, and it is only by the use of scientific 

 methods, that is, by a careful examination and comparison of 

 the phenomena with known facts and principles, and by a 

 systematic record of the results, that we can gradually approach, 

 step by step, to a truer and more accurate knowledge of the facts. 



It is hardly necessary for me to point out to this audience, 

 the majority of whom were present at the time of the great 

 earthquake of April 18, 1906, the general effects of a great 

 earthquake and, moreover, they have been described in sufficient 

 detail in several well-written books. I shall endeavor in these 

 lectures to put before you, as clearly as possible, a conception 

 of what actually takes place at the time of a tectonic earthquake, 

 the nature and purposes of the instrumental observations at 

 distant stations, the methods of study, the problems awaiting 

 solution, and the revelations regarding the interior of the earth 

 which earthquake study, up to the present time, has made. 



Let me first, however, call your attention to the essential 

 phenomena of earthquakes. It has long been recognized that 

 earthquakes were due to a rapid to-ancl-fro motion of the earth, 

 and that these vibrations were propagated from a center of dis- 

 turbance. It has also been recognized for some time that these 



