THE EARTHQUAKE PEOBLEM HECK 377 



If we know the acceleration, amplitude, and period, other desired 

 information can be deduced. The proposed instruments will be 

 capable of recording accelerations up to at least 1/5 g and simple 

 devices will also be available which will record accelerations up to 

 the value of ^, though with no such complete record as for the 

 instruments just mentioned. The instruments themselves will have 

 to be safeguarded to resist destruction from earthquake. 



Earthquakes are in many cases related to movements of the earth's 

 crust which may occur at the time of a severe earthquake or during 

 the interval between great earthquakes. The only satisfactory way 

 to determine such movements is by precise triangulation and level- 

 ing repeated at suitable intervals. Much work of this sort has been 

 done in Japan, especially leveling with significant results. In this 

 country triangulation was executed in California after the earth- 

 quake of 1906 which determined the local movements. During the 

 last few years, as the result of a special appropriation by Congress, 

 triangulation has been executed in California with connection to 

 undisturbed regions to the east, and the plan includes repetition of 

 the observations from time to time. A similar situation exists in 

 regard to precise levels, though more of this work remains to be 

 done. It has been pointed out by geologists that though practically 

 all of the movement in the 1868 and 1906 California earthquakes 

 was horizontal, geological studies indicate prevailing vertical move- 

 ments in the past. 



This work has required high accuracy, and it has been strength- 

 ened both by complete adjustment of all the triangulation of the 

 western half of the United States and also by the establishment 

 of Laplace stations in the region of the special triangulation. These 

 are stations where complete astronomical observations are made and 

 these are combined with the geodetic observations in such a way as 

 to improve the azimuths of lines and consequently the geographical 

 positions of points throughout the scheme. While there are a few 

 cases of large movements, there seems little doubt that at the Point 

 Reyes station there has been horizontal movement of the station 

 from its former position of 10^/2 feet, implying a shift of this amount 

 of the earth's crust. This is the greatest amount observed any- 

 where in California. 



The Japanese have added another type of investigation that they 

 have found very useful. Even before the days of instrumental ob- 

 servation, as far back as 1793,^ the inhabitants of a coastal village 

 noticed a sudden movement of the shore beneath their feet, and as- 

 suming that it meant the arrival of a tidal wave they rushed to the 

 hills. Nothing occurred for four hours. Then came a great earth- 



' Topographical changes accompanying earthqualjos or volcanic eruptions, Pulil. Earthq. 

 luvestig. Comm. in Foreign Languages, No. 25, Tokyo, 1930. 



