SEISMOLOGY. 



(For previous reports see Year Books Nos. 20, 21.) 

 REPORT OF THE ADVISORY COMMITTEE. 



In reviewing the advances made during 1923 toward the study of Cahfornia 

 earth-movements, with which the Advisory Committee in Seismology has 

 been charged, it is profitable to recall the fact that the procedure recom- 

 mended by the committee and adopted by the President and Trustees of the 

 Institution differs from the one usually followed in the pursuit of seismologic 

 research. In the judgment of the committee, a single observing station, how- 

 ever carefully located, completely equipped, or competently served, would by 

 itself yield little more than a routine record of local or distant disturbances. 

 It is rather through a comprehensive study of surface features, of geologic 

 structure, of the physical, particularly the elastic, properties of the rocks, of 

 the distribution, direction, and magnitude of the tremors and displacements 

 which occur and their relation to permanent zones of structural weakness, that 

 an understanding of these movements and their proximate causes is to be 

 sought. Of all these things the conventional seismograph, even of stations 

 of the first class, can tell us only a part; of local earthquakes of short period, 

 such as the California earth-movements for the most part have been shown to 

 be, it can tell us extremely little. 



It is for these reasons that your committee has departed from the prac- 

 tice which has been developed in other countries where earth-movements 

 are studied, and sought the cooperation of those agencies which, more 

 competently than any others, are able to furnish precise information 

 about the surface and the structure of the portion of the earth's crust which it 

 is proposed to stud3^ These are the Seismological Society of America, the 

 U. S. Coast and Geodetic Survey, the California Institute of Technology, the 

 Hydrographic Office of the U. S. Navy, the U. S. Geological Survey, the 

 University of California, Stanford University, the Observatories at Mount 

 Hamilton, Ukiah, and Mount Wilson, the U. S. Bureau of Standards, and the 

 Geophysical Laboratory of the Carnegie Institution of Washington. 



The plain purpose of such collaboration is to establish, with the highest 

 precision attainable, the present position of all useful surface landmarks, both 

 above and beneath the sea; to measure the aggregate displacements of the 

 past on active faults; to discriminate strained portions of the earth's crust 

 from unstrained and to seek out the forces which operate to produce 

 strains; to discover the composition, the physical structure, the geo- 

 logic age, the density, and the elastic properties of the rocks which com- 

 pose this composite crust, and in particular the zones of structural weakness 

 in it; to develop instruments competent to analyze and measure elastic 

 tremors, differences in pressure, and density; and to study any other factors 

 which may affect the stability of the structure under observation. In this 

 manner it is hoped to attain to a competent knowledge of the character, the 

 proximate causes, and trend of development (in both time and space) of the 

 earth-movements of the region selected for the initial studies (California). 



With this brief statement of purpose in mind, the details of the year's 

 progress will be readily understood. 



1. Fault-zone Geology. 



The local subcommittee on California fault-zone geology, under the chair- 

 manship of Professor Bailey Willis, of Stanford University, including in its 



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