REPORT OF ADVISORY COMMITTEE IN SEISMOLOGY. 177 



importance to carry the system of primary triangulation and precise 

 levels across the mountains to a region of unquestioned stability. 

 This organization alone possesses the requisite equipment and trained 

 personnel for this work. 



3. Southern California. 



We have in southern California a region of intricate faulting, in 

 which many of the faults are still active. Nevertheless, there is no prim- 

 ary triangulation in this region through which the magnitude and 

 displacement of the land-slips which occur there can be determined. 



The plan prepared by Mr. H. 0. Wood and printed in the Journal of 

 Seismology, to which reference has been made above, is concerned 

 primarily with studies in this region and the opportunity here open for 

 the systematic study of seismic disturbances has been adequately 

 emphasized. The reasons for selecting this region for detailed study 

 need not be repeated here. Mr. Wood's plan is elaborate beyond any 

 resources now available for its prosecution, but your Committee rec- 

 ommends that, as soon as suitable instruments are available, a be- 

 ginning be made of continuous seismologic observations at selected 

 points in this region for the study of local earth movements, both tre- 

 mors and displacements. 



4. Development of Instruments. 



It happens that no instrument appropriate for recording local earth- 

 quakes of short period and locating of their sources has yet been 

 developed, although instruments for recording of shocks of distant 

 origin have been in continuous operation at many stations for a 

 number of years. It also happens that one of the necessities of war 

 promoted the development of a receiving instrument for vibrations of 

 extremely short period through water (the supersonic experiments). 

 Somewhere between these two systems there is required an instrument 

 of considerable elasticity through which to record local vibrations of 

 intermediate period, such as are found in the California region and in 

 the countries where volcanism is active. 



Your committee is of the opinion that for the study of local earth 

 movements it is indispensable that appropriate agencies be invited to 

 take up this instrument problem as soon as practicable. It also ap- 

 pears desirable that more than one type of instrument be considered in 

 view of the considerable variety in the character of the shocks to be 

 studied and the great variation in amplitude to be expected. 



5. Isostast. 



Your committee is of the opinion that the gravity observations of 

 the Coast Survey should be continued in connection with the observa- 

 tions of crustal displacement, particularly in view of the immense 

 differences in land elevation in California and also of density between 

 the mountains and the foot of the deep-sea fault off the West Coast. 

 The changes in elevation are so abrupt and the densities hitherto meas- 



