1921] Lawson: The Mobility of the Coast Ranges of California 473 



strain to a limit and that this accumulation involves a slow creep of 

 the region affected. In the long periods between great earthquakes 

 the energy necessary for such shocks is being stored up in the rocks 

 as elastic compression. The relief from such compression by fault slip 

 of course indicates a sudden shift of ground, and therefore a sudden 

 change of geographic position, which is a matter of great concern 

 to the Coast and Geodetic Survey. But the changes in latitude and 

 longitude due to the preliminary strain creep are not less in magnitude 

 and of no less concern to the Survey than the sudden shifts at times of 

 faulting and earthquake. In order to render the surveys consistent 

 and to make proper adjustments, it will be necessary to ascertain both 

 the rate and direction of strain creep in this mobile portion of the 

 earth's crust. In the course of their regular duties, therefore, officers 

 of the Survey will acquire the data necessary not only to establish on 

 a firm basis the fact of strain creep, but also its rate and direction, 

 with their variations. 



