964 



SCIENCE. 



[N. S. Vol. XXIII. No. 600. 



from this are due to local causes such as 

 drag of the mantle of soil upon the rocks, 

 or the excessive movement of soft incoher- 

 ent deposits. Besides this general hori- 

 zontal displacement of about ten feet there 

 is observable in Sonoma and Mendocino 

 counties a differential vertical movement 

 not exceeding four feet, so far as at present 

 known, whereby the southwest side of the 

 rift Avas raised relatively to the northeast 

 side, so as to present a low scarp facing the 

 northeast. This vertical movement di- 

 minishes to the southeast along the rift 

 line and in San Mateo County is scarcely 

 if at all observable. Still farther south 

 there are suggestions that this movement 

 may have been in the reverse direction, but 

 this needs further field study. 



As a consequence of the movement it is 

 probable that the latitudes and longitudes 

 of all points in the Coast Ranges have been 

 permanently changed a few feet, and that 

 the stations occupied by the Coast and 

 Geodetic Survey in their triangulation 

 Avork have been changed in position. It 

 is hoped that a reoccupation of some of 

 these stations by the Coast and Geodetic 

 Survey may contribute data to the final 

 estimate of the amount of movement. 



The great length of the rift upon which 

 movement has occurred makes this earth- 

 quake unique. Such length implies great 

 depth of rupture, and the study of the 

 question of depth will, it is believed, con- 

 tribute much to current geophysical con- 

 ceptions. 



The time of the beginning of the earth- 

 quake as recorded in the Observatory at 

 Berkeley was 5^ 12™ 6" a.m.. Pacific stan- 

 dard time. The end of the shock was 

 5h j3m 11^ ^ jj^ |.j,g duration being 1" 5^ 

 Within an hour of the main shock twelve 

 minor shocks were observed by Mr. S. 

 Albrecht of the Observatory and their time 

 accurately noted. Before 6^ 52™ p.m. of 

 the same day thirty-one shocks were noted 



in addition to the main disturbance. These 

 minor shocks continued for many days 

 after April 18, and in this respect the 

 earthquake accords in behavior with other 

 notable earthquakes in the past. The 

 minor shocks which succeed the main one 

 are interpreted generally as due to sub- 

 ordinate adjustments of the garth's crust 

 in the tendency to reach equilibrium after 

 the chief movement. 



The collection of time records necessarily 

 proceeds slowly. The purpose of the 

 coseismal curves based upon these records 

 is in general two-fold. In ordinary earth- 

 quakes it is one of the means of locating 

 the seat of the disturbance when there is 

 no surface manifestation of the rupture in 

 the earth's crust. In the present instance, 

 however, the rupture has declared itself 

 in an unmistakable rift observable at the 

 surface, and coseismals are, therefore, un- 

 necessary for the determination of this im- 

 portant factor in the general problem, so 

 far at least as regards the main disturb- 

 ance. It is probable, however, that so 

 radical a change in the equilibrium of the 

 stresses of the earth's crust would induce 

 secondary ruptures and consequently sec- 

 ondary earthquakes closely associated with 

 the chief shock. The careful plotting of 

 the time records may, therefore, be useful 

 in revealing the location of these secondary 

 disturbances, such for example as the one 

 which aft'ected southern California on the 

 afternoon of the eighteenth of April. The 

 second purpose of securing time records is 

 the determination of the velocity of propa- 

 gation of the earth wave ; and the data for 

 this which are likely to be most serviceable 

 are the records obtained at various quite 

 distant seismographic stations. 



The destructive efl:'ects of the earthquake 

 are in the main distributed with reference 

 to the line of rift. The exact limits of the 

 area of destruction have not yet been 

 mapped, but it is known to extend out 



