THE SAX FRANCISCO EARTHQUAKE 



I2 5 



a point on the opposite side of the earth from the origin in about 

 20 minutes; the latter are supposed to pass directly through the earth. 

 In computing the velocities I have taken provisionally as the average 



Hor Intens 

 Pecln. 



Hoi: In fens 

 Decl'n. 



Vert\ Intens. 



Temc 



April 18 

 4AM 



Noon 

 12 



Vert. Intens. 



4- 



12 



Fig. 2. Record of the San Francisco Earthquake on the Eschenhagen Magneto- 

 graph at the Magnetic Observatory, Cheltenham, Maryland, reduced 2 times. [The hours as 

 marked are approximately local mean time. The earthquake effect will be noti ed on the 

 three magnetic elements, horizontal intensity, declination and vertical intensity in the shape 

 of a trumpet formation between 8 and 9 a. m., local mean time. The range or double ampli- 

 tude of the disturbance was about 1,1000 part of the horizontal intensity and about 1 3000 part ot 

 the vertical intensity. On account of the intersecting of the curves, the range in the magnetic 

 declination cannot be given.] 



time of the shocks on the Pacific Coast which gave rise to the effects 

 recorded at distant places as occurring at 5 h 12 m Pacific time or 

 13 h 12 m , April 18, Greenwich time. There may have been earlier 

 preliminary shocks. 2 



2 Professor George Davidson, of the University of California, determined 

 the time of first shock at his home in San Francisco by counting the number 

 of seconds it took him upon awakening and going to his watch and noting the 

 time. Owing to his large experience in the work of the U. S. Coast and Geodetic 

 Survey, the time which he gives he deems to be correct within two seconds, 

 via., 5: 12: 00 Pacific time. 



Professor A. O. Leuschner, of the University of California, according to his 

 article in the Berkeley Reporter, of Berkeley, Gal., April 20, 1906, says: 4 " The 

 best record of the beginning of the heaviest shocks is furnished by the standard 

 clock of the Student Observatory, which stopped at 5 b 12 m 38 s Pacific standard 

 time, while less severe shocks were recorded by Mr. S. Albrecht some 35 seconds 

 earlier. The principal part of the earthquake came in two sections, the first 

 series of vibrations lasting about 40 seconds. The vibration diminished con- 

 siderably during the following 10 seconds and then continued with renewed 

 vigor for about 25 seconds more. But even at this writing, about noon, the 

 disturbance has not as yet subsided, as slight shocks are being recorded at 

 frequent intervals on the Ewing seismograph, which has been restored to working 

 order. [This seismograph was thrown out of action at the beginning of the 

 earthquake; however, a fairly complete record was obtained with the duplex 

 instrument.] The principal direction of motion was from south-southeast to 



