The San Francisco Earthquake 



299 



The corresponding data as communi- 

 cated in a cable dispatch from the Sitka 

 magnetic observatory will be found in 

 the tables below. 



The air-line distance of Sitka from 

 San Francisco is 1,455 miles, whereas the 

 distance along the chord is about 1,447 

 miles. Accepting the time of the first 

 shock felt at San Francisco as given 

 above by Professor Leuschner, it took 4 

 minutes 18 seconds for the preliminary 

 tremors to reach Sitka, against 6 minutes 

 46 seconds to reach Cheltenham. The 

 velocity along the chord connecting Sitka 

 and San Francisco is 5.6 miles, or 9.0 

 kilometers ; hence about the same as that 

 deduced for the corresponding waves at 

 Cheltenham. 



The present indications are that the 

 source of the earthquake disturbance was 

 not far from San Francisco, though more 

 definite information on this point will be 

 had as soon as the records from the 

 Honolulu observatory have been received. 



According to Professor Leuschner, 

 quoted above, who had in operation dur- 

 ing the earthquake a Ewing seismograph, 

 the principal direction of motion was 

 S.SE. to N.NW. He says that "the re- 

 markable feature of this earthquake, 

 aside from its intensity, was its rotary 

 motion." Comparing the three severest 

 earthquakes on record at San Francisco, 

 viz, those of October 21, 1868, March 30, 

 1898, and the present one, he finds that 

 they have several features in common, 

 namely, that the heaviest shocks are gen- 

 erally in the direction S.SE. to N.NW. ; 

 and, again, while the displacements are 

 very large, the vibration period is com- 

 paratively slow, amounting to about one 

 second in the last two big earthquakes. 

 He says "the slowness of the vibration is 

 the only redeeming feature of these 

 calamities." 



This earthquake, besides being re- 

 corded the world over on seismographs, 

 likewise affected the self-recording mag- 

 netographs at the three magnetic observ- 

 atories of the Coast and Geodetic Sur- 

 vey thus far heard from. 



At the magnetic observatory, Chelten- 



ham, Maryland, the disturbance began 

 about 8:30 a. m., Eastern time, on April 

 18, and continued for about half an hour. 

 It Will be noticed in the table giving the 

 time as recorded by the seismograph that 

 while this disturbance began some time 

 later than the preliminary tremors, it 

 coincides with the principal portion of the 

 disturbance as recorded on the seismo- 

 graph. It affected principally the hori- 

 zontal and vertical components of the 

 earth's magnetic intensity; the greatest 

 disturbance amounts to one-one-thou- 

 sandth part of the horizontal intensity 

 and about one-two-thousandth part of the 

 vertical intensity. It was not of the same 

 character as that due to a cosmic mag- 

 netic storm or as that recorded in connec- 

 tion with the Mont Pele eruption, but 

 appears to be chiefly, if not entirely, me- 

 chanical. 



[At the present moment it cannot be 

 definitely stated whether there was a gen- 

 eral magnetic disturbance similar to that 

 of the Mont Pele eruption in connection 

 with the recent eruptions of Vesuvius. A 

 very similar disturbance began on April 

 8 at 9I1. 39m. a. m., Eastern time. How- 

 ever, this is 12 hours later than the re- 

 ported time of the beginning of the Vesu- 

 vius eruption. It will be necessary to 

 await the full reports from Professor 

 Matteuci's Vesuvius observatory. Mag- 

 netic disturbances affecting simultane- 

 ously points over the entire earth do not 

 necessarily accompany volcanic eruptions. 

 For example, the mighty eruption of 

 Krakatoa was not accompanied by a mag- 

 netic disturbance of the character as- 

 sociated with the Mont Pele eruption ; its 

 effect on magnetic instruments was 

 local.] 



The illustration on page 294 shows the 

 earthquake disturbance as recorded on 

 the magnetograph at the Cheltenham ob- 

 servatory. 



At Baldwin, Kansas, where there is no 

 seismograph, the magnetic instruments 

 also recorded a similar disturbance, last- 

 ing from 8 :24 to 8 131, Eastern time, some 

 time after the preliminary tremors of the 

 earthquake had reached Cheltenham. At 



