298 



The National Geographic Magazine 



long needle or lever, L, is pivoted very 

 delicately to the heavy yoke piece G, se- 

 cured to the ground. The short end of 

 the lever rests very gently against a pro- 

 jection from the steady mass C. The 



long end is tipped with a light stylus, 

 which writes the record on the smoked 

 surface of the drum R. The magnet M 

 serves, by the aid of a clock, to mark 

 minutes of time on the record sheet. 



THE SAN FRANCISCO EARTHQUAKE OF 



APRIL 18, 1906, AS RECORDED BY THE 



COAST AND GEODETIC SURVEY 



MAGNETIC OBSERVATORIES* 



By L. A. Bauer and J. E. Burbank 



THE recent severe earthquake, 

 which caused such serious dam- 

 age in San Francisco and vicin- 

 ity, was recorded on the seismograph at 

 the principal magnetic observatory of 

 the United States Coast and Geodetic 

 Survey at Cheltenham, Maryland, 16 

 miles southeast of Washington, as the 

 largest earthquake recorded since the in- 

 strument was mounted, in December, 

 1904. This observatory is 2,450 miles in 

 an air line from San Francisco. 



The seismograph is of the Bosch- 

 Omori type, especially adapted to detect 

 and record any vibration of the ground. 

 It consists of two parts, one adjustable to 

 record the north-south component of the 

 earth's motion and the other adjusted to 

 record the east-west component. 



At Cheltenham (see table) the pre- 

 liminary tremors began at 8h. 19m. 24s. 

 at a distance of" 2,450 miles from San 

 Francisco. Assuming now the time of 

 the first shock as 5I1. 12m. 38s. Pacific 

 time, or 8h. 12m. 38s. Eastern time, as 

 given by Prof. A. O. Leuschner, of the 

 University of California, the velocity of 

 these tremors, along the chord f connect- 



tlt is a mooted question as to the precise 

 path followed by the preliminary tremors; it is 

 believed, however, that the distance traversed 

 will correspond much more nearly to that of 

 the chord than to that along the surface. Ac- 



ing San Francisco and Cheltenham, is 

 found to be 5.9 miles, or 9.6 kilometers, 

 per second, about 30 times the velocity of 

 sound. The time taken for these waves 

 to cross the continent was 6 minutes 46 

 seconds. The large waves began about 

 8h. 30m. 13s., or at an interval of 17m. 

 35s. after the first shock was felt at San 

 Francisco ; hence the velocity of these 

 waves appears to be about 2.32 miles per 

 second. 



As will be seen from the accompanying 

 table (I), the duration at Cheltenham of 

 the earthquake was about 4 hours. The 

 duration of the strongest motion, how- 

 ever, was only from 8h. 30m. to about 8h. 

 40m. ; during this period the amplitude of 

 the motion was too large to be wholly re- 

 corded by the seismograph. 



The period of vibration in the pre- 

 liminary tremors was about two to four 

 seconds ; in the principal portion it varied 

 from ten to twenty seconds. 



cording to the distinguished seismologist, Prof. 

 E. Wiechert, director of the Geophysical Insti- 

 tute at Gottingen, Germany, who is making at 

 present a brief visit in the United States and 

 to whom was shown the records obtained by 

 the Coast and Geodetic Survey, the large dis- 

 turbances, traveling much less rapidly than the 

 preliminary tremors, proceed most likely along 

 the surface or at least at no very great depth 

 below it. 



* Communicated by Mr O. H. Tittmann, Superintendent of the U. S. Coast and Geodetic Survey. 



