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POPULAR SCIENCE MONTHLY 



origin is about the same, but the injury to its buildings was decidedly 

 less; and Santa Rosa, standing on ground apparently firmer than that 

 at Oakland or San Jose and having a somewhat greater distance from 

 the fault, was nevertheless shaken with extreme violence. 



It is too early to discuss these anomalies. With the data now in 

 hand it seems to be true that there are outlying tracts of high intensity 

 surrounded by areas of relatively low intensity; and these features, if 

 they shall be fully established, will doubtless affect in some important 

 way the general theory of the earthquake. 



One of the chief uses of time observations in connection with most 

 earthquakes has been to determine the position of the origin. As the 

 elastic wave travels outward in all directions from the initial point it 

 reaches successively points on the earth's surface which are more and 

 more remote. Coseismal lines, or lines of simultaneous arrival, are, 

 therefore, closed curves circling about the region of the initial fracture. 

 In the case of the San Francisco earthquake this particular function 

 of the coseismals is not required, because the fracture is visible at the 

 surface ; but they are not therefore without value. It is not to be sup- 

 posed that the yielding of the earth occurred at the same instant 

 throughout the entire extent of the fault plane. We should assume, 

 rather, that the fracture, beginning at some point, was extended thence 

 to the remainder of the tract, a certain amount of time being consumed 

 in its propagation. When the time data have been collected and 

 studied, it may be possible to determine at what point the fracture 

 began and at what rate it was extended. It is hoped also that when 

 the time records and intensity records shall have been systematically 

 discussed there may result some conclusion as to the depth to which 

 the fault extended and also as to its subterranean form. 



Mention has already been made of the question whether the perma- 

 nent dislocation or change of absolute position involved in the faulting 

 was divided between the tracts of land on the two sides or was con- 

 fined to one or the other of them. At first sight it would appear that 

 the only thing susceptible of actual determination is the relative dis- 

 placement, and that the absolute displacement, or the real movement 

 with reference to the earth as a whole, must remain a matter of theory 

 only. Nevertheless, it happens that in this particular instance the 

 real changes in geographic position are not only susceptible of deter- 

 mination, but are actually to be investigated. To illustrate the prob- 

 lem, let XY represent, in ground plan, a portion of the fault line, and 

 let ABB'C be the original position of a straight line intersected by the 

 fault. Assuming for the moment that the dislocation was equal on 

 the two sides of the fault, then the line AB was carried to the position 

 DE, and the line B'C to the position FG. We may think of the dis- 

 tances BE and B'F as each equal to five feet. The dislocation of five 



