252 



jVA TURE 



[January i6, 1908 



The earthquake, Dr. Gilbert tells us, had its origin 

 chiefly along the line of an old fault. This can be 

 traced from San Juan, about ninety miles south-east 

 of .San Francisco, to Point .\rena, about 120 miles 

 n,,rth-\\est of that city, the total length being approxi- 



j. I. — Fence parted by Earthquak'; Fault, 

 quake is inconspicuous althougb the tli^riz 

 U. K. Gilbert.) 



Tbe fault trac; 

 ..ntal displa;em;l 



matelv 200 miles. There are, however, good reasons 

 for believing that the fracture extends very much 

 farther to the north. The total length of the line of 

 \ielding would therefore have been joo or even 400 

 miles. Its general appearance is 

 that of a huge furrow, the dis- 

 placement of one side of which 

 relatively to that of the other side 

 has varied from 2 to 20 feet. In 

 Fig. I the furrow-like appearance 

 is not visible, but the fence, which 

 is broken across and shifted 

 8i feet, indicates the existence of 

 a sheer, the trace of which is 

 hidden. 



The vertical displacements were 

 comparatively small. Passing out 

 from the main fault are branching 

 cracks. These are particularly 

 noticeable in soft ground. The 

 depth to which this shattering 

 lias extended cannot be directly 

 measured, but that it has descended 

 to a considerable depth is indi- 

 cated by alteration in the general 

 circulation of underground waters. 

 New springs have been created, 

 w'hilst old springs have been closed 

 or altered in their flow. The great 

 length of the main fault suggests 

 that it hid a great depth. Further, 

 ■IS the initial impulse was sufficient 

 III send earth waves round the 

 world it is reasonable to suppose 

 that this was occasioned by the sudden dis- 

 placement of a very large earth block. We 

 know something about its length. Its breadth 

 may be estimated from its distance from more or less 

 parallel faults which yielded or from the width 



NO. 1994, VOL. 77] 



of the area of the molar displacement. Its thicknc^s 

 mav be that of the earth's crust. .A suboccanic mass 

 movement of this size might disturb the Pacific Ocean 

 for twenty-four hours, or shake the world from pole to 

 pole. Were it verv much less it is difficult to imagine 

 that such far-reaching commotions 

 ~ could be originated. This may be 

 mere speculation, but to shake the 

 world a heavv blow needs to be 

 applied over a considerable area. .\ 

 carious observation relating to the 

 length of earthquake waves was 

 made in Tcmales Bay, where, be- 

 fore the earthquake, there was the 

 usual smooth mud plain commonly 

 seen on tidal flats. After the earth- 

 quake this plain was ridged, the 

 crests of the ridges being ten to 

 twenty metres apart. Whether 

 these represent the solidification of 

 gravity waves we are told is not 

 quite clear, but whatever their his- 

 torv may have been, they illustrate 

 the response of a mud flat to earth- 

 quake motion. 



The major and most important 

 part of the publication is written 

 by three engineers, Messrs. R. L. 

 Humphrey, J. .S. Sewell, and Prof. 

 Soule. All have had experience of 

 lires, whilst Prof. .Soule has for 

 many years watched the growth of 

 ali'ie ' (Hlu:' gr-M'ii i-y ■^^" Francisco and was present at 

 its fall. We are told that the 

 whole secret of earthquake-resist- 

 ing power depends upon " proper design, first- 

 class materials, and honest workmanship." The 

 first condition, inasmuch as it involve> radical 

 changes in methods of ordinary construction. 



\ 



1 Arch, Lel.i 



liffen the walls were not tied to them, and helped I 



(Fhotogiaph by Richatd L. Humphrey.) 



night have been underlined. The greatest 

 destruction came from fire, and the modern structures 

 which best resisted both fire and earthquake seem to 

 have been those made of reinforced concrete. Tunnels, 

 flumes, wrought and cast-iron pipes, particularly where 



