DAMAGE TO BRIDGES DURING EARTHQUAKES 651 



explained the cracks as due to earth waves, and assumed that a 

 single crack, if one only was formed, opened just a half-wave-length 

 back from the river's edge on either side. 



When we take into account the observed effect of the earth move- 

 ments upon bridges, it is clear that something quite different from the 

 mere passage of an earth wave must be invoked in order to explain 

 these geological changes. Not only is the space between valley walls 

 in part closed up, but for a considerable distance back from the 

 banks the bridge approaches show the effect of compression (see 

 especially Fig. 1, A). The piers of the bridge which rest upon the 

 stream bed also suffer changes not explainable, upon Oldham's 

 theory (see Fig. 3, A and B). 'V\%ether occupied by streams or not 

 (see Fig. 3, D) it seems to be clear that the vicinity of valleys is marked 

 by unusual surface compression in a direction at right angles to the 

 valleys. 



It is perhaps unnecessary to here bring forward the evidence 

 obtained from observations of a different kind that local compression 

 of the ground actually occurs during earthquakes. On the one 

 hand, there are the continuous stone curbings and buried metal pipes 

 which are found buckled up from the surface of the ground; and on 

 the other, there are the many variations in the longitudinal shear 

 along fault lines which must be accounted for either through differ- 

 ential contraction, expansion, or both.' On the Baishiko fault 

 opened in Formosa on March 17, 1906, a change in the direction of 

 the longitudinal shear between two stations less than three-fourths of 

 a mile apart, indicated a change in linear distance between the two 

 points before equally distant of about fourteen fcet.^ 



That the changes of superficies to which we have called attention 

 are wholly restricted to the mantle of unconsolidated rock material is 

 most improbable; though, as we shall see, there is reason for sup- 

 posing that it is locally much larger within the mantle than within the 

 underlying rock. To account for an extension of surface of any por- 

 tion of the consolidated outer shell of the lithosphere, it is only neces- 

 sary to assume a .very slight increase of each of the joint spaces 

 present within the rock. A contraction of the surface area may, 



1 For examples see the author's Earthquakes, pp. 62, 66, 73-75, 228-31. 



2 Omori, Bull. E. I. C, Vol. I, No. 2, PL xvii. 



