288 Scientific Intelligence. 



1851-1865, the elastic strain, which caused the rupture in 1006, 

 had ah'cadyaccnmulated to nearly half its final amount. It seems 

 not improbable, therefore, tiiat the strain was accumulating for 

 one hundred years. 



The distribution of the distortion of the rock resulting from 

 the earthquake shows that the previous strain was highly concen- 

 trated within a few kilometers of the fault line. Tlie strain per 

 unit area must rise to a certain value before fracture results. If 

 the strain had extended over a wider zone, a greater displace- 

 ment would have occurred at the time of fracture. Since, how- 

 ever, the throw of six meters is a large one for an earthquake, it 

 seems conclusive that in fault movements in general the preceding 

 strain between the adjacent crust blocks is confined to a relatively 

 narrow zone. 



Reid next considers the possibility of the prediction of earth- 

 quakes by measuring the growing strains in a fault zone. For 

 this purpose a line of piers should be built, say a kilometer apart, 

 at right angles to the direction of the fault line. Starting from a 

 time when the zone is comparatively free from strain, as is to a 

 large extent true in the San Andreas rift at the present time, geo- 

 detic measurements at intervals of some years would determine 

 the amount of growing distortion in this line at right angles to 

 the fault. When the sm-face becomes strained through an angle 

 of about 1/2000 we should expect a strong shock to relieve this 

 strain. It seems probable that a very long period will elapse 

 before another important earthquake occurs along that part of 

 the San Andreas rift which broke in 1906 ; for we have seen that 

 the strains causing the slip were probably accumulating for one 

 hundred years. It is quite possible, however, for strong earth- 

 quakes to occur on neighboring faults after short intei'vals. The 

 ruptures of the Haywai'ds fault in 1868 and of the San Andreas 

 fault in 1906 are fair examples, though the interval is rather 

 long. The Haywards fault passes through Berkeley, and is 30 

 kilometers northeast of the San Andreas fault. 



The second part of this volume deals with the instrumental 

 records of the earthquake. On account of the great energy of 

 this earthquake it was recorded by seismographs in all parts of 

 the world, and these seismograms supply a large amount of data. 

 The first topic considered is that of the increasing duration of 

 the disturbance as the distance of the station from the origin is 

 greater. The major reason for this is that the longitudinal waves 

 travel faster through the earth than do the transverse waves, but 

 Reid suggests in addition the analogy of a clap of thunder, which 

 although a sudden crash at its point of origin is heard at a dis- 

 tance as a prolonged reverberation. Reflections of the elastic 

 waves from the many surfaces in the outer zone of the crust serve 

 to prolong and mix the vibrations, but with each reflection a 

 partial transformation of the wave motions is produced, a wave 

 motion of a single character being reflected as partly longitudinal, 

 partly transverse vibrations. Consequently the waves become 



