I909-] REID— SEISMOLOGICAL NOTES. 307 



and caused the California earthquake, were the consequences not of 

 special conditions but of the general properties of rock, we may 

 make the general statement that tectonic earthquakes are caused by 

 the gradual relative displacement of neighboring regions, which sets 

 tip elastic strains so great that the rock is ruptured ; and that at the 

 time of the rupture no displacements of large areas take place, but 

 there occurs merely an elastic rebound, to an unstrained position, of 

 the lips of the fault extending but a fetv miles on each side of it. 



It is not necessary of course that the slow displacement should 

 set up a simple horizontal shear, as in the case of the California 

 earthquake, but simply that an elastic strain of some kind should be 

 produced by the relative displacement of adjoining regions. This 

 may be due, for instance, to the slow sinking of a large region with 

 the production of vertical elastic shears around its boundary, and 

 when these shears become sufficiently strong a break will occur and 

 the movement of the two lips will be vertical and in opposite direc- 

 tions, thus producing a fault-scarp. The main, sinking region, 

 however, would not suddenly drop at the time of the break ; there 

 would only be an elastic rebound around its boundaries ; its own 

 displacement having taken place slowly over a long period of time. 

 The elastic strains might also be set up by a horizontal compression, 

 in which case the rock would be folded upward, and when the 

 curvature became too great it would break like a bent stick, both 

 sides of the broken surface flying upwards under the elastic forces 

 and leaving an open fissure between them. Examples of this kind 

 of ruptvire are only known on a small scale. 



It is possible that the rupture may not be confined to a single 

 surface, but may be distributed over a number of neighboring 

 surfaces, and a small block between these surfaces may be displaced 

 as a whole ; but this must be looked upon as a minor phenomenon of 

 the fault-zone, and is not an example of the readjustment of large 

 blocks. 



(b) Some Characteristics of Seismological Instruments. 

 When efforts began to be made, some thirty or forty years ago, 

 to produce an instrument that would record the actual movement 

 of the ground caused by an earthquake, the object aimed at was to 



