ON MASS-MOVEMENTS IN TECTONIC EARTHQUAKES, 
THE MOVEMENTS BEFORE AND DURING EARTHQUAKES. 
‘The following is the conception of the events leading up to a tectonic earthquake and of 
the earth-movements which take place at the time of the rupture, as developed by the 
observations and study of the California earthquake and by the comparison of these 
observations with what has been observed in other great earthquakes. 
It is impossible for rock to rupture without first being subjected to elastic strains 
greater than it can endure; the only imaginable ways of rapidly setting up these strains 
are by an explosion or by the rapid withdrawal, or accumulation, of material below a por- 
tion of the crust. Both explosions and the rapid flow of molten rock are associated with 
volcanic eruptions and with a class of earthquakes not under present discussion; since 
earthquakes occur not associated with volcanic action, we conclude that the crust, in 
many parts of the earth, is being slowly displaced, and the difference between displacements 
in neighboring regions sets up elastic strains, which may become greater than the rock can 
endure; a rupture then takes place and the strained rock rebounds under its own elastic 
stresses, until the strain is largely or wholly relieved. In the majority of cases, such as 
when there is a general differential elevation or depression of adjoining areas, or where there 
are horizontal displacements, the elastic rebounds on opposite sides of the fault are in 
opposite directions. The directions of the slow relative displacements on the two sides 
of the rupture and of the elastic rebounds, all of which are practically parallel with each 
other, may be vertical, horizontal, or inclined. 
The sudden displacements, which occur at the time of an earthquake, are confined to a 
zone within a few kilometers of the fault-plane, beyond which only the disturbances due 
to elastic vibrations are experienced. The distribution of the distortion of the rock at the 
time of the California earthquake shows that the elastic rebound and consequently the 
elastic shear was greatly concentrated near the fault-plane and was much reduced in 
intensity at even short distances from it; this concentration of the shear brought about a 
strain sufficient to cause rupture after a comparatively small relative displacement of the 
surrounding regions; if the shear had been more uniformly distributed over a wider 
region, a larger relative displacement would have been necessary to cause a rupture and 
there would have been a greater slip at the fault-plane. Therefore, altho it is quite con- 
ceivable that regions at a distance apart of, let us say, several times 20 km., might be 
relatively displaced and set up a state of elastic strain in the broad intervening area, it 
would be necessary that the relative displacements of the distant regions should be at 
least several times 6 meters, in order that the strain should become great enough to cause 
a rupture; and if the strain were less concentrated than it was in California, the relative 
displacements would have to begreater still. Itisonly inthe case of very large earthquakes 
that a slip as great as 6 meters occurs; and we may therefore infer that it is only in the 
case of large earthquakes that the sudden elastic rebound is appreciable as far as 8 or 
10 km. from the fault-plane. 
The rupture does not occur simultaneously at all parts of the fault-plane; but, on 
account of the elastic qualities of the rock, it begins in a very limited area and spreads at 
a rate not exceeding the velocity of compressional waves in the rock. 
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