NOTES ON THE JAMAICA EARTHQUAKE 721 
3. The sharpness and intensity of the vibrations seem to be such 
as would characterize shocks arising from rock fractures rather than 
from disturbances due to the slipping of unconsolidated deposits on 
the inclined sea bottom as suggested by Dr. J. W. Spencer.!' The 
shock was distinctly felt by the seismographs at the Weather Bureau 
at Washington, D. C., the amplitude of movement being about 
one-fiftieth of an inch, which it is believed would hardly have been 
the case if due to a landslide in superficial deposits. 
4. The elevation of the east end of Jamaica, near which the fault- 
ing occurred, is about 7,400 feet, while the sea to the south is approxi- 
mately 18,000 feet, making a total difference in level of over 25,000 
feet. It can easily be conceived that isostatic readjustments, such 
as would tend to take place wherever such differences of level exist, 
at closely adjacent points, might have given rise to the fracture. 
It may be noted in this connection that each of the heavy earthquakes 
occurring in the western hemisphere in the last year—San Francisco, 
Valparaiso, Jamaica, and Mexico—all took place in regions of 
similar sharp differences in elevation. On the other hand, all are 
in regions characterized by more or less recent distortion, as brought 
out by the elevated beaches, warped and faulted beds, etc., and 
folding must be regarded as a possible source of the stresses giving 
rise to the fractures. 
tJ. W. Spencer, Science, N. S., Vol. 25, 1907, pp. 966, 967. 
