12 THE SAN FRANCISCO EARTHQUAKE AND FIRE. 
pulses from remoter parts of the zone of origin ceased to be over- 
powered by those from the nearest parts. The motion in that closing 
phase of the violent part of the earthquake has been compared by an 
observer to the motion of a vessel in a choppy sea ; and I conceive 
that this comparison is the expression of a veritable analogy. 
DISTRIBUTION OF INTENSITY. 
When the isoseismal curves for this earthquake are drawn those 
for the higher intensities will show a remarkable elongation in the 
northwest-southeast direction. They will also show irregularties 
expressive of high intensity in the Santa Rosa Valley and other 
outlying areas. These features are related to the position and form 
of the zone or zones of origin. If drawn in detail they will show 
also the great influence of peculiarities of geologic formations. De- 
tailed surveying has been attempted by the State -commission only in 
San Francisco, partly because the results will there have the greatest 
practical value, and partly because the data are there most available. 
Notwithstanding the abundance of cracks, landslides, and broken 
trees in the region of high intensity, it is nevertheless true that 
natural structures in general are much less sensitive to earthquake 
violence than artificial structures, and for that reason grades of in- 
tensity are most easily mapped in cities. 
The word " intensity " has various meanings as applied to earth- 
quakes. As technically defined, it is the acceleration of the earth 
particle and is a quantity to be measured by the seismograph. But 
the field of instrumental observation is so limited that another defini- 
tion practically obtains — that of power to destroy, a property which 
depends on the duration and direction of the motion as well as its 
acceleration, and which may have other factors. 
It has long been known that buildings and other structures on 
ground of certain kinds are more susceptible to earthquake injury 
than on ground of other kinds, and these differences were strikingly 
illustrated in San Francisco. The general fact appears to be that 
the amplitude of vibration and the acceleration are greater in loose, 
unconsolidated formations than in solid rock. The firmer and more 
elastic a rock formation, the less the intensity of the earthquake 
shocks it transmits to buildings standing on it ; and there is a grada- 
tion in this quality from the firmest bed rock to the loosest gravel, 
sand, and mud. For strong shocks, at least, the intensity is greater 
in loose formations saturated with water than in those that are dry. 
Closely related to the control of intensity by the peculiarities of 
formations is the subject of surface undulations. Observers of strong- 
earthquakes sometimes report visible progressh^e undulations of the 
ground, similar to water waves, and such observations are usually 
