GENERAL DISCUSSION OF THE EARTHQUAKE CONDITIONS. 15 
which successfully withstood the first test failed signally under the 
second, by reason of inadequate fireproofing. A very few withstood 
both tests successfully. 
It is a generally accepted fact that no structure could have with- 
stood the stresses produced by the movement of the earth at the 
"fault trace," along which the maximum intensity of disturbance 
was localized. The stresses produced by the vibrations at other points, 
however, could have been resisted with safety if proper design, first- 
class materials, and honest workmanship — constituting the whole 
secret of earthquake-resisting power — had been employed in the struc- 
tures so located. In tall structures rigidity of construction, attained 
by adequate diagonal and portal bracing, is absolutely essential. In 
such buildings, owing to the inertia of the mass of the upper portion, 
the maximum bending moment exerted by the earthquake was ap- 
plied at some point between the foundation and the top — generally 
just below the middle. While reenforced-concrete structures were 
few in the zone of seismic disturbances, these few stood the test by 
earthquake and fire in a highly satisfactory manner. 'Rigidity and 
stiffness and a high fire resistance are inherent qualities of concrete, 
and this material proved admirably suited to resist these extraordi- 
nary tests. 
The destruction was greatest in structures built on filled ground, 
or on alluvial soils in the valleys of rivers, with foundations which 
did not go through to solid ground, the settling of the earth caused 
by the vibrations resulting in permanent displacement or distortion 
of such structures. At many places where great destruction took 
place, as at Palo Alto, San Jose, Salinas, Santa Rosa, etc., the struc- 
tures were built on soft ground. In structures built on solid ground 
or rock formation the action was much less severe and was confined 
to shaking, producing a maximum oscillation in the upper portions 
of the structure. 
In order to understand properly the conditions under which such 
destruction as that caused by the earthquake could occur, one should 
study the geologic conditions prevalent just prior to the earthquake. 
The history of the Pacific coast is replete with records of seismic 
disturbances, the entire region being in a condition of unstable 
equilibrium and cut by long rifts in the surface, called " faults," 
which have produced a series of long, narrow valleys. The forces 
which produce elevations and subsidence of the surface also produce 
stresses, which finally overcome the adhesion of the opposing rift 
walls, and earthquakes take place in the slipping of these walls, 
through a few inches or a few feet, in the effort to adjust the stresses. 
These earthquakes are of two classes — volcanic and tectonic. The 
former occur in regions of volcanic activity, are shallow in extent, 
