16 THE SAN FRANCISCO EARTHQUAKE AND FIRE. 
and affect comparatively small areas; the latter not only extend to 
a much greater depth, but also affect much greater areas. The 
region north and south of San Francisco is regarded as particularly 
susceptible to earthquakes of the latter type. 
A. C. Lawson has traced several of the " faults " referred to, and 
about ten years prior to the San Francisco earthquake he indicated 
the location of the present fault trace south of the Golden Gate. 
His relief map of California serves to show the lines of known faults 
by the parallelism of the ridges and valleys. In following these 
valleys one finds evidence of faulting on every hand — the scars and 
markings on the earth and rocks, and the presence of little lakes or 
ponds without visible source of supply other than the watershed of 
the adjacent ridges. According to Lawson, the coast of California 
is rising, and the seismic disturbances whose record is found in the 
rocks have been produced by movements in the process of upheaval 
and subsidence, of folding and faulting, which are, perhaps, greater 
along the coast of California than in any other part of the world. 
Whatever may be the causes of these movements, it is apparent that 
the resultant stresses relieve themselves by producing these faults 
or rifts in the earth's surface. 
The average Californian becomes accustomed to the earthquakes 
which produce " temblors " of sufficient intensity to rattle windows. 
Prior to the great earthquake of April 18, 1906, these temblors 
were of frequent occurrence, but occasioned no alarm and, indeed, 
scarcely excited a passing interest. Over two hundred earthquakes 
were recorded during the period from 1850 to 1886, being more 
prevalent in the vicinity of San Francisco Bay than elsewhere. The 
writer is informed that during the period just prior to April 18 
few if any noticeable temblors occurred. It is fair to assume, there- 
fore, that the great earthquake resulted from an accumulation of 
stresses which would ordinarily have been relieved by smaller move- 
ments. 
The relative intensities of earthquakes are indicated, on the Rossi - 
Forel scale, by Nos. I to X. All under V produce no visible destruc- 
tion, and from V the destruction increases up to X, which represents 
those in "which complete destruction takes place. The writer under- 
stands that the earthquake of April 18 is rated at IX on this scale. 
This earthquake, the most severe in the history of the State, took 
place at 5 hours 13 minutes and 38 seconds a. m., Pacific time, the 
main shock lasting one minute and five seconds. Between this time 
and 7 a. m. about thirty minor shocks were recorded. A zone of 
destruction 50 miles wide was produced, extending for a distance of 
150 to 200 miles north and south of San Francisco (see the map, fig. 1, 
p. 3), beginning at Point Arena, paralleling the coast to Fort Ross, 
