ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 223 
were relatively small districts, however, in which brick and frame buildings of ordinary con- 
struction were badly wrecked or quite destroyed. Pavements were fissured, buckled, and 
arched. Sewers and water-mains were broken. In places, portions of streets were moved 
laterally several feet out of place. Well-ballasted street-car tracks, equipped with 8, 10, 
or 11 inch rails, were arched and flexed or thrown into shallow wave forms. The whole 
land surface, sometimes for several blocks together, was deformed into shallow waves of 
irregular extension, length, and amplitude. Effects of this degree of violence were pretty 
closely confined, as has been stated already, to areas of “filled” or ‘‘made” land. Such 
characterize, therefore, only a small portion of the city; but, as it happens, areas of 
commercial importance and of special interest for the scientific purposes of this inquiry. 
In consequence they will require a relatively large share of attention. 
These destructive effects vary in degree from place to place thru the whole range be- 
tween the extremes cited. In some cases this variation is best shown by the character 
of these effects; again by the frequency of their occurrence. The change from strong 
effects to weak sometimes takes place rather abruptly within the distance of a block 
or two, or less. Commonly the localities where very violent effects were produced are 
themselves pretty sharply limited. In such cases, however, there is still a noticeable 
variation in the sort and amount of damage resulting at different points just outside their 
limits, along their peripheries. At other places the destructive effects change gradually 
thru a distance of several blocks. 
This areal variation in the degree of damage indicates clearly a like variation in the 
intensity of the shock. The effects produced are the direct results of the intensity mani- 
fested, since where nearly all kinds of structures are to be found in all districts, of whatever 
intensity, such factors as the individual strength of the injured structures must practically 
cancel in the aggregate result. Consequently the destructive effects furnish a measure of 
the intensity, not very precise, it is true, but the best available, since no seismographic 
instruments were maintained in the city. By a classification of these effects different 
grades of intensity can be recognized and defined. 
Several such classifications have been made by seismologists for this purpose. The 
best known of these is the Rossi-Forel intensity scale, which provides ten scale numbers. 
The first defines a shock just barely perceptible to a sensitive observer, or one recorded by 
a sensitive seismograph; the tenth, a great disaster. The four highest numbers of this 
scale, as republished by the present Commission in its Preliminary Report, are as follows: 
VII. Violent shock, overturning of loose objects; falling of plaster; striking of church 
bells; some chimneys fall. 
VIII. Fall of chimneys; cracks in the walls of buildings. 
IX. Partial or total destruction of some buildings. 
X. Great disasters; overturning of rocks; fissures in the surface of the earth; 
mountain slides. 
The range of intensity in the city did not exceed these limits. Probably it did not reach 
the higher numbers recognized by the scale number X. In only a few small localities were 
the minimum values of scale number VII prevalent. It is easy to see, however, that this 
scale distinguishes its three upper scale numbers in vague terms, particularly with regard 
to effects likely to be produced in a modern city. For this reason it was found unsatis- 
factory for the investigations in San Francisco. 
A scale of greater merit is that devised by Professor Omori, of Tokyo, given below: 
No. 1. Maximum acceleration is 300 mm. per sec. per sec. People run out of houses; 
brick walls of bad construction are slightly cracked; plaster of some old dozo (godowns) 
shaken down; wooden houses so much shaken that cracking noises are produced; trees 
visibly shaken; water in ponds rendered slightly turbid in consequence of the disturbance 
in the mud. 
